# The Netwide Assembler: NASM

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## Appendix B: x86 Instruction Reference

This appendix provides a complete list of the machine instructions which NASM will assemble, and a short description of the function of each one.

It is not intended to be exhaustive documentation on the fine details of the instructions' function, such as which exceptions they can trigger: for such documentation, you should go to Intel's Web site, `http://developer.intel.com/design/Pentium4/manuals/`.

Instead, this appendix is intended primarily to provide documentation on the way the instructions may be used within NASM. For example, looking up `LOOP` will tell you that NASM allows `CX` or `ECX` to be specified as an optional second argument to the `LOOP` instruction, to enforce which of the two possible counter registers should be used if the default is not the one desired.

The instructions are not quite listed in alphabetical order, since groups of instructions with similar functions are lumped together in the same entry. Most of them don't move very far from their alphabetic position because of this.

### B.1 Key to Operand Specifications

The instruction descriptions in this appendix specify their operands using the following notation:

- Registers: `reg8` denotes an 8-bit general purpose register, `reg16` denotes a 16-bit general purpose register, and `reg32` a 32-bit one. `fpureg` denotes one of the eight FPU stack registers, `mmxreg` denotes one of the eight 64-bit MMX registers, and `segreg` denotes a segment register. In addition, some registers (such as `AL`, `DX` or `ECX`) may be specified explicitly.
- Immediate operands: `imm` denotes a generic immediate operand. `imm8`, `imm16` and `imm32` are used when the operand is intended to be a specific size. For some of these instructions, NASM needs an explicit specifier: for example, `ADD ESP,16` could be interpreted as either `ADD r/m32,imm32` or `ADD r/m32,imm8`. NASM chooses the former by default, and so you must specify `ADD ESP,BYTE 16` for the latter.
- Memory references: `mem` denotes a generic memory reference; `mem8`, `mem16`, `mem32`, `mem64` and `mem80` are used when the operand needs to be a specific size. Again, a specifier is needed in some cases: `DEC [address]` is ambiguous and will be rejected by NASM. You must specify `DEC BYTE [address]`, `DEC WORD [address]` or `DEC DWORD [address]` instead.
- Restricted memory references: one form of the `MOV` instruction allows a memory address to be specified without allowing the normal range of register combinations and effective address processing. This is denoted by `memoffs8`, `memoffs16` and `memoffs32`.
- Register or memory choices: many instructions can accept either a register or a memory reference as an operand. `r/m8` is a shorthand for `reg8/mem8`; similarly `r/m16` and `r/m32`. `r/m64` is MMX-related, and is a shorthand for `mmxreg/mem64`.

### B.2 Key to Opcode Descriptions

This appendix also provides the opcodes which NASM will generate for each form of each instruction. The opcodes are listed in the following way:

- A hex number, such as `3F`, indicates a fixed byte containing that number.
- A hex number followed by `+r`, such as `C8+r`, indicates that one of the operands to the instruction is a register, and the `register value' of that register should be added to the hex number to produce the generated byte. For example, EDX has register value 2, so the code `C8+r`, when the register operand is EDX, generates the hex byte `CA`. Register values for specific registers are given in section B.2.1.
- A hex number followed by `+cc`, such as `40+cc`, indicates that the instruction name has a condition code suffix, and the numeric representation of the condition code should be added to the hex number to produce the generated byte. For example, the code `40+cc`, when the instruction contains the `NE` condition, generates the hex byte `45`. Condition codes and their numeric representations are given in section B.2.2.
- A slash followed by a digit, such as `/2`, indicates that one of the operands to the instruction is a memory address or register (denoted `mem` or `r/m`, with an optional size). This is to be encoded as an effective address, with a ModR/M byte, an optional SIB byte, and an optional displacement, and the spare (register) field of the ModR/M byte should be the digit given (which will be from 0 to 7, so it fits in three bits). The encoding of effective addresses is given in section B.2.5.
- The code `/r` combines the above two: it indicates that one of the operands is a memory address or `r/m`, and another is a register, and that an effective address should be generated with the spare (register) field in the ModR/M byte being equal to the `register value' of the register operand. The encoding of effective addresses is given in section B.2.5; register values are given in section B.2.1.
- The codes `ib`, `iw` and `id` indicate that one of the operands to the instruction is an immediate value, and that this is to be encoded as a byte, little-endian word or little-endian doubleword respectively.
- The codes `rb`, `rw` and `rd` indicate that one of the operands to the instruction is an immediate value, and that the difference between this value and the address of the end of the instruction is to be encoded as a byte, word or doubleword respectively. Where the form `rw/rd` appears, it indicates that either `rw` or `rd` should be used according to whether assembly is being performed in `BITS 16` or `BITS 32` state respectively.
- The codes `ow` and `od` indicate that one of the operands to the instruction is a reference to the contents of a memory address specified as an immediate value: this encoding is used in some forms of the `MOV` instruction in place of the standard effective-address mechanism. The displacement is encoded as a word or doubleword. Again, `ow/od` denotes that `ow` or `od` should be chosen according to the `BITS` setting.
- The codes `o16` and `o32` indicate that the given form of the instruction should be assembled with operand size 16 or 32 bits. In other words, `o16` indicates a `66` prefix in `BITS 32` state, but generates no code in `BITS 16` state; and `o32` indicates a `66` prefix in `BITS 16` state but generates nothing in `BITS 32`.
- The codes `a16` and `a32`, similarly to `o16` and `o32`, indicate the address size of the given form of the instruction. Where this does not match the `BITS` setting, a `67` prefix is required.

#### B.2.1 Register Values

Where an instruction requires a register value, it is already implicit in the encoding of the rest of the instruction what type of register is intended: an 8-bit general-purpose register, a segment register, a debug register, an MMX register, or whatever. Therefore there is no problem with registers of different types sharing an encoding value.

The encodings for the various classes of register are:

- 8-bit general registers: `AL` is 0, `CL` is 1, `DL` is 2, `BL` is 3, `AH` is 4, `CH` is 5, `DH` is 6, and `BH` is 7.
- 16-bit general registers: `AX` is 0, `CX` is 1, `DX` is 2, `BX` is 3, `SP` is 4, `BP` is 5, `SI` is 6, and `DI` is 7.
- 32-bit general registers: `EAX` is 0, `ECX` is 1, `EDX` is 2, `EBX` is 3, `ESP` is 4, `EBP` is 5, `ESI` is 6, and `EDI` is 7.
- Segment registers: `ES` is 0, `CS` is 1, `SS` is 2, `DS` is 3, `FS` is 4, and `GS` is 5.
- Floating-point registers: `ST0` is 0, `ST1` is 1, `ST2` is 2, `ST3` is 3, `ST4` is 4, `ST5` is 5, `ST6` is 6, and `ST7` is 7.
- 64-bit MMX registers: `MM0` is 0, `MM1` is 1, `MM2` is 2, `MM3` is 3, `MM4` is 4, `MM5` is 5, `MM6` is 6, and `MM7` is 7.
- Control registers: `CR0` is 0, `CR2` is 2, `CR3` is 3, and `CR4` is 4.
- Debug registers: `DR0` is 0, `DR1` is 1, `DR2` is 2, `DR3` is 3, `DR6` is 6, and `DR7` is 7.
- Test registers: `TR3` is 3, `TR4` is 4, `TR5` is 5, `TR6` is 6, and `TR7` is 7.

(Note that wherever a register name contains a number, that number is also the register value for that register.)

#### B.2.2 Condition Codes

The available condition codes are given here, along with their numeric representations as part of opcodes. Many of these condition codes have synonyms, so several will be listed at a time.

In the following descriptions, the word `either', when applied to two possible trigger conditions, is used to mean `either or both'. If `either but not both' is meant, the phrase `exactly one of' is used.

- `O` is 0 (trigger if the overflow flag is set); `NO` is 1.
- `B`, `C` and `NAE` are 2 (trigger if the carry flag is set); `AE`, `NB` and `NC` are 3.
- `E` and `Z` are 4 (trigger if the zero flag is set); `NE` and `NZ` are 5.
- `BE` and `NA` are 6 (trigger if either of the carry or zero flags is set); `A` and `NBE` are 7.
- `S` is 8 (trigger if the sign flag is set); `NS` is 9.
- `P` and `PE` are 10 (trigger if the parity flag is set); `NP` and `PO` are 11.
- `L` and `NGE` are 12 (trigger if exactly one of the sign and overflow flags is set); `GE` and `NL` are 13.
- `LE` and `NG` are 14 (trigger if either the zero flag is set, or exactly one of the sign and overflow flags is set); `G` and `NLE` are 15.

Note that in all cases, the sense of a condition code may be reversed by changing the low bit of the numeric representation.

For details of when an instruction sets each of the status flags, see the individual instruction, plus the Status Flags reference in section B.2.4

#### B.2.3 SSE Condition Predicates

The condition predicates for SSE comparison instructions are the codes used as part of the opcode, to determine what form of comparison is being carried out. In each case, the imm8 value is the final byte of the opcode encoding, and the predicate is the code used as part of the mnemonic for the instruction (equivalent to the "cc" in an integer instruction that used a condition code). The instructions that use this will give details of what the various mnemonics are, this table is used to help you work out details of what is happening.

    Predi-  imm8  Description Relation where:   Emula- Result   QNaN
     cate  Encod-             A Is 1st Operand  tion   if NaN   Signal
            ing               B Is 2nd Operand         Operand  Invalid

    EQ     000B   equal       A = B                    False     No

    LT     001B   less-than   A < B                    False     Yes

    LE     010B   less-than-  A <= B                   False     Yes
                   or-equal

    ---    ----   greater     A > B             Swap   False     Yes
                  than                          Operands,
                                                Use LT

    ---    ----   greater-    A >= B            Swap   False     Yes
                  than-or-equal                 Operands,
                                                Use LE

    UNORD  011B   unordered   A, B = Unordered         True      No

    NEQ    100B   not-equal   A != B                   True      No

    NLT    101B   not-less-   NOT(A < B)               True      Yes
                  than

    NLE    110B   not-less-   NOT(A <= B)              True      Yes
                  than-or-
                  equal

    ---    ----   not-greater NOT(A > B)        Swap   True      Yes
                  than                          Operands,
                                                Use NLT

    ---    ----   not-greater NOT(A >= B)       Swap   True      Yes
                  than-                         Operands,
                  or-equal                      Use NLE

    ORD    111B   ordered      A , B = Ordered         False     No

The unordered relationship is true when at least one of the two values being compared is a NaN or in an unsupported format.

Note that the comparisons which are listed as not having a predicate or encoding can only be achieved through software emulation, as described in the "emulation" column. Note in particular that an instruction such as `greater-than` is not the same as `NLE`, as, unlike with the `CMP` instruction, it has to take into account the possibility of one operand containing a NaN or an unsupported numeric format.

#### B.2.4 Status Flags

The status flags provide some information about the result of the arithmetic instructions. This information can be used by conditional instructions (such a `Jcc` and `CMOVcc`) as well as by some of the other instructions (such as `ADC` and `INTO`).

There are 6 status flags:

    CF - Carry flag.

Set if an arithmetic operation generates a carry or a borrow out of the most-significant bit of the result; cleared otherwise. This flag indicates an overflow condition for unsigned-integer arithmetic. It is also used in multiple-precision arithmetic.

    PF - Parity flag.

Set if the least-significant byte of the result contains an even number of 1 bits; cleared otherwise.

    AF - Adjust flag.

Set if an arithmetic operation generates a carry or a borrow out of bit 3 of the result; cleared otherwise. This flag is used in binary-coded decimal (BCD) arithmetic.

    ZF - Zero flag.

Set if the result is zero; cleared otherwise.

    SF - Sign flag.

Set equal to the most-significant bit of the result, which is the sign bit of a signed integer. (0 indicates a positive value and 1 indicates a negative value.)

    OF - Overflow flag.

Set if the integer result is too large a positive number or too small a negative number (excluding the sign-bit) to fit in the destination operand; cleared otherwise. This flag indicates an overflow condition for signed-integer (two's complement) arithmetic.

#### B.2.5 Effective Address Encoding: ModR/M and SIB

An effective address is encoded in up to three parts: a ModR/M byte, an optional SIB byte, and an optional byte, word or doubleword displacement field.

The ModR/M byte consists of three fields: the `mod` field, ranging from 0 to 3, in the upper two bits of the byte, the `r/m` field, ranging from 0 to 7, in the lower three bits, and the spare (register) field in the middle (bit 3 to bit 5). The spare field is not relevant to the effective address being encoded, and either contains an extension to the instruction opcode or the register value of another operand.

The ModR/M system can be used to encode a direct register reference rather than a memory access. This is always done by setting the `mod` field to 3 and the `r/m` field to the register value of the register in question (it must be a general-purpose register, and the size of the register must already be implicit in the encoding of the rest of the instruction). In this case, the SIB byte and displacement field are both absent.

In 16-bit addressing mode (either `BITS 16` with no `67` prefix, or `BITS 32` with a `67` prefix), the SIB byte is never used. The general rules for `mod` and `r/m` (there is an exception, given below) are:

- The `mod` field gives the length of the displacement field: 0 means no displacement, 1 means one byte, and 2 means two bytes.
- The `r/m` field encodes the combination of registers to be added to the displacement to give the accessed address: 0 means `BX+SI`, 1 means `BX+DI`, 2 means `BP+SI`, 3 means `BP+DI`, 4 means `SI` only, 5 means `DI` only, 6 means `BP` only, and 7 means `BX` only.

However, there is a special case:

- If `mod` is 0 and `r/m` is 6, the effective address encoded is not `[BP]` as the above rules would suggest, but instead `[disp16]`: the displacement field is present and is two bytes long, and no registers are added to the displacement.

Therefore the effective address `[BP]` cannot be encoded as efficiently as `[BX]`; so if you code `[BP]` in a program, NASM adds a notional 8-bit zero displacement, and sets `mod` to 1, `r/m` to 6, and the one-byte displacement field to 0.

In 32-bit addressing mode (either `BITS 16` with a `67` prefix, or `BITS 32` with no `67` prefix) the general rules (again, there are exceptions) for `mod` and `r/m` are:

- The `mod` field gives the length of the displacement field: 0 means no displacement, 1 means one byte, and 2 means four bytes.
- If only one register is to be added to the displacement, and it is not `ESP`, the `r/m` field gives its register value, and the SIB byte is absent. If the `r/m` field is 4 (which would encode `ESP`), the SIB byte is present and gives the combination and scaling of registers to be added to the displacement.

If the SIB byte is present, it describes the combination of registers (an optional base register, and an optional index register scaled by multiplication by 1, 2, 4 or 8) to be added to the displacement. The SIB byte is divided into the `scale` field, in the top two bits, the `index` field in the next three, and the `base` field in the bottom three. The general rules are:

- The `base` field encodes the register value of the base register.
- The `index` field encodes the register value of the index register, unless it is 4, in which case no index register is used (so `ESP` cannot be used as an index register).
- The `scale` field encodes the multiplier by which the index register is scaled before adding it to the base and displacement: 0 encodes a multiplier of 1, 1 encodes 2, 2 encodes 4 and 3 encodes 8.

The exceptions to the 32-bit encoding rules are:

- If `mod` is 0 and `r/m` is 5, the effective address encoded is not `[EBP]` as the above rules would suggest, but instead `[disp32]`: the displacement field is present and is four bytes long, and no registers are added to the displacement.
- If `mod` is 0, `r/m` is 4 (meaning the SIB byte is present) and `base` is 4, the effective address encoded is not `[EBP+index]` as the above rules would suggest, but instead `[disp32+index]`: the displacement field is present and is four bytes long, and there is no base register (but the index register is still processed in the normal way).

### B.3 Key to Instruction Flags

Given along with each instruction in this appendix is a set of flags, denoting the type of the instruction. The types are as follows:

- `8086`, `186`, `286`, `386`, `486`, `PENT` and `P6` denote the lowest processor type that supports the instruction. Most instructions run on all processors above the given type; those that do not are documented. The Pentium II contains no additional instructions beyond the P6 (Pentium Pro); from the point of view of its instruction set, it can be thought of as a P6 with MMX capability.
- `3DNOW` indicates that the instruction is a 3DNow! one, and will run on the AMD K6-2 and later processors. ATHLON extensions to the 3DNow! instruction set are documented as such.
- `CYRIX` indicates that the instruction is specific to Cyrix processors, for example the extra MMX instructions in the Cyrix extended MMX instruction set.
- `FPU` indicates that the instruction is a floating-point one, and will only run on machines with a coprocessor (automatically including 486DX, Pentium and above).
- `KATMAI` indicates that the instruction was introduced as part of the Katmai New Instruction set. These instructions are available on the Pentium III and later processors. Those which are not specifically SSE instructions are also available on the AMD Athlon.
- `MMX` indicates that the instruction is an MMX one, and will run on MMX-capable Pentium processors and the Pentium II.
- `PRIV` indicates that the instruction is a protected-mode management instruction. Many of these may only be used in protected mode, or only at privilege level zero.
- `SSE` and `SSE2` indicate that the instruction is a Streaming SIMD Extension instruction. These instructions operate on multiple values in a single operation. SSE was introduced with the Pentium III and SSE2 was introduced with the Pentium 4.
- `UNDOC` indicates that the instruction is an undocumented one, and not part of the official Intel Architecture; it may or may not be supported on any given machine.
- `WILLAMETTE` indicates that the instruction was introduced as part of the new instruction set in the Pentium 4 and Intel Xeon processors. These instructions are also known as SSE2 instructions.

### B.4 x86 Instruction Set

#### B.4.1 `AAA`, `AAS`, `AAM`, `AAD`: ASCII Adjustments

    AAA                           ; 37                   [8086]

    AAS                           ; 3F                   [8086]

    AAD                           ; D5 0A                [8086]
    AAD imm                       ; D5 ib                [8086]

    AAM                           ; D4 0A                [8086]
    AAM imm                       ; D4 ib                [8086]

These instructions are used in conjunction with the add, subtract, multiply and divide instructions to perform binary-coded decimal arithmetic in unpacked (one BCD digit per byte - easy to translate to and from `ASCII`, hence the instruction names) form. There are also packed BCD instructions `DAA` and `DAS`: see section B.4.57.

- `AAA` (ASCII Adjust After Addition) should be used after a one-byte `ADD` instruction whose destination was the `AL` register: by means of examining the value in the low nibble of `AL` and also the auxiliary carry flag `AF`, it determines whether the addition has overflowed, and adjusts it (and sets the carry flag) if so. You can add long BCD strings together by doing `ADD`/`AAA` on the low digits, then doing `ADC`/`AAA` on each subsequent digit.
- `AAS` (ASCII Adjust AL After Subtraction) works similarly to `AAA`, but is for use after `SUB` instructions rather than `ADD`.
- `AAM` (ASCII Adjust AX After Multiply) is for use after you have multiplied two decimal digits together and left the result in `AL`: it divides `AL` by ten and stores the quotient in `AH`, leaving the remainder in `AL`. The divisor 10 can be changed by specifying an operand to the instruction: a particularly handy use of this is `AAM 16`, causing the two nibbles in `AL` to be separated into `AH` and `AL`.
- `AAD` (ASCII Adjust AX Before Division) performs the inverse operation to `AAM`: it multiplies `AH` by ten, adds it to `AL`, and sets `AH` to zero. Again, the multiplier 10 can be changed.

#### B.4.2 `ADC`: Add with Carry

    ADC r/m8,reg8                 ; 10 /r                [8086]
    ADC r/m16,reg16               ; o16 11 /r            [8086]
    ADC r/m32,reg32               ; o32 11 /r            [386]

    ADC reg8,r/m8                 ; 12 /r                [8086]
    ADC reg16,r/m16               ; o16 13 /r            [8086]
    ADC reg32,r/m32               ; o32 13 /r            [386]

    ADC r/m8,imm8                 ; 80 /2 ib             [8086]
    ADC r/m16,imm16               ; o16 81 /2 iw         [8086]
    ADC r/m32,imm32               ; o32 81 /2 id         [386]

    ADC r/m16,imm8                ; o16 83 /2 ib         [8086]
    ADC r/m32,imm8                ; o32 83 /2 ib         [386]

    ADC AL,imm8                   ; 14 ib                [8086]
    ADC AX,imm16                  ; o16 15 iw            [8086]
    ADC EAX,imm32                 ; o32 15 id            [386]

`ADC` performs integer addition: it adds its two operands together, plus the value of the carry flag, and leaves the result in its destination (first) operand. The destination operand can be a register or a memory location. The source operand can be a register, a memory location or an immediate value.

The flags are set according to the result of the operation: in particular, the carry flag is affected and can be used by a subsequent `ADC` instruction.

In the forms with an 8-bit immediate second operand and a longer first operand, the second operand is considered to be signed, and is sign-extended to the length of the first operand. In these cases, the `BYTE` qualifier is necessary to force NASM to generate this form of the instruction.

To add two numbers without also adding the contents of the carry flag, use `ADD` (section B.4.3).

#### B.4.3 `ADD`: Add Integers

    ADD r/m8,reg8                 ; 00 /r                [8086]
    ADD r/m16,reg16               ; o16 01 /r            [8086]
    ADD r/m32,reg32               ; o32 01 /r            [386]

    ADD reg8,r/m8                 ; 02 /r                [8086]
    ADD reg16,r/m16               ; o16 03 /r            [8086]
    ADD reg32,r/m32               ; o32 03 /r            [386]

    ADD r/m8,imm8                 ; 80 /0 ib             [8086]
    ADD r/m16,imm16               ; o16 81 /0 iw         [8086]
    ADD r/m32,imm32               ; o32 81 /0 id         [386]

    ADD r/m16,imm8                ; o16 83 /0 ib         [8086]
    ADD r/m32,imm8                ; o32 83 /0 ib         [386]

    ADD AL,imm8                   ; 04 ib                [8086]
    ADD AX,imm16                  ; o16 05 iw            [8086]
    ADD EAX,imm32                 ; o32 05 id            [386]

`ADD` performs integer addition: it adds its two operands together, and leaves the result in its destination (first) operand. The destination operand can be a register or a memory location. The source operand can be a register, a memory location or an immediate value.

The flags are set according to the result of the operation: in particular, the carry flag is affected and can be used by a subsequent `ADC` instruction.

In the forms with an 8-bit immediate second operand and a longer first operand, the second operand is considered to be signed, and is sign-extended to the length of the first operand. In these cases, the `BYTE` qualifier is necessary to force NASM to generate this form of the instruction.

#### B.4.4 `ADDPD`: ADD Packed Double-Precision FP Values

    ADDPD xmm1,xmm2/mem128        ; 66 0F 58 /r     [WILLAMETTE,SSE2]

`ADDPD` performs addition on each of two packed double-precision FP value pairs.

       dst[0-63]   := dst[0-63]   + src[0-63],
       dst[64-127] := dst[64-127] + src[64-127].

The destination is an `XMM` register. The source operand can be either an `XMM` register or a 128-bit memory location.

#### B.4.5 `ADDPS`: ADD Packed Single-Precision FP Values

    ADDPS xmm1,xmm2/mem128        ; 0F 58 /r        [KATMAI,SSE]

`ADDPS` performs addition on each of four packed single-precision FP value pairs

       dst[0-31]   := dst[0-31]   + src[0-31],
       dst[32-63]  := dst[32-63]  + src[32-63],
       dst[64-95]  := dst[64-95]  + src[64-95],
       dst[96-127] := dst[96-127] + src[96-127].

The destination is an `XMM` register. The source operand can be either an `XMM` register or a 128-bit memory location.

#### B.4.6 `ADDSD`: ADD Scalar Double-Precision FP Values

    ADDSD xmm1,xmm2/mem64         ; F2 0F 58 /r     [KATMAI,SSE]

`ADDSD` adds the low double-precision FP values from the source and destination operands and stores the double-precision FP result in the destination operand.

       dst[0-63]   := dst[0-63] + src[0-63],
       dst[64-127) remains unchanged.

The destination is an `XMM` register. The source operand can be either an `XMM` register or a 64-bit memory location.

#### B.4.7 `ADDSS`: ADD Scalar Single-Precision FP Values

    ADDSS xmm1,xmm2/mem32         ; F3 0F 58 /r     [WILLAMETTE,SSE2]

`ADDSS` adds the low single-precision FP values from the source and destination operands and stores the single-precision FP result in the destination operand.

       dst[0-31]   := dst[0-31] + src[0-31],
       dst[32-127] remains unchanged.

The destination is an `XMM` register. The source operand can be either an `XMM` register or a 32-bit memory location.

#### B.4.8 `AND`: Bitwise AND

    AND r/m8,reg8                 ; 20 /r                [8086]
    AND r/m16,reg16               ; o16 21 /r            [8086]
    AND r/m32,reg32               ; o32 21 /r            [386]

    AND reg8,r/m8                 ; 22 /r                [8086]
    AND reg16,r/m16               ; o16 23 /r            [8086]
    AND reg32,r/m32               ; o32 23 /r            [386]

    AND r/m8,imm8                 ; 80 /4 ib             [8086]
    AND r/m16,imm16               ; o16 81 /4 iw         [8086]
    AND r/m32,imm32               ; o32 81 /4 id         [386]

    AND r/m16,imm8                ; o16 83 /4 ib         [8086]
    AND r/m32,imm8                ; o32 83 /4 ib         [386]

    AND AL,imm8                   ; 24 ib                [8086]
    AND AX,imm16                  ; o16 25 iw            [8086]
    AND EAX,imm32                 ; o32 25 id            [386]

`AND` performs a bitwise AND operation between its two operands (i.e. each bit of the result is 1 if and only if the corresponding bits of the two inputs were both 1), and stores the result in the destination (first) operand. The destination operand can be a register or a memory location. The source operand can be a register, a memory location or an immediate value.

In the forms with an 8-bit immediate second operand and a longer first operand, the second operand is considered to be signed, and is sign-extended to the length of the first operand. In these cases, the `BYTE` qualifier is necessary to force NASM to generate this form of the instruction.

The `MMX` instruction `PAND` (see section B.4.202) performs the same operation on the 64-bit `MMX` registers.

#### B.4.9 `ANDNPD`: Bitwise Logical AND NOT of Packed Double-Precision FP Values

    ANDNPD xmm1,xmm2/mem128       ; 66 0F 55 /r     [WILLAMETTE,SSE2]

`ANDNPD` inverts the bits of the two double-precision floating-point values in the destination register, and then performs a logical AND between the two double-precision floating-point values in the source operand and the temporary inverted result, storing the result in the destination register.

       dst[0-63]   := src[0-63]   AND NOT dst[0-63],
       dst[64-127] := src[64-127] AND NOT dst[64-127].

The destination is an `XMM` register. The source operand can be either an `XMM` register or a 128-bit memory location.

#### B.4.10 `ANDNPS`: Bitwise Logical AND NOT of Packed Single-Precision FP Values

    ANDNPS xmm1,xmm2/mem128       ; 0F 55 /r        [KATMAI,SSE]

`ANDNPS` inverts the bits of the four single-precision floating-point values in the destination register, and then performs a logical AND between the four single-precision floating-point values in the source operand and the temporary inverted result, storing the result in the destination register.

       dst[0-31]   := src[0-31]   AND NOT dst[0-31],
       dst[32-63]  := src[32-63]  AND NOT dst[32-63],
       dst[64-95]  := src[64-95]  AND NOT dst[64-95],
       dst[96-127] := src[96-127] AND NOT dst[96-127].

The destination is an `XMM` register. The source operand can be either an `XMM` register or a 128-bit memory location.

#### B.4.11 `ANDPD`: Bitwise Logical AND For Single FP

    ANDPD xmm1,xmm2/mem128        ; 66 0F 54 /r     [WILLAMETTE,SSE2]

`ANDPD` performs a bitwise logical AND of the two double-precision floating point values in the source and destination operand, and stores the result in the destination register.

       dst[0-63]   := src[0-63]   AND dst[0-63],
       dst[64-127] := src[64-127] AND dst[64-127].

The destination is an `XMM` register. The source operand can be either an `XMM` register or a 128-bit memory location.

#### B.4.12 `ANDPS`: Bitwise Logical AND For Single FP

    ANDPS xmm1,xmm2/mem128        ; 0F 54 /r        [KATMAI,SSE]

`ANDPS` performs a bitwise logical AND of the four single-precision floating point values in the source and destination operand, and stores the result in the destination register.

       dst[0-31]   := src[0-31]   AND dst[0-31],
       dst[32-63]  := src[32-63]  AND dst[32-63],
       dst[64-95]  := src[64-95]  AND dst[64-95],
       dst[96-127] := src[96-127] AND dst[96-127].

The destination is an `XMM` register. The source operand can be either an `XMM` register or a 128-bit memory location.

#### B.4.13 `ARPL`: Adjust RPL Field of Selector

    ARPL r/m16,reg16              ; 63 /r                [286,PRIV]

`ARPL` expects its two word operands to be segment selectors. It adjusts the `RPL` (requested privilege level - stored in the bottom two bits of the selector) field of the destination (first) operand to ensure that it is no less (i.e. no more privileged than) the `RPL` field of the source operand. The zero flag is set if and only if a change had to be made.

#### B.4.14 `BOUND`: Check Array Index against Bounds

    BOUND reg16,mem               ; o16 62 /r            [186]
    BOUND reg32,mem               ; o32 62 /r            [386]

`BOUND` expects its second operand to point to an area of memory containing two signed values of the same size as its first operand (i.e. two words for the 16-bit form; two doublewords for the 32-bit form). It performs two signed comparisons: if the value in the register passed as its first operand is less than the first of the in-memory values, or is greater than or equal to the second, it throws a `BR` exception. Otherwise, it does nothing.

#### B.4.15 `BSF`, `BSR`: Bit Scan

    BSF reg16,r/m16               ; o16 0F BC /r         [386]
    BSF reg32,r/m32               ; o32 0F BC /r         [386]

    BSR reg16,r/m16               ; o16 0F BD /r         [386]
    BSR reg32,r/m32               ; o32 0F BD /r         [386]

- `BSF` searches for the least significant set bit in its source (second) operand, and if it finds one, stores the index in its destination (first) operand. If no set bit is found, the contents of the destination operand are undefined. If the source operand is zero, the zero flag is set.
- `BSR` performs the same function, but searches from the top instead, so it finds the most significant set bit.

Bit indices are from 0 (least significant) to 15 or 31 (most significant). The destination operand can only be a register. The source operand can be a register or a memory location.

#### B.4.16 `BSWAP`: Byte Swap

    BSWAP reg32                   ; o32 0F C8+r          [486]

`BSWAP` swaps the order of the four bytes of a 32-bit register: bits 0-7 exchange places with bits 24-31, and bits 8-15 swap with bits 16-23. There is no explicit 16-bit equivalent: to byte-swap `AX`, `BX`, `CX` or `DX`, `XCHG` can be used. When `BSWAP` is used with a 16-bit register, the result is undefined.

#### B.4.17 `BT`, `BTC`, `BTR`, `BTS`: Bit Test

    BT r/m16,reg16                ; o16 0F A3 /r         [386]
    BT r/m32,reg32                ; o32 0F A3 /r         [386]
    BT r/m16,imm8                 ; o16 0F BA /4 ib      [386]
    BT r/m32,imm8                 ; o32 0F BA /4 ib      [386]

    BTC r/m16,reg16               ; o16 0F BB /r         [386]
    BTC r/m32,reg32               ; o32 0F BB /r         [386]
    BTC r/m16,imm8                ; o16 0F BA /7 ib      [386]
    BTC r/m32,imm8                ; o32 0F BA /7 ib      [386]

    BTR r/m16,reg16               ; o16 0F B3 /r         [386]
    BTR r/m32,reg32               ; o32 0F B3 /r         [386]
    BTR r/m16,imm8                ; o16 0F BA /6 ib      [386]
    BTR r/m32,imm8                ; o32 0F BA /6 ib      [386]

    BTS r/m16,reg16               ; o16 0F AB /r         [386]
    BTS r/m32,reg32               ; o32 0F AB /r         [386]
    BTS r/m16,imm                 ; o16 0F BA /5 ib      [386]
    BTS r/m32,imm                 ; o32 0F BA /5 ib      [386]

These instructions all test one bit of their first operand, whose index is given by the second operand, and store the value of that bit into the carry flag. Bit indices are from 0 (least significant) to 15 or 31 (most significant).

In addition to storing the original value of the bit into the carry flag, `BTR` also resets (clears) the bit in the operand itself. `BTS` sets the bit, and `BTC` complements the bit. `BT` does not modify its operands.

The destination can be a register or a memory location. The source can be a register or an immediate value.

If the destination operand is a register, the bit offset should be in the range 0-15 (for 16-bit operands) or 0-31 (for 32-bit operands). An immediate value outside these ranges will be taken modulo 16/32 by the processor.

If the destination operand is a memory location, then an immediate bit offset follows the same rules as for a register. If the bit offset is in a register, then it can be anything within the signed range of the register used (ie, for a 32-bit operand, it can be (-2^31) to (2^31 - 1)

#### B.4.18 `CALL`: Call Subroutine

    CALL imm                      ; E8 rw/rd             [8086]
    CALL imm:imm16                ; o16 9A iw iw         [8086]
    CALL imm:imm32                ; o32 9A id iw         [386]
    CALL FAR mem16                ; o16 FF /3            [8086]
    CALL FAR mem32                ; o32 FF /3            [386]
    CALL r/m16                    ; o16 FF /2            [8086]
    CALL r/m32                    ; o32 FF /2            [386]

`CALL` calls a subroutine, by means of pushing the current instruction pointer (`IP`) and optionally `CS` as well on the stack, and then jumping to a given address.

`CS` is pushed as well as `IP` if and only if the call is a far call, i.e. a destination segment address is specified in the instruction. The forms involving two colon-separated arguments are far calls; so are the `CALL FAR mem` forms.

The immediate near call takes one of two forms (`call imm16/imm32`, determined by the current segment size limit. For 16-bit operands, you would use `CALL 0x1234`, and for 32-bit operands you would use `CALL 0x12345678`. The value passed as an operand is a relative offset.

You can choose between the two immediate far call forms (`CALL imm:imm`) by the use of the `WORD` and `DWORD` keywords: `CALL WORD 0x1234:0x5678`) or `CALL DWORD 0x1234:0x56789abc`.

The `CALL FAR mem` forms execute a far call by loading the destination address out of memory. The address loaded consists of 16 or 32 bits of offset (depending on the operand size), and 16 bits of segment. The operand size may be overridden using `CALL WORD FAR mem` or `CALL DWORD FAR mem`.

The `CALL r/m` forms execute a near call (within the same segment), loading the destination address out of memory or out of a register. The keyword `NEAR` may be specified, for clarity, in these forms, but is not necessary. Again, operand size can be overridden using `CALL WORD mem` or `CALL DWORD mem`.

As a convenience, NASM does not require you to call a far procedure symbol by coding the cumbersome `CALL SEG routine:routine`, but instead allows the easier synonym `CALL FAR routine`.

The `CALL r/m` forms given above are near calls; NASM will accept the `NEAR` keyword (e.g. `CALL NEAR [address]`), even though it is not strictly necessary.

#### B.4.19 `CBW`, `CWD`, `CDQ`, `CWDE`: Sign Extensions

    CBW                           ; o16 98               [8086]
    CWDE                          ; o32 98               [386]

    CWD                           ; o16 99               [8086]
    CDQ                           ; o32 99               [386]

All these instructions sign-extend a short value into a longer one, by replicating the top bit of the original value to fill the extended one.

`CBW` extends `AL` into `AX` by repeating the top bit of `AL` in every bit of `AH`. `CWDE` extends `AX` into `EAX`. `CWD` extends `AX` into `DX:AX` by repeating the top bit of `AX` throughout `DX`, and `CDQ` extends `EAX` into `EDX:EAX`.

#### B.4.20 `CLC`, `CLD`, `CLI`, `CLTS`: Clear Flags

    CLC                           ; F8                   [8086]
    CLD                           ; FC                   [8086]
    CLI                           ; FA                   [8086]
    CLTS                          ; 0F 06                [286,PRIV]

These instructions clear various flags. `CLC` clears the carry flag; `CLD` clears the direction flag; `CLI` clears the interrupt flag (thus disabling interrupts); and `CLTS` clears the task-switched (`TS`) flag in `CR0`.

To set the carry, direction, or interrupt flags, use the `STC`, `STD` and `STI` instructions (section B.4.301). To invert the carry flag, use `CMC` (section B.4.22).

#### B.4.21 `CLFLUSH`: Flush Cache Line

    CLFLUSH mem                   ; 0F AE /7        [WILLAMETTE,SSE2]

`CLFLUSH` invalidates the cache line that contains the linear address specified by the source operand from all levels of the processor cache hierarchy (data and instruction). If, at any level of the cache hierarchy, the line is inconsistent with memory (dirty) it is written to memory before invalidation. The source operand points to a byte-sized memory location.

Although `CLFLUSH` is flagged `SSE2` and above, it may not be present on all processors which have `SSE2` support, and it may be supported on other processors; the `CPUID` instruction (section B.4.34) will return a bit which indicates support for the `CLFLUSH` instruction.

#### B.4.22 `CMC`: Complement Carry Flag

    CMC                           ; F5                   [8086]

`CMC` changes the value of the carry flag: if it was 0, it sets it to 1, and vice versa.

#### B.4.23 `CMOVcc`: Conditional Move

    CMOVcc reg16,r/m16            ; o16 0F 40+cc /r      [P6]
    CMOVcc reg32,r/m32            ; o32 0F 40+cc /r      [P6]

`CMOV` moves its source (second) operand into its destination (first) operand if the given condition code is satisfied; otherwise it does nothing.

For a list of condition codes, see section B.2.2.

Although the `CMOV` instructions are flagged `P6` and above, they may not be supported by all Pentium Pro processors; the `CPUID` instruction (section B.4.34) will return a bit which indicates whether conditional moves are supported.

#### B.4.24 `CMP`: Compare Integers

    CMP r/m8,reg8                 ; 38 /r                [8086]
    CMP r/m16,reg16               ; o16 39 /r            [8086]
    CMP r/m32,reg32               ; o32 39 /r            [386]

    CMP reg8,r/m8                 ; 3A /r                [8086]
    CMP reg16,r/m16               ; o16 3B /r            [8086]
    CMP reg32,r/m32               ; o32 3B /r            [386]

    CMP r/m8,imm8                 ; 80 /0 ib             [8086]
    CMP r/m16,imm16               ; o16 81 /0 iw         [8086]
    CMP r/m32,imm32               ; o32 81 /0 id         [386]

    CMP r/m16,imm8                ; o16 83 /0 ib         [8086]
    CMP r/m32,imm8                ; o32 83 /0 ib         [386]

    CMP AL,imm8                   ; 3C ib                [8086]
    CMP AX,imm16                  ; o16 3D iw            [8086]
    CMP EAX,imm32                 ; o32 3D id            [386]

`CMP` performs a `mental' subtraction of its second operand from its first operand, and affects the flags as if the subtraction had taken place, but does not store the result of the subtraction anywhere.

In the forms with an 8-bit immediate second operand and a longer first operand, the second operand is considered to be signed, and is sign-extended to the length of the first operand. In these cases, the `BYTE` qualifier is necessary to force NASM to generate this form of the instruction.

The destination operand can be a register or a memory location. The source can be a register, memory location or an immediate value of the same size as the destination.

#### B.4.25 `CMPccPD`: Packed Double-Precision FP Compare

    CMPPD xmm1,xmm2/mem128,imm8   ; 66 0F C2 /r ib  [WILLAMETTE,SSE2]

    CMPEQPD xmm1,xmm2/mem128      ; 66 0F C2 /r 00  [WILLAMETTE,SSE2]
    CMPLTPD xmm1,xmm2/mem128      ; 66 0F C2 /r 01  [WILLAMETTE,SSE2]
    CMPLEPD xmm1,xmm2/mem128      ; 66 0F C2 /r 02  [WILLAMETTE,SSE2]
    CMPUNORDPD xmm1,xmm2/mem128   ; 66 0F C2 /r 03  [WILLAMETTE,SSE2]
    CMPNEQPD xmm1,xmm2/mem128     ; 66 0F C2 /r 04  [WILLAMETTE,SSE2]
    CMPNLTPD xmm1,xmm2/mem128     ; 66 0F C2 /r 05  [WILLAMETTE,SSE2]
    CMPNLEPD xmm1,xmm2/mem128     ; 66 0F C2 /r 06  [WILLAMETTE,SSE2]
    CMPORDPD xmm1,xmm2/mem128     ; 66 0F C2 /r 07  [WILLAMETTE,SSE2]

The `CMPccPD` instructions compare the two packed double-precision FP values in the source and destination operands, and returns the result of the comparison in the destination register. The result of each comparison is a quadword mask of all 1s (comparison true) or all 0s (comparison false).

The destination is an `XMM` register. The source can be either an `XMM` register or a 128-bit memory location.

The third operand is an 8-bit immediate value, of which the low 3 bits define the type of comparison. For ease of programming, the 8 two-operand pseudo-instructions are provided, with the third operand already filled in. The `Condition Predicates` are:

    EQ     0   Equal
    LT     1   Less-than
    LE     2   Less-than-or-equal
    UNORD  3   Unordered
    NE     4   Not-equal
    NLT    5   Not-less-than
    NLE    6   Not-less-than-or-equal
    ORD    7   Ordered

For more details of the comparison predicates, and details of how to emulate the "greater-than" equivalents, see section B.2.3

#### B.4.26 `CMPccPS`: Packed Single-Precision FP Compare

    CMPPS xmm1,xmm2/mem128,imm8   ; 0F C2 /r ib     [KATMAI,SSE]

    CMPEQPS xmm1,xmm2/mem128      ; 0F C2 /r 00     [KATMAI,SSE]
    CMPLTPS xmm1,xmm2/mem128      ; 0F C2 /r 01     [KATMAI,SSE]
    CMPLEPS xmm1,xmm2/mem128      ; 0F C2 /r 02     [KATMAI,SSE]
    CMPUNORDPS xmm1,xmm2/mem128   ; 0F C2 /r 03     [KATMAI,SSE]
    CMPNEQPS xmm1,xmm2/mem128     ; 0F C2 /r 04     [KATMAI,SSE]
    CMPNLTPS xmm1,xmm2/mem128     ; 0F C2 /r 05     [KATMAI,SSE]
    CMPNLEPS xmm1,xmm2/mem128     ; 0F C2 /r 06     [KATMAI,SSE]
    CMPORDPS xmm1,xmm2/mem128     ; 0F C2 /r 07     [KATMAI,SSE]

The `CMPccPS` instructions compare the two packed single-precision FP values in the source and destination operands, and returns the result of the comparison in the destination register. The result of each comparison is a doubleword mask of all 1s (comparison true) or all 0s (comparison false).

The destination is an `XMM` register. The source can be either an `XMM` register or a 128-bit memory location.

The third operand is an 8-bit immediate value, of which the low 3 bits define the type of comparison. For ease of programming, the 8 two-operand pseudo-instructions are provided, with the third operand already filled in. The `Condition Predicates` are:

    EQ     0   Equal
    LT     1   Less-than
    LE     2   Less-than-or-equal
    UNORD  3   Unordered
    NE     4   Not-equal
    NLT    5   Not-less-than
    NLE    6   Not-less-than-or-equal
    ORD    7   Ordered

For more details of the comparison predicates, and details of how to emulate the "greater-than" equivalents, see section B.2.3

#### B.4.27 `CMPSB`, `CMPSW`, `CMPSD`: Compare Strings

    CMPSB                         ; A6                   [8086]
    CMPSW                         ; o16 A7               [8086]
    CMPSD                         ; o32 A7               [386]

`CMPSB` compares the byte at `[DS:SI]` or `[DS:ESI]` with the byte at `[ES:DI]` or `[ES:EDI]`, and sets the flags accordingly. It then increments or decrements (depending on the direction flag: increments if the flag is clear, decrements if it is set) `SI` and `DI` (or `ESI` and `EDI`).

The registers used are `SI` and `DI` if the address size is 16 bits, and `ESI` and `EDI` if it is 32 bits. If you need to use an address size not equal to the current `BITS` setting, you can use an explicit `a16` or `a32` prefix.

The segment register used to load from `[SI]` or `[ESI]` can be overridden by using a segment register name as a prefix (for example, `ES CMPSB`). The use of `ES` for the load from `[DI]` or `[EDI]` cannot be overridden.

`CMPSW` and `CMPSD` work in the same way, but they compare a word or a doubleword instead of a byte, and increment or decrement the addressing registers by 2 or 4 instead of 1.

The `REPE` and `REPNE` prefixes (equivalently, `REPZ` and `REPNZ`) may be used to repeat the instruction up to `CX` (or `ECX` - again, the address size chooses which) times until the first unequal or equal byte is found.

#### B.4.28 `CMPccSD`: Scalar Double-Precision FP Compare

    CMPSD xmm1,xmm2/mem64,imm8    ; F2 0F C2 /r ib  [WILLAMETTE,SSE2]

    CMPEQSD xmm1,xmm2/mem64       ; F2 0F C2 /r 00  [WILLAMETTE,SSE2]
    CMPLTSD xmm1,xmm2/mem64       ; F2 0F C2 /r 01  [WILLAMETTE,SSE2]
    CMPLESD xmm1,xmm2/mem64       ; F2 0F C2 /r 02  [WILLAMETTE,SSE2]
    CMPUNORDSD xmm1,xmm2/mem64    ; F2 0F C2 /r 03  [WILLAMETTE,SSE2]
    CMPNEQSD xmm1,xmm2/mem64      ; F2 0F C2 /r 04  [WILLAMETTE,SSE2]
    CMPNLTSD xmm1,xmm2/mem64      ; F2 0F C2 /r 05  [WILLAMETTE,SSE2]
    CMPNLESD xmm1,xmm2/mem64      ; F2 0F C2 /r 06  [WILLAMETTE,SSE2]
    CMPORDSD xmm1,xmm2/mem64      ; F2 0F C2 /r 07  [WILLAMETTE,SSE2]

The `CMPccSD` instructions compare the low-order double-precision FP values in the source and destination operands, and returns the result of the comparison in the destination register. The result of each comparison is a quadword mask of all 1s (comparison true) or all 0s (comparison false).

The destination is an `XMM` register. The source can be either an `XMM` register or a 128-bit memory location.

The third operand is an 8-bit immediate value, of which the low 3 bits define the type of comparison. For ease of programming, the 8 two-operand pseudo-instructions are provided, with the third operand already filled in. The `Condition Predicates` are:

    EQ     0   Equal
    LT     1   Less-than
    LE     2   Less-than-or-equal
    UNORD  3   Unordered
    NE     4   Not-equal
    NLT    5   Not-less-than
    NLE    6   Not-less-than-or-equal
    ORD    7   Ordered

For more details of the comparison predicates, and details of how to emulate the "greater-than" equivalents, see section B.2.3

#### B.4.29 `CMPccSS`: Scalar Single-Precision FP Compare

    CMPSS xmm1,xmm2/mem32,imm8    ; F3 0F C2 /r ib  [KATMAI,SSE]

    CMPEQSS xmm1,xmm2/mem32       ; F3 0F C2 /r 00  [KATMAI,SSE]
    CMPLTSS xmm1,xmm2/mem32       ; F3 0F C2 /r 01  [KATMAI,SSE]
    CMPLESS xmm1,xmm2/mem32       ; F3 0F C2 /r 02  [KATMAI,SSE]
    CMPUNORDSS xmm1,xmm2/mem32    ; F3 0F C2 /r 03  [KATMAI,SSE]
    CMPNEQSS xmm1,xmm2/mem32      ; F3 0F C2 /r 04  [KATMAI,SSE]
    CMPNLTSS xmm1,xmm2/mem32      ; F3 0F C2 /r 05  [KATMAI,SSE]
    CMPNLESS xmm1,xmm2/mem32      ; F3 0F C2 /r 06  [KATMAI,SSE]
    CMPORDSS xmm1,xmm2/mem32      ; F3 0F C2 /r 07  [KATMAI,SSE]

The `CMPccSS` instructions compare the low-order single-precision FP values in the source and destination operands, and returns the result of the comparison in the destination register. The result of each comparison is a doubleword mask of all 1s (comparison true) or all 0s (comparison false).

The destination is an `XMM` register. The source can be either an `XMM` register or a 128-bit memory location.

The third operand is an 8-bit immediate value, of which the low 3 bits define the type of comparison. For ease of programming, the 8 two-operand pseudo-instructions are provided, with the third operand already filled in. The `Condition Predicates` are:

    EQ     0   Equal
    LT     1   Less-than
    LE     2   Less-than-or-equal
    UNORD  3   Unordered
    NE     4   Not-equal
    NLT    5   Not-less-than
    NLE    6   Not-less-than-or-equal
    ORD    7   Ordered

For more details of the comparison predicates, and details of how to emulate the "greater-than" equivalents, see section B.2.3

#### B.4.30 `CMPXCHG`, `CMPXCHG486`: Compare and Exchange

    CMPXCHG r/m8,reg8             ; 0F B0 /r             [PENT]
    CMPXCHG r/m16,reg16           ; o16 0F B1 /r         [PENT]
    CMPXCHG r/m32,reg32           ; o32 0F B1 /r         [PENT]

    CMPXCHG486 r/m8,reg8          ; 0F A6 /r             [486,UNDOC]
    CMPXCHG486 r/m16,reg16        ; o16 0F A7 /r         [486,UNDOC]
    CMPXCHG486 r/m32,reg32        ; o32 0F A7 /r         [486,UNDOC]

These two instructions perform exactly the same operation; however, apparently some (not all) 486 processors support it under a non-standard opcode, so NASM provides the undocumented `CMPXCHG486` form to generate the non-standard opcode.

`CMPXCHG` compares its destination (first) operand to the value in `AL`, `AX` or `EAX` (depending on the operand size of the instruction). If they are equal, it copies its source (second) operand into the destination and sets the zero flag. Otherwise, it clears the zero flag and copies the destination register to AL, AX or EAX.

The destination can be either a register or a memory location. The source is a register.

`CMPXCHG` is intended to be used for atomic operations in multitasking or multiprocessor environments. To safely update a value in shared memory, for example, you might load the value into `EAX`, load the updated value into `EBX`, and then execute the instruction `LOCK CMPXCHG [value],EBX`. If `value` has not changed since being loaded, it is updated with your desired new value, and the zero flag is set to let you know it has worked. (The `LOCK` prefix prevents another processor doing anything in the middle of this operation: it guarantees atomicity.) However, if another processor has modified the value in between your load and your attempted store, the store does not happen, and you are notified of the failure by a cleared zero flag, so you can go round and try again.

#### B.4.31 `CMPXCHG8B`: Compare and Exchange Eight Bytes

    CMPXCHG8B mem                 ; 0F C7 /1             [PENT]

This is a larger and more unwieldy version of `CMPXCHG`: it compares the 64-bit (eight-byte) value stored at `[mem]` with the value in `EDX:EAX`. If they are equal, it sets the zero flag and stores `ECX:EBX` into the memory area. If they are unequal, it clears the zero flag and stores the memory contents into `EDX:EAX`.

`CMPXCHG8B` can be used with the `LOCK` prefix, to allow atomic execution. This is useful in multi-processor and multi-tasking environments.

#### B.4.32 `COMISD`: Scalar Ordered Double-Precision FP Compare and Set EFLAGS

    COMISD xmm1,xmm2/mem64        ; 66 0F 2F /r     [WILLAMETTE,SSE2]

`COMISD` compares the low-order double-precision FP value in the two source operands. ZF, PF and CF are set according to the result. OF, AF and AF are cleared. The unordered result is returned if either source is a NaN (QNaN or SNaN).

The destination operand is an `XMM` register. The source can be either an `XMM` register or a memory location.

The flags are set according to the following rules:

       Result          Flags        Values

       UNORDERED:      ZF,PF,CF <-- 111;
       GREATER_THAN:   ZF,PF,CF <-- 000;
       LESS_THAN:      ZF,PF,CF <-- 001;
       EQUAL:          ZF,PF,CF <-- 100;

#### B.4.33 `COMISS`: Scalar Ordered Single-Precision FP Compare and Set EFLAGS

    COMISS xmm1,xmm2/mem32        ; 66 0F 2F /r     [KATMAI,SSE]

`COMISS` compares the low-order single-precision FP value in the two source operands. ZF, PF and CF are set according to the result. OF, AF and AF are cleared. The unordered result is returned if either source is a NaN (QNaN or SNaN).

The destination operand is an `XMM` register. The source can be either an `XMM` register or a memory location.

The flags are set according to the following rules:

       Result          Flags        Values

       UNORDERED:      ZF,PF,CF <-- 111;
       GREATER_THAN:   ZF,PF,CF <-- 000;
       LESS_THAN:      ZF,PF,CF <-- 001;
       EQUAL:          ZF,PF,CF <-- 100;

#### B.4.34 `CPUID`: Get CPU Identification Code

    CPUID                         ; 0F A2                [PENT]

`CPUID` returns various information about the processor it is being executed on. It fills the four registers `EAX`, `EBX`, `ECX` and `EDX` with information, which varies depending on the input contents of `EAX`.

`CPUID` also acts as a barrier to serialise instruction execution: executing the `CPUID` instruction guarantees that all the effects (memory modification, flag modification, register modification) of previous instructions have been completed before the next instruction gets fetched.

The information returned is as follows:

- If `EAX` is zero on input, `EAX` on output holds the maximum acceptable input value of `EAX`, and `EBX:EDX:ECX` contain the string `"GenuineIntel"` (or not, if you have a clone processor). That is to say, `EBX` contains `"Genu"` (in NASM's own sense of character constants, described in section 3.4.2), `EDX` contains `"ineI"` and `ECX` contains `"ntel"`.
- If `EAX` is one on input, `EAX` on output contains version information about the processor, and `EDX` contains a set of feature flags, showing the presence and absence of various features. For example, bit 8 is set if the `CMPXCHG8B` instruction (section B.4.31) is supported, bit 15 is set if the conditional move instructions (section B.4.23 and section B.4.72) are supported, and bit 23 is set if `MMX` instructions are supported.
- If `EAX` is two on input, `EAX`, `EBX`, `ECX` and `EDX` all contain information about caches and TLBs (Translation Lookahead Buffers).

For more information on the data returned from `CPUID`, see the documentation from Intel and other processor manufacturers.

#### B.4.35 `CVTDQ2PD`: Packed Signed INT32 to Packed Double-Precision FP Conversion

    CVTDQ2PD xmm1,xmm2/mem64      ; F3 0F E6 /r     [WILLAMETTE,SSE2]

`CVTDQ2PD` converts two packed signed doublewords from the source operand to two packed double-precision FP values in the destination operand.

The destination operand is an `XMM` register. The source can be either an `XMM` register or a 64-bit memory location. If the source is a register, the packed integers are in the low quadword.

#### B.4.36 `CVTDQ2PS`: Packed Signed INT32 to Packed Single-Precision FP Conversion

    CVTDQ2PS xmm1,xmm2/mem128     ; 0F 5B /r        [WILLAMETTE,SSE2]

`CVTDQ2PS` converts four packed signed doublewords from the source operand to four packed single-precision FP values in the destination operand.

The destination operand is an `XMM` register. The source can be either an `XMM` register or a 128-bit memory location.

For more details of this instruction, see the Intel Processor manuals.

#### B.4.37 `CVTPD2DQ`: Packed Double-Precision FP to Packed Signed INT32 Conversion

    CVTPD2DQ xmm1,xmm2/mem128     ; F2 0F E6 /r     [WILLAMETTE,SSE2]

`CVTPD2DQ` converts two packed double-precision FP values from the source operand to two packed signed doublewords in the low quadword of the destination operand. The high quadword of the destination is set to all 0s.

The destination operand is an `XMM` register. The source can be either an `XMM` register or a 128-bit memory location.

For more details of this instruction, see the Intel Processor manuals.

#### B.4.38 `CVTPD2PI`: Packed Double-Precision FP to Packed Signed INT32 Conversion

    CVTPD2PI mm,xmm/mem128        ; 66 0F 2D /r     [WILLAMETTE,SSE2]

`CVTPD2PI` converts two packed double-precision FP values from the source operand to two packed signed doublewords in the destination operand.

The destination operand is an `MMX` register. The source can be either an `XMM` register or a 128-bit memory location.

For more details of this instruction, see the Intel Processor manuals.

#### B.4.39 `CVTPD2PS`: Packed Double-Precision FP to Packed Single-Precision FP Conversion

    CVTPD2PS xmm1,xmm2/mem128     ; 66 0F 5A /r     [WILLAMETTE,SSE2]

`CVTPD2PS` converts two packed double-precision FP values from the source operand to two packed single-precision FP values in the low quadword of the destination operand. The high quadword of the destination is set to all 0s.

The destination operand is an `XMM` register. The source can be either an `XMM` register or a 128-bit memory location.

For more details of this instruction, see the Intel Processor manuals.

#### B.4.40 `CVTPI2PD`: Packed Signed INT32 to Packed Double-Precision FP Conversion

    CVTPI2PD xmm,mm/mem64         ; 66 0F 2A /r     [WILLAMETTE,SSE2]

`CVTPI2PD` converts two packed signed doublewords from the source operand to two packed double-precision FP values in the destination operand.

The destination operand is an `XMM` register. The source can be either an `MMX` register or a 64-bit memory location.

For more details of this instruction, see the Intel Processor manuals.

#### B.4.41 `CVTPI2PS`: Packed Signed INT32 to Packed Single-FP Conversion

    CVTPI2PS xmm,mm/mem64         ; 0F 2A /r        [KATMAI,SSE]

`CVTPI2PS` converts two packed signed doublewords from the source operand to two packed single-precision FP values in the low quadword of the destination operand. The high quadword of the destination remains unchanged.

The destination operand is an `XMM` register. The source can be either an `MMX` register or a 64-bit memory location.

For more details of this instruction, see the Intel Processor manuals.

#### B.4.42 `CVTPS2DQ`: Packed Single-Precision FP to Packed Signed INT32 Conversion

    CVTPS2DQ xmm1,xmm2/mem128     ; 66 0F 5B /r     [WILLAMETTE,SSE2]

`CVTPS2DQ` converts four packed single-precision FP values from the source operand to four packed signed doublewords in the destination operand.

The destination operand is an `XMM` register. The source can be either an `XMM` register or a 128-bit memory location.

For more details of this instruction, see the Intel Processor manuals.

#### B.4.43 `CVTPS2PD`: Packed Single-Precision FP to Packed Double-Precision FP Conversion

    CVTPS2PD xmm1,xmm2/mem64      ; 0F 5A /r        [WILLAMETTE,SSE2]

`CVTPS2PD` converts two packed single-precision FP values from the source operand to two packed double-precision FP values in the destination operand.

The destination operand is an `XMM` register. The source can be either an `XMM` register or a 64-bit memory location. If the source is a register, the input values are in the low quadword.

For more details of this instruction, see the Intel Processor manuals.

#### B.4.44 `CVTPS2PI`: Packed Single-Precision FP to Packed Signed INT32 Conversion

    CVTPS2PI mm,xmm/mem64         ; 0F 2D /r        [KATMAI,SSE]

`CVTPS2PI` converts two packed single-precision FP values from the source operand to two packed signed doublewords in the destination operand.

The destination operand is an `MMX` register. The source can be either an `XMM` register or a 64-bit memory location. If the source is a register, the input values are in the low quadword.

For more details of this instruction, see the Intel Processor manuals.

#### B.4.45 `CVTSD2SI`: Scalar Double-Precision FP to Signed INT32 Conversion

    CVTSD2SI reg32,xmm/mem64      ; F2 0F 2D /r     [WILLAMETTE,SSE2]

`CVTSD2SI` converts a double-precision FP value from the source operand to a signed doubleword in the destination operand.

The destination operand is a general purpose register. The source can be either an `XMM` register or a 64-bit memory location. If the source is a register, the input value is in the low quadword.

For more details of this instruction, see the Intel Processor manuals.

#### B.4.46 `CVTSD2SS`: Scalar Double-Precision FP to Scalar Single-Precision FP Conversion

    CVTSD2SS xmm1,xmm2/mem64      ; F2 0F 5A /r     [KATMAI,SSE]

`CVTSD2SS` converts a double-precision FP value from the source operand to a single-precision FP value in the low doubleword of the destination operand. The upper 3 doublewords are left unchanged.

The destination operand is an `XMM` register. The source can be either an `XMM` register or a 64-bit memory location. If the source is a register, the input value is in the low quadword.

For more details of this instruction, see the Intel Processor manuals.

#### B.4.47 `CVTSI2SD`: Signed INT32 to Scalar Double-Precision FP Conversion

    CVTSI2SD xmm,r/m32            ; F2 0F 2A /r     [WILLAMETTE,SSE2]

`CVTSI2SD` converts a signed doubleword from the source operand to a double-precision FP value in the low quadword of the destination operand. The high quadword is left unchanged.

The destination operand is an `XMM` register. The source can be either a general purpose register or a 32-bit memory location.

For more details of this instruction, see the Intel Processor manuals.

#### B.4.48 `CVTSI2SS`: Signed INT32 to Scalar Single-Precision FP Conversion

    CVTSI2SS xmm,r/m32            ; F3 0F 2A /r     [KATMAI,SSE]

`CVTSI2SS` converts a signed doubleword from the source operand to a single-precision FP value in the low doubleword of the destination operand. The upper 3 doublewords are left unchanged.

The destination operand is an `XMM` register. The source can be either a general purpose register or a 32-bit memory location.

For more details of this instruction, see the Intel Processor manuals.

#### B.4.49 `CVTSS2SD`: Scalar Single-Precision FP to Scalar Double-Precision FP Conversion

    CVTSS2SD xmm1,xmm2/mem32      ; F3 0F 5A /r     [WILLAMETTE,SSE2]

`CVTSS2SD` converts a single-precision FP value from the source operand to a double-precision FP value in the low quadword of the destination operand. The upper quadword is left unchanged.

The destination operand is an `XMM` register. The source can be either an `XMM` register or a 32-bit memory location. If the source is a register, the input value is contained in the low doubleword.

For more details of this instruction, see the Intel Processor manuals.

#### B.4.50 `CVTSS2SI`: Scalar Single-Precision FP to Signed INT32 Conversion

    CVTSS2SI reg32,xmm/mem32      ; F3 0F 2D /r     [KATMAI,SSE]

`CVTSS2SI` converts a single-precision FP value from the source operand to a signed doubleword in the destination operand.

The destination operand is a general purpose register. The source can be either an `XMM` register or a 32-bit memory location. If the source is a register, the input value is in the low doubleword.

For more details of this instruction, see the Intel Processor manuals.

#### B.4.51 `CVTTPD2DQ`: Packed Double-Precision FP to Packed Signed INT32 Conversion with Truncation

    CVTTPD2DQ xmm1,xmm2/mem128    ; 66 0F E6 /r     [WILLAMETTE,SSE2]

`CVTTPD2DQ` converts two packed double-precision FP values in the source operand to two packed single-precision FP values in the destination operand. If the result is inexact, it is truncated (rounded toward zero). The high quadword is set to all 0s.

The destination operand is an `XMM` register. The source can be either an `XMM` register or a 128-bit memory location.

For more details of this instruction, see the Intel Processor manuals.

#### B.4.52 `CVTTPD2PI`: Packed Double-Precision FP to Packed Signed INT32 Conversion with Truncation

    CVTTPD2PI mm,xmm/mem128        ; 66 0F 2C /r     [WILLAMETTE,SSE2]

`CVTTPD2PI` converts two packed double-precision FP values in the source operand to two packed single-precision FP values in the destination operand. If the result is inexact, it is truncated (rounded toward zero).

The destination operand is an `MMX` register. The source can be either an `XMM` register or a 128-bit memory location.

For more details of this instruction, see the Intel Processor manuals.

#### B.4.53 `CVTTPS2DQ`: Packed Single-Precision FP to Packed Signed INT32 Conversion with Truncation

    CVTTPS2DQ xmm1,xmm2/mem128    ; F3 0F 5B /r     [WILLAMETTE,SSE2]

`CVTTPS2DQ` converts four packed single-precision FP values in the source operand to four packed signed doublewords in the destination operand. If the result is inexact, it is truncated (rounded toward zero).

The destination operand is an `XMM` register. The source can be either an `XMM` register or a 128-bit memory location.

For more details of this instruction, see the Intel Processor manuals.

#### B.4.54 `CVTTPS2PI`: Packed Single-Precision FP to Packed Signed INT32 Conversion with Truncation

    CVTTPS2PI mm,xmm/mem64         ; 0F 2C /r       [KATMAI,SSE]

`CVTTPS2PI` converts two packed single-precision FP values in the source operand to two packed signed doublewords in the destination operand. If the result is inexact, it is truncated (rounded toward zero). If the source is a register, the input values are in the low quadword.

The destination operand is an `MMX` register. The source can be either an `XMM` register or a 64-bit memory location. If the source is a register, the input value is in the low quadword.

For more details of this instruction, see the Intel Processor manuals.

#### B.4.55 `CVTTSD2SI`: Scalar Double-Precision FP to Signed INT32 Conversion with Truncation

    CVTTSD2SI reg32,xmm/mem64      ; F2 0F 2C /r    [WILLAMETTE,SSE2]

`CVTTSD2SI` converts a double-precision FP value in the source operand to a signed doubleword in the destination operand. If the result is inexact, it is truncated (rounded toward zero).

The destination operand is a general purpose register. The source can be either an `XMM` register or a 64-bit memory location. If the source is a register, the input value is in the low quadword.

For more details of this instruction, see the Intel Processor manuals.

#### B.4.56 `CVTTSS2SI`: Scalar Single-Precision FP to Signed INT32 Conversion with Truncation

    CVTTSD2SI reg32,xmm/mem32      ; F3 0F 2C /r    [KATMAI,SSE]

`CVTTSS2SI` converts a single-precision FP value in the source operand to a signed doubleword in the destination operand. If the result is inexact, it is truncated (rounded toward zero).

The destination operand is a general purpose register. The source can be either an `XMM` register or a 32-bit memory location. If the source is a register, the input value is in the low doubleword.

For more details of this instruction, see the Intel Processor manuals.

#### B.4.57 `DAA`, `DAS`: Decimal Adjustments

    DAA                           ; 27                   [8086]
    DAS                           ; 2F                   [8086]

These instructions are used in conjunction with the add and subtract instructions to perform binary-coded decimal arithmetic in packed (one BCD digit per nibble) form. For the unpacked equivalents, see section B.4.1.

`DAA` should be used after a one-byte `ADD` instruction whose destination was the `AL` register: by means of examining the value in the `AL` and also the auxiliary carry flag `AF`, it determines whether either digit of the addition has overflowed, and adjusts it (and sets the carry and auxiliary-carry flags) if so. You can add long BCD strings together by doing `ADD`/`DAA` on the low two digits, then doing `ADC`/`DAA` on each subsequent pair of digits.

`DAS` works similarly to `DAA`, but is for use after `SUB` instructions rather than `ADD`.

#### B.4.58 `DEC`: Decrement Integer

    DEC reg16                     ; o16 48+r             [8086]
    DEC reg32                     ; o32 48+r             [386]
    DEC r/m8                      ; FE /1                [8086]
    DEC r/m16                     ; o16 FF /1            [8086]
    DEC r/m32                     ; o32 FF /1            [386]

`DEC` subtracts 1 from its operand. It does not affect the carry flag: to affect the carry flag, use `SUB something,1` (see section B.4.305). `DEC` affects all the other flags according to the result.

This instruction can be used with a `LOCK` prefix to allow atomic execution.

See also `INC` (section B.4.120).

#### B.4.59 `DIV`: Unsigned Integer Divide

    DIV r/m8                      ; F6 /6                [8086]
    DIV r/m16                     ; o16 F7 /6            [8086]
    DIV r/m32                     ; o32 F7 /6            [386]

`DIV` performs unsigned integer division. The explicit operand provided is the divisor; the dividend and destination operands are implicit, in the following way:

- For `DIV r/m8`, `AX` is divided by the given operand; the quotient is stored in `AL` and the remainder in `AH`.
- For `DIV r/m16`, `DX:AX` is divided by the given operand; the quotient is stored in `AX` and the remainder in `DX`.
- For `DIV r/m32`, `EDX:EAX` is divided by the given operand; the quotient is stored in `EAX` and the remainder in `EDX`.

Signed integer division is performed by the `IDIV` instruction: see section B.4.117.

#### B.4.60 `DIVPD`: Packed Double-Precision FP Divide

    DIVPD xmm1,xmm2/mem128        ; 66 0F 5E /r     [WILLAMETTE,SSE2]

`DIVPD` divides the two packed double-precision FP values in the destination operand by the two packed double-precision FP values in the source operand, and stores the packed double-precision results in the destination register.

The destination is an `XMM` register. The source operand can be either an `XMM` register or a 128-bit memory location.

       dst[0-63]   := dst[0-63]   / src[0-63],
       dst[64-127] := dst[64-127] / src[64-127].

#### B.4.61 `DIVPS`: Packed Single-Precision FP Divide

    DIVPS xmm1,xmm2/mem128        ; 0F 5E /r        [KATMAI,SSE]

`DIVPS` divides the four packed single-precision FP values in the destination operand by the four packed single-precision FP values in the source operand, and stores the packed single-precision results in the destination register.

The destination is an `XMM` register. The source operand can be either an `XMM` register or a 128-bit memory location.

       dst[0-31]   := dst[0-31]   / src[0-31],
       dst[32-63]  := dst[32-63]  / src[32-63],
       dst[64-95]  := dst[64-95]  / src[64-95],
       dst[96-127] := dst[96-127] / src[96-127].

#### B.4.62 `DIVSD`: Scalar Double-Precision FP Divide

    DIVSD xmm1,xmm2/mem64         ; F2 0F 5E /r     [WILLAMETTE,SSE2]

`DIVSD` divides the low-order double-precision FP value in the destination operand by the low-order double-precision FP value in the source operand, and stores the double-precision result in the destination register.

The destination is an `XMM` register. The source operand can be either an `XMM` register or a 64-bit memory location.

       dst[0-63]   := dst[0-63] / src[0-63],
       dst[64-127] remains unchanged.

#### B.4.63 `DIVSS`: Scalar Single-Precision FP Divide

    DIVSS xmm1,xmm2/mem32         ; F3 0F 5E /r     [KATMAI,SSE]

`DIVSS` divides the low-order single-precision FP value in the destination operand by the low-order single-precision FP value in the source operand, and stores the single-precision result in the destination register.

The destination is an `XMM` register. The source operand can be either an `XMM` register or a 32-bit memory location.

       dst[0-31]   := dst[0-31] / src[0-31],
       dst[32-127] remains unchanged.

#### B.4.64 `EMMS`: Empty MMX State

    EMMS                          ; 0F 77                [PENT,MMX]

`EMMS` sets the FPU tag word (marking which floating-point registers are available) to all ones, meaning all registers are available for the FPU to use. It should be used after executing `MMX` instructions and before executing any subsequent floating-point operations.

#### B.4.65 `ENTER`: Create Stack Frame

    ENTER imm,imm                 ; C8 iw ib             [186]

`ENTER` constructs a `stack frame` for a high-level language procedure call. The first operand (the `iw` in the opcode definition above refers to the first operand) gives the amount of stack space to allocate for local variables; the second (the `ib` above) gives the nesting level of the procedure (for languages like Pascal, with nested procedures).

The function of `ENTER`, with a nesting level of zero, is equivalent to

              PUSH EBP            ; or PUSH BP         in 16 bits
              MOV EBP,ESP         ; or MOV BP,SP       in 16 bits
              SUB ESP,operand1    ; or SUB SP,operand1 in 16 bits

This creates a stack frame with the procedure parameters accessible upwards from `EBP`, and local variables accessible downwards from `EBP`.

With a nesting level of one, the stack frame created is 4 (or 2) bytes bigger, and the value of the final frame pointer `EBP` is accessible in memory at `[EBP-4]`.

This allows `ENTER`, when called with a nesting level of two, to look at the stack frame described by the previous value of `EBP`, find the frame pointer at offset -4 from that, and push it along with its new frame pointer, so that when a level-two procedure is called from within a level-one procedure, `[EBP-4]` holds the frame pointer of the most recent level-one procedure call and `[EBP-8]` holds that of the most recent level-two call. And so on, for nesting levels up to 31.

Stack frames created by `ENTER` can be destroyed by the `LEAVE` instruction: see section B.4.136.

#### B.4.66 `F2XM1`: Calculate 2**X-1

    F2XM1                         ; D9 F0                [8086,FPU]

`F2XM1` raises 2 to the power of `ST0`, subtracts one, and stores the result back into `ST0`. The initial contents of `ST0` must be a number in the range -1.0 to +1.0.

#### B.4.67 `FABS`: Floating-Point Absolute Value

    FABS                          ; D9 E1                [8086,FPU]

`FABS` computes the absolute value of `ST0`,by clearing the sign bit, and stores the result back in `ST0`.

#### B.4.68 `FADD`, `FADDP`: Floating-Point Addition

    FADD mem32                    ; D8 /0                [8086,FPU]
    FADD mem64                    ; DC /0                [8086,FPU]

    FADD fpureg                   ; D8 C0+r              [8086,FPU]
    FADD ST0,fpureg               ; D8 C0+r              [8086,FPU]

    FADD TO fpureg                ; DC C0+r              [8086,FPU]
    FADD fpureg,ST0               ; DC C0+r              [8086,FPU]

    FADDP fpureg                  ; DE C0+r              [8086,FPU]
    FADDP fpureg,ST0              ; DE C0+r              [8086,FPU]

- `FADD`, given one operand, adds the operand to `ST0` and stores the result back in `ST0`. If the operand has the `TO` modifier, the result is stored in the register given rather than in `ST0`.
- `FADDP` performs the same function as `FADD TO`, but pops the register stack after storing the result.

The given two-operand forms are synonyms for the one-operand forms.

To add an integer value to `ST0`, use the c{FIADD} instruction (section B.4.80)

#### B.4.69 `FBLD`, `FBSTP`: BCD Floating-Point Load and Store

    FBLD mem80                    ; DF /4                [8086,FPU]
    FBSTP mem80                   ; DF /6                [8086,FPU]

`FBLD` loads an 80-bit (ten-byte) packed binary-coded decimal number from the given memory address, converts it to a real, and pushes it on the register stack. `FBSTP` stores the value of `ST0`, in packed BCD, at the given address and then pops the register stack.

#### B.4.70 `FCHS`: Floating-Point Change Sign

    FCHS                          ; D9 E0                [8086,FPU]

`FCHS` negates the number in `ST0`, by inverting the sign bit: negative numbers become positive, and vice versa.

#### B.4.71 `FCLEX`, `FNCLEX`: Clear Floating-Point Exceptions

    FCLEX                         ; 9B DB E2             [8086,FPU]
    FNCLEX                        ; DB E2                [8086,FPU]

`FCLEX` clears any floating-point exceptions which may be pending. `FNCLEX` does the same thing but doesn't wait for previous floating-point operations (including the handling of pending exceptions) to finish first.

#### B.4.72 `FCMOVcc`: Floating-Point Conditional Move

    FCMOVB fpureg                 ; DA C0+r              [P6,FPU]
    FCMOVB ST0,fpureg             ; DA C0+r              [P6,FPU]

    FCMOVE fpureg                 ; DA C8+r              [P6,FPU]
    FCMOVE ST0,fpureg             ; DA C8+r              [P6,FPU]

    FCMOVBE fpureg                ; DA D0+r              [P6,FPU]
    FCMOVBE ST0,fpureg            ; DA D0+r              [P6,FPU]

    FCMOVU fpureg                 ; DA D8+r              [P6,FPU]
    FCMOVU ST0,fpureg             ; DA D8+r              [P6,FPU]

    FCMOVNB fpureg                ; DB C0+r              [P6,FPU]
    FCMOVNB ST0,fpureg            ; DB C0+r              [P6,FPU]

    FCMOVNE fpureg                ; DB C8+r              [P6,FPU]
    FCMOVNE ST0,fpureg            ; DB C8+r              [P6,FPU]

    FCMOVNBE fpureg               ; DB D0+r              [P6,FPU]
    FCMOVNBE ST0,fpureg           ; DB D0+r              [P6,FPU]

    FCMOVNU fpureg                ; DB D8+r              [P6,FPU]
    FCMOVNU ST0,fpureg            ; DB D8+r              [P6,FPU]

The `FCMOV` instructions perform conditional move operations: each of them moves the contents of the given register into `ST0` if its condition is satisfied, and does nothing if not.

The conditions are not the same as the standard condition codes used with conditional jump instructions. The conditions `B`, `BE`, `NB`, `NBE`, `E` and `NE` are exactly as normal, but none of the other standard ones are supported. Instead, the condition `U` and its counterpart `NU` are provided; the `U` condition is satisfied if the last two floating-point numbers compared were unordered, i.e. they were not equal but neither one could be said to be greater than the other, for example if they were NaNs. (The flag state which signals this is the setting of the parity flag: so the `U` condition is notionally equivalent to `PE`, and `NU` is equivalent to `PO`.)

The `FCMOV` conditions test the main processor's status flags, not the FPU status flags, so using `FCMOV` directly after `FCOM` will not work. Instead, you should either use `FCOMI` which writes directly to the main CPU flags word, or use `FSTSW` to extract the FPU flags.

Although the `FCMOV` instructions are flagged `P6` above, they may not be supported by all Pentium Pro processors; the `CPUID` instruction (section B.4.34) will return a bit which indicates whether conditional moves are supported.

#### B.4.73 `FCOM`, `FCOMP`, `FCOMPP`, `FCOMI`, `FCOMIP`: Floating-Point Compare

    FCOM mem32                    ; D8 /2                [8086,FPU]
    FCOM mem64                    ; DC /2                [8086,FPU]
    FCOM fpureg                   ; D8 D0+r              [8086,FPU]
    FCOM ST0,fpureg               ; D8 D0+r              [8086,FPU]

    FCOMP mem32                   ; D8 /3                [8086,FPU]
    FCOMP mem64                   ; DC /3                [8086,FPU]
    FCOMP fpureg                  ; D8 D8+r              [8086,FPU]
    FCOMP ST0,fpureg              ; D8 D8+r              [8086,FPU]

    FCOMPP                        ; DE D9                [8086,FPU]

    FCOMI fpureg                  ; DB F0+r              [P6,FPU]
    FCOMI ST0,fpureg              ; DB F0+r              [P6,FPU]

    FCOMIP fpureg                 ; DF F0+r              [P6,FPU]
    FCOMIP ST0,fpureg             ; DF F0+r              [P6,FPU]

`FCOM` compares `ST0` with the given operand, and sets the FPU flags accordingly. `ST0` is treated as the left-hand side of the comparison, so that the carry flag is set (for a `less-than' result) if `ST0` is less than the given operand.

`FCOMP` does the same as `FCOM`, but pops the register stack afterwards. `FCOMPP` compares `ST0` with `ST1` and then pops the register stack twice.

`FCOMI` and `FCOMIP` work like the corresponding forms of `FCOM` and `FCOMP`, but write their results directly to the CPU flags register rather than the FPU status word, so they can be immediately followed by conditional jump or conditional move instructions.

The `FCOM` instructions differ from the `FUCOM` instructions (section B.4.108) only in the way they handle quiet NaNs: `FUCOM` will handle them silently and set the condition code flags to an `unordered' result, whereas `FCOM` will generate an exception.

#### B.4.74 `FCOS`: Cosine

    FCOS                          ; D9 FF                [386,FPU]

`FCOS` computes the cosine of `ST0` (in radians), and stores the result in `ST0`. The absolute value of `ST0` must be less than 2**63.

See also `FSINCOS` (section B.4.100).

#### B.4.75 `FDECSTP`: Decrement Floating-Point Stack Pointer

    FDECSTP                       ; D9 F6                [8086,FPU]

`FDECSTP` decrements the `top' field in the floating-point status word. This has the effect of rotating the FPU register stack by one, as if the contents of `ST7` had been pushed on the stack. See also `FINCSTP` (section B.4.85).

#### B.4.76 `FxDISI`, `FxENI`: Disable and Enable Floating-Point Interrupts

    FDISI                         ; 9B DB E1             [8086,FPU]
    FNDISI                        ; DB E1                [8086,FPU]

    FENI                          ; 9B DB E0             [8086,FPU]
    FNENI                         ; DB E0                [8086,FPU]

`FDISI` and `FENI` disable and enable floating-point interrupts. These instructions are only meaningful on original 8087 processors: the 287 and above treat them as no-operation instructions.

`FNDISI` and `FNENI` do the same thing as `FDISI` and `FENI` respectively, but without waiting for the floating-point processor to finish what it was doing first.

#### B.4.77 `FDIV`, `FDIVP`, `FDIVR`, `FDIVRP`: Floating-Point Division

    FDIV mem32                    ; D8 /6                [8086,FPU]
    FDIV mem64                    ; DC /6                [8086,FPU]

    FDIV fpureg                   ; D8 F0+r              [8086,FPU]
    FDIV ST0,fpureg               ; D8 F0+r              [8086,FPU]

    FDIV TO fpureg                ; DC F8+r              [8086,FPU]
    FDIV fpureg,ST0               ; DC F8+r              [8086,FPU]

    FDIVR mem32                   ; D8 /0                [8086,FPU]
    FDIVR mem64                   ; DC /0                [8086,FPU]

    FDIVR fpureg                  ; D8 F8+r              [8086,FPU]
    FDIVR ST0,fpureg              ; D8 F8+r              [8086,FPU]

    FDIVR TO fpureg               ; DC F0+r              [8086,FPU]
    FDIVR fpureg,ST0              ; DC F0+r              [8086,FPU]

    FDIVP fpureg                  ; DE F8+r              [8086,FPU]
    FDIVP fpureg,ST0              ; DE F8+r              [8086,FPU]

    FDIVRP fpureg                 ; DE F0+r              [8086,FPU]
    FDIVRP fpureg,ST0             ; DE F0+r              [8086,FPU]

- `FDIV` divides `ST0` by the given operand and stores the result back in `ST0`, unless the `TO` qualifier is given, in which case it divides the given operand by `ST0` and stores the result in the operand.
- `FDIVR` does the same thing, but does the division the other way up: so if `TO` is not given, it divides the given operand by `ST0` and stores the result in `ST0`, whereas if `TO` is given it divides `ST0` by its operand and stores the result in the operand.
- `FDIVP` operates like `FDIV TO`, but pops the register stack once it has finished.
- `FDIVRP` operates like `FDIVR TO`, but pops the register stack once it has finished.

For FP/Integer divisions, see `FIDIV` (section B.4.82).

#### B.4.78 `FEMMS`: Faster Enter/Exit of the MMX or floating-point state

    FEMMS                         ; 0F 0E           [PENT,3DNOW]

`FEMMS` can be used in place of the `EMMS` instruction on processors which support the 3DNow! instruction set. Following execution of `FEMMS`, the state of the `MMX/FP` registers is undefined, and this allows a faster context switch between `FP` and `MMX` instructions. The `FEMMS` instruction can also be used before executing `MMX` instructions

#### B.4.79 `FFREE`: Flag Floating-Point Register as Unused

    FFREE fpureg                  ; DD C0+r              [8086,FPU]
    FFREEP fpureg                 ; DF C0+r              [286,FPU,UNDOC]

`FFREE` marks the given register as being empty.

`FFREEP` marks the given register as being empty, and then pops the register stack.

#### B.4.80 `FIADD`: Floating-Point/Integer Addition

    FIADD mem16                   ; DE /0                [8086,FPU]
    FIADD mem32                   ; DA /0                [8086,FPU]

`FIADD` adds the 16-bit or 32-bit integer stored in the given memory location to `ST0`, storing the result in `ST0`.

#### B.4.81 `FICOM`, `FICOMP`: Floating-Point/Integer Compare

    FICOM mem16                   ; DE /2                [8086,FPU]
    FICOM mem32                   ; DA /2                [8086,FPU]

    FICOMP mem16                  ; DE /3                [8086,FPU]
    FICOMP mem32                  ; DA /3                [8086,FPU]

`FICOM` compares `ST0` with the 16-bit or 32-bit integer stored in the given memory location, and sets the FPU flags accordingly. `FICOMP` does the same, but pops the register stack afterwards.

#### B.4.82 `FIDIV`, `FIDIVR`: Floating-Point/Integer Division

    FIDIV mem16                   ; DE /6                [8086,FPU]
    FIDIV mem32                   ; DA /6                [8086,FPU]

    FIDIVR mem16                  ; DE /7                [8086,FPU]
    FIDIVR mem32                  ; DA /7                [8086,FPU]

`FIDIV` divides `ST0` by the 16-bit or 32-bit integer stored in the given memory location, and stores the result in `ST0`. `FIDIVR` does the division the other way up: it divides the integer by `ST0`, but still stores the result in `ST0`.

#### B.4.83 `FILD`, `FIST`, `FISTP`: Floating-Point/Integer Conversion

    FILD mem16                    ; DF /0                [8086,FPU]
    FILD mem32                    ; DB /0                [8086,FPU]
    FILD mem64                    ; DF /5                [8086,FPU]

    FIST mem16                    ; DF /2                [8086,FPU]
    FIST mem32                    ; DB /2                [8086,FPU]

    FISTP mem16                   ; DF /3                [8086,FPU]
    FISTP mem32                   ; DB /3                [8086,FPU]
    FISTP mem64                   ; DF /7                [8086,FPU]

`FILD` loads an integer out of a memory location, converts it to a real, and pushes it on the FPU register stack. `FIST` converts `ST0` to an integer and stores that in memory; `FISTP` does the same as `FIST`, but pops the register stack afterwards.

#### B.4.84 `FIMUL`: Floating-Point/Integer Multiplication

    FIMUL mem16                   ; DE /1                [8086,FPU]
    FIMUL mem32                   ; DA /1                [8086,FPU]

`FIMUL` multiplies `ST0` by the 16-bit or 32-bit integer stored in the given memory location, and stores the result in `ST0`.

#### B.4.85 `FINCSTP`: Increment Floating-Point Stack Pointer

    FINCSTP                       ; D9 F7                [8086,FPU]

`FINCSTP` increments the `top' field in the floating-point status word. This has the effect of rotating the FPU register stack by one, as if the register stack had been popped; however, unlike the popping of the stack performed by many FPU instructions, it does not flag the new `ST7` (previously `ST0`) as empty. See also `FDECSTP` (section B.4.75).

#### B.4.86 `FINIT`, `FNINIT`: Initialise Floating-Point Unit

    FINIT                         ; 9B DB E3             [8086,FPU]
    FNINIT                        ; DB E3                [8086,FPU]

`FINIT` initialises the FPU to its default state. It flags all registers as empty, without actually change their values, clears the top of stack pointer. `FNINIT` does the same, without first waiting for pending exceptions to clear.

#### B.4.87 `FISUB`: Floating-Point/Integer Subtraction

    FISUB mem16                   ; DE /4                [8086,FPU]
    FISUB mem32                   ; DA /4                [8086,FPU]

    FISUBR mem16                  ; DE /5                [8086,FPU]
    FISUBR mem32                  ; DA /5                [8086,FPU]

`FISUB` subtracts the 16-bit or 32-bit integer stored in the given memory location from `ST0`, and stores the result in `ST0`. `FISUBR` does the subtraction the other way round, i.e. it subtracts `ST0` from the given integer, but still stores the result in `ST0`.

#### B.4.88 `FLD`: Floating-Point Load

    FLD mem32                     ; D9 /0                [8086,FPU]
    FLD mem64                     ; DD /0                [8086,FPU]
    FLD mem80                     ; DB /5                [8086,FPU]
    FLD fpureg                    ; D9 C0+r              [8086,FPU]

`FLD` loads a floating-point value out of the given register or memory location, and pushes it on the FPU register stack.

#### B.4.89 `FLDxx`: Floating-Point Load Constants

    FLD1                          ; D9 E8                [8086,FPU]
    FLDL2E                        ; D9 EA                [8086,FPU]
    FLDL2T                        ; D9 E9                [8086,FPU]
    FLDLG2                        ; D9 EC                [8086,FPU]
    FLDLN2                        ; D9 ED                [8086,FPU]
    FLDPI                         ; D9 EB                [8086,FPU]
    FLDZ                          ; D9 EE                [8086,FPU]

These instructions push specific standard constants on the FPU register stack.

     Instruction    Constant pushed

     FLD1           1
     FLDL2E         base-2 logarithm of e
     FLDL2T         base-2 log of 10
     FLDLG2         base-10 log of 2
     FLDLN2         base-e log of 2
     FLDPI          pi
     FLDZ           zero

#### B.4.90 `FLDCW`: Load Floating-Point Control Word

    FLDCW mem16                   ; D9 /5                [8086,FPU]

`FLDCW` loads a 16-bit value out of memory and stores it into the FPU control word (governing things like the rounding mode, the precision, and the exception masks). See also `FSTCW` (section B.4.103). If exceptions are enabled and you don't want to generate one, use `FCLEX` or `FNCLEX` (section B.4.71) before loading the new control word.

#### B.4.91 `FLDENV`: Load Floating-Point Environment

    FLDENV mem                    ; D9 /4                [8086,FPU]

`FLDENV` loads the FPU operating environment (control word, status word, tag word, instruction pointer, data pointer and last opcode) from memory. The memory area is 14 or 28 bytes long, depending on the CPU mode at the time. See also `FSTENV` (section B.4.104).

#### B.4.92 `FMUL`, `FMULP`: Floating-Point Multiply

    FMUL mem32                    ; D8 /1                [8086,FPU]
    FMUL mem64                    ; DC /1                [8086,FPU]

    FMUL fpureg                   ; D8 C8+r              [8086,FPU]
    FMUL ST0,fpureg               ; D8 C8+r              [8086,FPU]

    FMUL TO fpureg                ; DC C8+r              [8086,FPU]
    FMUL fpureg,ST0               ; DC C8+r              [8086,FPU]

    FMULP fpureg                  ; DE C8+r              [8086,FPU]
    FMULP fpureg,ST0              ; DE C8+r              [8086,FPU]

`FMUL` multiplies `ST0` by the given operand, and stores the result in `ST0`, unless the `TO` qualifier is used in which case it stores the result in the operand. `FMULP` performs the same operation as `FMUL TO`, and then pops the register stack.

#### B.4.93 `FNOP`: Floating-Point No Operation

    FNOP                          ; D9 D0                [8086,FPU]

`FNOP` does nothing.

#### B.4.94 `FPATAN`, `FPTAN`: Arctangent and Tangent

    FPATAN                        ; D9 F3                [8086,FPU]
    FPTAN                         ; D9 F2                [8086,FPU]

`FPATAN` computes the arctangent, in radians, of the result of dividing `ST1` by `ST0`, stores the result in `ST1`, and pops the register stack. It works like the C `atan2` function, in that changing the sign of both `ST0` and `ST1` changes the output value by pi (so it performs true rectangular-to-polar coordinate conversion, with `ST1` being the Y coordinate and `ST0` being the X coordinate, not merely an arctangent).

`FPTAN` computes the tangent of the value in `ST0` (in radians), and stores the result back into `ST0`.

The absolute value of `ST0` must be less than 2**63.

#### B.4.95 `FPREM`, `FPREM1`: Floating-Point Partial Remainder

    FPREM                         ; D9 F8                [8086,FPU]
    FPREM1                        ; D9 F5                [386,FPU]

These instructions both produce the remainder obtained by dividing `ST0` by `ST1`. This is calculated, notionally, by dividing `ST0` by `ST1`, rounding the result to an integer, multiplying by `ST1` again, and computing the value which would need to be added back on to the result to get back to the original value in `ST0`.

The two instructions differ in the way the notional round-to-integer operation is performed. `FPREM` does it by rounding towards zero, so that the remainder it returns always has the same sign as the original value in `ST0`; `FPREM1` does it by rounding to the nearest integer, so that the remainder always has at most half the magnitude of `ST1`.

Both instructions calculate partial remainders, meaning that they may not manage to provide the final result, but might leave intermediate results in `ST0` instead. If this happens, they will set the C2 flag in the FPU status word; therefore, to calculate a remainder, you should repeatedly execute `FPREM` or `FPREM1` until C2 becomes clear.

#### B.4.96 `FRNDINT`: Floating-Point Round to Integer

    FRNDINT                       ; D9 FC                [8086,FPU]

`FRNDINT` rounds the contents of `ST0` to an integer, according to the current rounding mode set in the FPU control word, and stores the result back in `ST0`.

#### B.4.97 `FSAVE`, `FRSTOR`: Save/Restore Floating-Point State

    FSAVE mem                     ; 9B DD /6             [8086,FPU]
    FNSAVE mem                    ; DD /6                [8086,FPU]

    FRSTOR mem                    ; DD /4                [8086,FPU]

`FSAVE` saves the entire floating-point unit state, including all the information saved by `FSTENV` (section B.4.104) plus the contents of all the registers, to a 94 or 108 byte area of memory (depending on the CPU mode). `FRSTOR` restores the floating-point state from the same area of memory.

`FNSAVE` does the same as `FSAVE`, without first waiting for pending floating-point exceptions to clear.

#### B.4.98 `FSCALE`: Scale Floating-Point Value by Power of Two

    FSCALE                        ; D9 FD                [8086,FPU]

`FSCALE` scales a number by a power of two: it rounds `ST1` towards zero to obtain an integer, then multiplies `ST0` by two to the power of that integer, and stores the result in `ST0`.

#### B.4.99 `FSETPM`: Set Protected Mode

    FSETPM                        ; DB E4                [286,FPU]

This instruction initialises protected mode on the 287 floating-point coprocessor. It is only meaningful on that processor: the 387 and above treat the instruction as a no-operation.

#### B.4.100 `FSIN`, `FSINCOS`: Sine and Cosine

    FSIN                          ; D9 FE                [386,FPU]
    FSINCOS                       ; D9 FB                [386,FPU]

`FSIN` calculates the sine of `ST0` (in radians) and stores the result in `ST0`. `FSINCOS` does the same, but then pushes the cosine of the same value on the register stack, so that the sine ends up in `ST1` and the cosine in `ST0`. `FSINCOS` is faster than executing `FSIN` and `FCOS` (see section B.4.74) in succession.

The absolute value of `ST0` must be less than 2**63.

#### B.4.101 `FSQRT`: Floating-Point Square Root

    FSQRT                         ; D9 FA                [8086,FPU]

`FSQRT` calculates the square root of `ST0` and stores the result in `ST0`.

#### B.4.102 `FST`, `FSTP`: Floating-Point Store

    FST mem32                     ; D9 /2                [8086,FPU]
    FST mem64                     ; DD /2                [8086,FPU]
    FST fpureg                    ; DD D0+r              [8086,FPU]

    FSTP mem32                    ; D9 /3                [8086,FPU]
    FSTP mem64                    ; DD /3                [8086,FPU]
    FSTP mem80                    ; DB /7                [8086,FPU]
    FSTP fpureg                   ; DD D8+r              [8086,FPU]

`FST` stores the value in `ST0` into the given memory location or other FPU register. `FSTP` does the same, but then pops the register stack.

#### B.4.103 `FSTCW`: Store Floating-Point Control Word

    FSTCW mem16                   ; 9B D9 /7             [8086,FPU]
    FNSTCW mem16                  ; D9 /7                [8086,FPU]

`FSTCW` stores the `FPU` control word (governing things like the rounding mode, the precision, and the exception masks) into a 2-byte memory area. See also `FLDCW` (section B.4.90).

`FNSTCW` does the same thing as `FSTCW`, without first waiting for pending floating-point exceptions to clear.

#### B.4.104 `FSTENV`: Store Floating-Point Environment

    FSTENV mem                    ; 9B D9 /6             [8086,FPU]
    FNSTENV mem                   ; D9 /6                [8086,FPU]

`FSTENV` stores the `FPU` operating environment (control word, status word, tag word, instruction pointer, data pointer and last opcode) into memory. The memory area is 14 or 28 bytes long, depending on the CPU mode at the time. See also `FLDENV` (section B.4.91).

`FNSTENV` does the same thing as `FSTENV`, without first waiting for pending floating-point exceptions to clear.

#### B.4.105 `FSTSW`: Store Floating-Point Status Word

    FSTSW mem16                   ; 9B DD /7             [8086,FPU]
    FSTSW AX                      ; 9B DF E0             [286,FPU]

    FNSTSW mem16                  ; DD /7                [8086,FPU]
    FNSTSW AX                     ; DF E0                [286,FPU]

`FSTSW` stores the `FPU` status word into `AX` or into a 2-byte memory area.

`FNSTSW` does the same thing as `FSTSW`, without first waiting for pending floating-point exceptions to clear.

#### B.4.106 `FSUB`, `FSUBP`, `FSUBR`, `FSUBRP`: Floating-Point Subtract

    FSUB mem32                    ; D8 /4                [8086,FPU]
    FSUB mem64                    ; DC /4                [8086,FPU]

    FSUB fpureg                   ; D8 E0+r              [8086,FPU]
    FSUB ST0,fpureg               ; D8 E0+r              [8086,FPU]

    FSUB TO fpureg                ; DC E8+r              [8086,FPU]
    FSUB fpureg,ST0               ; DC E8+r              [8086,FPU]

    FSUBR mem32                   ; D8 /5                [8086,FPU]
    FSUBR mem64                   ; DC /5                [8086,FPU]

    FSUBR fpureg                  ; D8 E8+r              [8086,FPU]
    FSUBR ST0,fpureg              ; D8 E8+r              [8086,FPU]

    FSUBR TO fpureg               ; DC E0+r              [8086,FPU]
    FSUBR fpureg,ST0              ; DC E0+r              [8086,FPU]

    FSUBP fpureg                  ; DE E8+r              [8086,FPU]
    FSUBP fpureg,ST0              ; DE E8+r              [8086,FPU]

    FSUBRP fpureg                 ; DE E0+r              [8086,FPU]
    FSUBRP fpureg,ST0             ; DE E0+r              [8086,FPU]

- `FSUB` subtracts the given operand from `ST0` and stores the result back in `ST0`, unless the `TO` qualifier is given, in which case it subtracts `ST0` from the given operand and stores the result in the operand.
- `FSUBR` does the same thing, but does the subtraction the other way up: so if `TO` is not given, it subtracts `ST0` from the given operand and stores the result in `ST0`, whereas if `TO` is given it subtracts its operand from `ST0` and stores the result in the operand.
- `FSUBP` operates like `FSUB TO`, but pops the register stack once it has finished.
- `FSUBRP` operates like `FSUBR TO`, but pops the register stack once it has finished.

#### B.4.107 `FTST`: Test `ST0` Against Zero

    FTST                          ; D9 E4                [8086,FPU]

`FTST` compares `ST0` with zero and sets the FPU flags accordingly. `ST0` is treated as the left-hand side of the comparison, so that a `less-than' result is generated if `ST0` is negative.

#### B.4.108 `FUCOMxx`: Floating-Point Unordered Compare

    FUCOM fpureg                  ; DD E0+r              [386,FPU]
    FUCOM ST0,fpureg              ; DD E0+r              [386,FPU]

    FUCOMP fpureg                 ; DD E8+r              [386,FPU]
    FUCOMP ST0,fpureg             ; DD E8+r              [386,FPU]

    FUCOMPP                       ; DA E9                [386,FPU]

    FUCOMI fpureg                 ; DB E8+r              [P6,FPU]
    FUCOMI ST0,fpureg             ; DB E8+r              [P6,FPU]

    FUCOMIP fpureg                ; DF E8+r              [P6,FPU]
    FUCOMIP ST0,fpureg            ; DF E8+r              [P6,FPU]

- `FUCOM` compares `ST0` with the given operand, and sets the FPU flags accordingly. `ST0` is treated as the left-hand side of the comparison, so that the carry flag is set (for a `less-than' result) if `ST0` is less than the given operand.
- `FUCOMP` does the same as `FUCOM`, but pops the register stack afterwards. `FUCOMPP` compares `ST0` with `ST1` and then pops the register stack twice.
- `FUCOMI` and `FUCOMIP` work like the corresponding forms of `FUCOM` and `FUCOMP`, but write their results directly to the CPU flags register rather than the FPU status word, so they can be immediately followed by conditional jump or conditional move instructions.

The `FUCOM` instructions differ from the `FCOM` instructions (section B.4.73) only in the way they handle quiet NaNs: `FUCOM` will handle them silently and set the condition code flags to an `unordered' result, whereas `FCOM` will generate an exception.

#### B.4.109 `FXAM`: Examine Class of Value in `ST0`

    FXAM                          ; D9 E5                [8086,FPU]

`FXAM` sets the FPU flags `C3`, `C2` and `C0` depending on the type of value stored in `ST0`:

     Register contents     Flags

     Unsupported format    000
     NaN                   001
     Finite number         010
     Infinity              011
     Zero                  100
     Empty register        101
     Denormal              110

Additionally, the `C1` flag is set to the sign of the number.

#### B.4.110 `FXCH`: Floating-Point Exchange

    FXCH                          ; D9 C9                [8086,FPU]
    FXCH fpureg                   ; D9 C8+r              [8086,FPU]
    FXCH fpureg,ST0               ; D9 C8+r              [8086,FPU]
    FXCH ST0,fpureg               ; D9 C8+r              [8086,FPU]

`FXCH` exchanges `ST0` with a given FPU register. The no-operand form exchanges `ST0` with `ST1`.

#### B.4.111 `FXRSTOR`: Restore `FP`, `MMX` and `SSE` State

    FXRSTOR memory                ; 0F AE /1               [P6,SSE,FPU]

The `FXRSTOR` instruction reloads the `FPU`, `MMX` and `SSE` state (environment and registers), from the 512 byte memory area defined by the source operand. This data should have been written by a previous `FXSAVE`.

#### B.4.112 `FXSAVE`: Store `FP`, `MMX` and `SSE` State

    FXSAVE memory                 ; 0F AE /0         [P6,SSE,FPU]

`FXSAVE`The FXSAVE instruction writes the current `FPU`, `MMX` and `SSE` technology states (environment and registers), to the 512 byte memory area defined by the destination operand. It does this without checking for pending unmasked floating-point exceptions (similar to the operation of `FNSAVE`).

Unlike the `FSAVE/FNSAVE` instructions, the processor retains the contents of the `FPU`, `MMX` and `SSE` state in the processor after the state has been saved. This instruction has been optimised to maximize floating-point save performance.

#### B.4.113 `FXTRACT`: Extract Exponent and Significand

    FXTRACT                       ; D9 F4                [8086,FPU]

`FXTRACT` separates the number in `ST0` into its exponent and significand (mantissa), stores the exponent back into `ST0`, and then pushes the significand on the register stack (so that the significand ends up in `ST0`, and the exponent in `ST1`).

#### B.4.114 `FYL2X`, `FYL2XP1`: Compute Y times Log2(X) or Log2(X+1)

    FYL2X                         ; D9 F1                [8086,FPU]
    FYL2XP1                       ; D9 F9                [8086,FPU]

`FYL2X` multiplies `ST1` by the base-2 logarithm of `ST0`, stores the result in `ST1`, and pops the register stack (so that the result ends up in `ST0`). `ST0` must be non-zero and positive.

`FYL2XP1` works the same way, but replacing the base-2 log of `ST0` with that of `ST0` plus one. This time, `ST0` must have magnitude no greater than 1 minus half the square root of two.

#### B.4.115 `HLT`: Halt Processor

    HLT                           ; F4                   [8086,PRIV]

`HLT` puts the processor into a halted state, where it will perform no more operations until restarted by an interrupt or a reset.

On the 286 and later processors, this is a privileged instruction.

#### B.4.116 `IBTS`: Insert Bit String

    IBTS r/m16,reg16              ; o16 0F A7 /r         [386,UNDOC]
    IBTS r/m32,reg32              ; o32 0F A7 /r         [386,UNDOC]

The implied operation of this instruction is:

    IBTS r/m16,AX,CL,reg16
    IBTS r/m32,EAX,CL,reg32

Writes a bit string from the source operand to the destination. `CL` indicates the number of bits to be copied, from the low bits of the source. `(E)AX` indicates the low order bit offset in the destination that is written to. For example, if `CL` is set to 4 and `AX` (for 16-bit code) is set to 5, bits 0-3 of `src` will be copied to bits 5-8 of `dst`. This instruction is very poorly documented, and I have been unable to find any official source of documentation on it.

`IBTS` is supported only on the early Intel 386s, and conflicts with the opcodes for `CMPXCHG486` (on early Intel 486s). NASM supports it only for completeness. Its counterpart is `XBTS` (see section B.4.332).

#### B.4.117 `IDIV`: Signed Integer Divide

    IDIV r/m8                     ; F6 /7                [8086]
    IDIV r/m16                    ; o16 F7 /7            [8086]
    IDIV r/m32                    ; o32 F7 /7            [386]

`IDIV` performs signed integer division. The explicit operand provided is the divisor; the dividend and destination operands are implicit, in the following way:

- For `IDIV r/m8`, `AX` is divided by the given operand; the quotient is stored in `AL` and the remainder in `AH`.
- For `IDIV r/m16`, `DX:AX` is divided by the given operand; the quotient is stored in `AX` and the remainder in `DX`.
- For `IDIV r/m32`, `EDX:EAX` is divided by the given operand; the quotient is stored in `EAX` and the remainder in `EDX`.

Unsigned integer division is performed by the `DIV` instruction: see section B.4.59.

#### B.4.118 `IMUL`: Signed Integer Multiply

    IMUL r/m8                     ; F6 /5                [8086]
    IMUL r/m16                    ; o16 F7 /5            [8086]
    IMUL r/m32                    ; o32 F7 /5            [386]

    IMUL reg16,r/m16              ; o16 0F AF /r         [386]
    IMUL reg32,r/m32              ; o32 0F AF /r         [386]

    IMUL reg16,imm8               ; o16 6B /r ib         [186]
    IMUL reg16,imm16              ; o16 69 /r iw         [186]
    IMUL reg32,imm8               ; o32 6B /r ib         [386]
    IMUL reg32,imm32              ; o32 69 /r id         [386]

    IMUL reg16,r/m16,imm8         ; o16 6B /r ib         [186]
    IMUL reg16,r/m16,imm16        ; o16 69 /r iw         [186]
    IMUL reg32,r/m32,imm8         ; o32 6B /r ib         [386]
    IMUL reg32,r/m32,imm32        ; o32 69 /r id         [386]

`IMUL` performs signed integer multiplication. For the single-operand form, the other operand and destination are implicit, in the following way:

- For `IMUL r/m8`, `AL` is multiplied by the given operand; the product is stored in `AX`.
- For `IMUL r/m16`, `AX` is multiplied by the given operand; the product is stored in `DX:AX`.
- For `IMUL r/m32`, `EAX` is multiplied by the given operand; the product is stored in `EDX:EAX`.

The two-operand form multiplies its two operands and stores the result in the destination (first) operand. The three-operand form multiplies its last two operands and stores the result in the first operand.

The two-operand form with an immediate second operand is in fact a shorthand for the three-operand form, as can be seen by examining the opcode descriptions: in the two-operand form, the code `/r` takes both its register and `r/m` parts from the same operand (the first one).

In the forms with an 8-bit immediate operand and another longer source operand, the immediate operand is considered to be signed, and is sign-extended to the length of the other source operand. In these cases, the `BYTE` qualifier is necessary to force NASM to generate this form of the instruction.

Unsigned integer multiplication is performed by the `MUL` instruction: see section B.4.184.

#### B.4.119 `IN`: Input from I/O Port

    IN AL,imm8                    ; E4 ib                [8086]
    IN AX,imm8                    ; o16 E5 ib            [8086]
    IN EAX,imm8                   ; o32 E5 ib            [386]
    IN AL,DX                      ; EC                   [8086]
    IN AX,DX                      ; o16 ED               [8086]
    IN EAX,DX                     ; o32 ED               [386]

`IN` reads a byte, word or doubleword from the specified I/O port, and stores it in the given destination register. The port number may be specified as an immediate value if it is between 0 and 255, and otherwise must be stored in `DX`. See also `OUT` (section B.4.194).

#### B.4.120 `INC`: Increment Integer

    INC reg16                     ; o16 40+r             [8086]
    INC reg32                     ; o32 40+r             [386]
    INC r/m8                      ; FE /0                [8086]
    INC r/m16                     ; o16 FF /0            [8086]
    INC r/m32                     ; o32 FF /0            [386]

`INC` adds 1 to its operand. It does not affect the carry flag: to affect the carry flag, use `ADD something,1` (see section B.4.3). `INC` affects all the other flags according to the result.

This instruction can be used with a `LOCK` prefix to allow atomic execution.

See also `DEC` (section B.4.58).

#### B.4.121 `INSB`, `INSW`, `INSD`: Input String from I/O Port

    INSB                          ; 6C                   [186]
    INSW                          ; o16 6D               [186]
    INSD                          ; o32 6D               [386]

`INSB` inputs a byte from the I/O port specified in `DX` and stores it at `[ES:DI]` or `[ES:EDI]`. It then increments or decrements (depending on the direction flag: increments if the flag is clear, decrements if it is set) `DI` or `EDI`.

The register used is `DI` if the address size is 16 bits, and `EDI` if it is 32 bits. If you need to use an address size not equal to the current `BITS` setting, you can use an explicit `a16` or `a32` prefix.

Segment override prefixes have no effect for this instruction: the use of `ES` for the load from `[DI]` or `[EDI]` cannot be overridden.

`INSW` and `INSD` work in the same way, but they input a word or a doubleword instead of a byte, and increment or decrement the addressing register by 2 or 4 instead of 1.

The `REP` prefix may be used to repeat the instruction `CX` (or `ECX` - again, the address size chooses which) times.

See also `OUTSB`, `OUTSW` and `OUTSD` (section B.4.195).

#### B.4.122 `INT`: Software Interrupt

    INT imm8                      ; CD ib                [8086]

`INT` causes a software interrupt through a specified vector number from 0 to 255.

The code generated by the `INT` instruction is always two bytes long: although there are short forms for some `INT` instructions, NASM does not generate them when it sees the `INT` mnemonic. In order to generate single-byte breakpoint instructions, use the `INT3` or `INT1` instructions (see section B.4.123) instead.

#### B.4.123 `INT3`, `INT1`, `ICEBP`, `INT01`: Breakpoints

    INT1                          ; F1                   [P6]
    ICEBP                         ; F1                   [P6]
    INT01                         ; F1                   [P6]

    INT3                          ; CC                   [8086]
    INT03                         ; CC                   [8086]

`INT1` and `INT3` are short one-byte forms of the instructions `INT 1` and `INT 3` (see section B.4.122). They perform a similar function to their longer counterparts, but take up less code space. They are used as breakpoints by debuggers.

- `INT1`, and its alternative synonyms `INT01` and `ICEBP`, is an instruction used by in-circuit emulators (ICEs). It is present, though not documented, on some processors down to the 286, but is only documented for the Pentium Pro. `INT3` is the instruction normally used as a breakpoint by debuggers.
- `INT3`, and its synonym `INT03`, is not precisely equivalent to `INT 3`: the short form, since it is designed to be used as a breakpoint, bypasses the normal `IOPL` checks in virtual-8086 mode, and also does not go through interrupt redirection.

#### B.4.124 `INTO`: Interrupt if Overflow

    INTO                          ; CE                   [8086]

`INTO` performs an `INT 4` software interrupt (see section B.4.122) if and only if the overflow flag is set.

#### B.4.125 `INVD`: Invalidate Internal Caches

    INVD                          ; 0F 08                [486]

`INVD` invalidates and empties the processor's internal caches, and causes the processor to instruct external caches to do the same. It does not write the contents of the caches back to memory first: any modified data held in the caches will be lost. To write the data back first, use `WBINVD` (section B.4.328).

#### B.4.126 `INVLPG`: Invalidate TLB Entry

    INVLPG mem                    ; 0F 01 /7             [486]

`INVLPG` invalidates the translation lookahead buffer (TLB) entry associated with the supplied memory address.

#### B.4.127 `IRET`, `IRETW`, `IRETD`: Return from Interrupt

    IRET                          ; CF                   [8086]
    IRETW                         ; o16 CF               [8086]
    IRETD                         ; o32 CF               [386]

`IRET` returns from an interrupt (hardware or software) by means of popping `IP` (or `EIP`), `CS` and the flags off the stack and then continuing execution from the new `CS:IP`.

`IRETW` pops `IP`, `CS` and the flags as 2 bytes each, taking 6 bytes off the stack in total. `IRETD` pops `EIP` as 4 bytes, pops a further 4 bytes of which the top two are discarded and the bottom two go into `CS`, and pops the flags as 4 bytes as well, taking 12 bytes off the stack.

`IRET` is a shorthand for either `IRETW` or `IRETD`, depending on the default `BITS` setting at the time.

#### B.4.128 `Jcc`: Conditional Branch

    Jcc imm                       ; 70+cc rb             [8086]
    Jcc NEAR imm                  ; 0F 80+cc rw/rd       [386]

The conditional jump instructions execute a near (same segment) jump if and only if their conditions are satisfied. For example, `JNZ` jumps only if the zero flag is not set.

The ordinary form of the instructions has only a 128-byte range; the `NEAR` form is a 386 extension to the instruction set, and can span the full size of a segment. NASM will not override your choice of jump instruction: if you want `Jcc NEAR`, you have to use the `NEAR` keyword.

The `SHORT` keyword is allowed on the first form of the instruction, for clarity, but is not necessary.

For details of the condition codes, see section B.2.2.

#### B.4.129 `JCXZ`, `JECXZ`: Jump if CX/ECX Zero

    JCXZ imm                      ; a16 E3 rb            [8086]
    JECXZ imm                     ; a32 E3 rb            [386]

`JCXZ` performs a short jump (with maximum range 128 bytes) if and only if the contents of the `CX` register is 0. `JECXZ` does the same thing, but with `ECX`.

#### B.4.130 `JMP`: Jump

    JMP imm                       ; E9 rw/rd             [8086]
    JMP SHORT imm                 ; EB rb                [8086]
    JMP imm:imm16                 ; o16 EA iw iw         [8086]
    JMP imm:imm32                 ; o32 EA id iw         [386]
    JMP FAR mem                   ; o16 FF /5            [8086]
    JMP FAR mem32                 ; o32 FF /5            [386]
    JMP r/m16                     ; o16 FF /4            [8086]
    JMP r/m32                     ; o32 FF /4            [386]

`JMP` jumps to a given address. The address may be specified as an absolute segment and offset, or as a relative jump within the current segment.

`JMP SHORT imm` has a maximum range of 128 bytes, since the displacement is specified as only 8 bits, but takes up less code space. NASM does not choose when to generate `JMP SHORT` for you: you must explicitly code `SHORT` every time you want a short jump.

You can choose between the two immediate far jump forms (`JMP imm:imm`) by the use of the `WORD` and `DWORD` keywords: `JMP WORD 0x1234:0x5678`) or `JMP DWORD 0x1234:0x56789abc`.

The `JMP FAR mem` forms execute a far jump by loading the destination address out of memory. The address loaded consists of 16 or 32 bits of offset (depending on the operand size), and 16 bits of segment. The operand size may be overridden using `JMP WORD FAR mem` or `JMP DWORD FAR mem`.

The `JMP r/m` forms execute a near jump (within the same segment), loading the destination address out of memory or out of a register. The keyword `NEAR` may be specified, for clarity, in these forms, but is not necessary. Again, operand size can be overridden using `JMP WORD mem` or `JMP DWORD mem`.

As a convenience, NASM does not require you to jump to a far symbol by coding the cumbersome `JMP SEG routine:routine`, but instead allows the easier synonym `JMP FAR routine`.

The `CALL r/m` forms given above are near calls; NASM will accept the `NEAR` keyword (e.g. `CALL NEAR [address]`), even though it is not strictly necessary.

#### B.4.131 `LAHF`: Load AH from Flags

    LAHF                          ; 9F                   [8086]

`LAHF` sets the `AH` register according to the contents of the low byte of the flags word.

The operation of `LAHF` is:

     AH <-- SF:ZF:0:AF:0:PF:1:CF

See also `SAHF` (section B.4.282).

#### B.4.132 `LAR`: Load Access Rights

    LAR reg16,r/m16               ; o16 0F 02 /r         [286,PRIV]
    LAR reg32,r/m32               ; o32 0F 02 /r         [286,PRIV]

`LAR` takes the segment selector specified by its source (second) operand, finds the corresponding segment descriptor in the GDT or LDT, and loads the access-rights byte of the descriptor into its destination (first) operand.

#### B.4.133 `LDMXCSR`: Load Streaming SIMD Extension Control/Status

    LDMXCSR mem32                 ; 0F AE /2        [KATMAI,SSE]

`LDMXCSR` loads 32-bits of data from the specified memory location into the `MXCSR` control/status register. `MXCSR` is used to enable masked/unmasked exception handling, to set rounding modes, to set flush-to-zero mode, and to view exception status flags.

For details of the `MXCSR` register, see the Intel processor docs.

See also `STMXCSR` (section B.4.302

#### B.4.134 `LDS`, `LES`, `LFS`, `LGS`, `LSS`: Load Far Pointer

    LDS reg16,mem                 ; o16 C5 /r            [8086]
    LDS reg32,mem                 ; o32 C5 /r            [386]

    LES reg16,mem                 ; o16 C4 /r            [8086]
    LES reg32,mem                 ; o32 C4 /r            [386]

    LFS reg16,mem                 ; o16 0F B4 /r         [386]
    LFS reg32,mem                 ; o32 0F B4 /r         [386]

    LGS reg16,mem                 ; o16 0F B5 /r         [386]
    LGS reg32,mem                 ; o32 0F B5 /r         [386]

    LSS reg16,mem                 ; o16 0F B2 /r         [386]
    LSS reg32,mem                 ; o32 0F B2 /r         [386]

These instructions load an entire far pointer (16 or 32 bits of offset, plus 16 bits of segment) out of memory in one go. `LDS`, for example, loads 16 or 32 bits from the given memory address into the given register (depending on the size of the register), then loads the next 16 bits from memory into `DS`. `LES`, `LFS`, `LGS` and `LSS` work in the same way but use the other segment registers.

#### B.4.135 `LEA`: Load Effective Address

    LEA reg16,mem                 ; o16 8D /r            [8086]
    LEA reg32,mem                 ; o32 8D /r            [386]

`LEA`, despite its syntax, does not access memory. It calculates the effective address specified by its second operand as if it were going to load or store data from it, but instead it stores the calculated address into the register specified by its first operand. This can be used to perform quite complex calculations (e.g. `LEA EAX,[EBX+ECX*4+100]`) in one instruction.

`LEA`, despite being a purely arithmetic instruction which accesses no memory, still requires square brackets around its second operand, as if it were a memory reference.

The size of the calculation is the current address size, and the size that the result is stored as is the current operand size. If the address and operand size are not the same, then if the addressing mode was 32-bits, the low 16-bits are stored, and if the address was 16-bits, it is zero-extended to 32-bits before storing.

#### B.4.136 `LEAVE`: Destroy Stack Frame

    LEAVE                         ; C9                   [186]

`LEAVE` destroys a stack frame of the form created by the `ENTER` instruction (see section B.4.65). It is functionally equivalent to `MOV ESP,EBP` followed by `POP EBP` (or `MOV SP,BP` followed by `POP BP` in 16-bit mode).

#### B.4.137 `LFENCE`: Load Fence

    LFENCE                        ; 0F AE /5        [WILLAMETTE,SSE2]

`LFENCE` performs a serialising operation on all loads from memory that were issued before the `LFENCE` instruction. This guarantees that all memory reads before the `LFENCE` instruction are visible before any reads after the `LFENCE` instruction.

`LFENCE` is ordered respective to other `LFENCE` instruction, `MFENCE`, any memory read and any other serialising instruction (such as `CPUID`).

Weakly ordered memory types can be used to achieve higher processor performance through such techniques as out-of-order issue and speculative reads. The degree to which a consumer of data recognizes or knows that the data is weakly ordered varies among applications and may be unknown to the producer of this data. The `LFENCE` instruction provides a performance-efficient way of ensuring load ordering between routines that produce weakly-ordered results and routines that consume that data.

`LFENCE` uses the following ModRM encoding:

              Mod (7:6)        = 11B
              Reg/Opcode (5:3) = 101B
              R/M (2:0)        = 000B

All other ModRM encodings are defined to be reserved, and use of these encodings risks incompatibility with future processors.

See also `SFENCE` (section B.4.288) and `MFENCE` (section B.4.151).

#### B.4.138 `LGDT`, `LIDT`, `LLDT`: Load Descriptor Tables

    LGDT mem                      ; 0F 01 /2             [286,PRIV]
    LIDT mem                      ; 0F 01 /3             [286,PRIV]
    LLDT r/m16                    ; 0F 00 /2             [286,PRIV]

`LGDT` and `LIDT` both take a 6-byte memory area as an operand: they load a 32-bit linear address and a 16-bit size limit from that area (in the opposite order) into the `GDTR` (global descriptor table register) or `IDTR` (interrupt descriptor table register). These are the only instructions which directly use linear addresses, rather than segment/offset pairs.

`LLDT` takes a segment selector as an operand. The processor looks up that selector in the GDT and stores the limit and base address given there into the `LDTR` (local descriptor table register).

See also `SGDT`, `SIDT` and `SLDT` (section B.4.289).

#### B.4.139 `LMSW`: Load/Store Machine Status Word

    LMSW r/m16                    ; 0F 01 /6             [286,PRIV]

`LMSW` loads the bottom four bits of the source operand into the bottom four bits of the `CR0` control register (or the Machine Status Word, on 286 processors). See also `SMSW` (section B.4.296).

#### B.4.140 `LOADALL`, `LOADALL286`: Load Processor State

    LOADALL                       ; 0F 07                [386,UNDOC]
    LOADALL286                    ; 0F 05                [286,UNDOC]

This instruction, in its two different-opcode forms, is apparently supported on most 286 processors, some 386 and possibly some 486. The opcode differs between the 286 and the 386.

The function of the instruction is to load all information relating to the state of the processor out of a block of memory: on the 286, this block is located implicitly at absolute address `0x800`, and on the 386 and 486 it is at `[ES:EDI]`.

#### B.4.141 `LODSB`, `LODSW`, `LODSD`: Load from String

    LODSB                         ; AC                   [8086]
    LODSW                         ; o16 AD               [8086]
    LODSD                         ; o32 AD               [386]

`LODSB` loads a byte from `[DS:SI]` or `[DS:ESI]` into `AL`. It then increments or decrements (depending on the direction flag: increments if the flag is clear, decrements if it is set) `SI` or `ESI`.

The register used is `SI` if the address size is 16 bits, and `ESI` if it is 32 bits. If you need to use an address size not equal to the current `BITS` setting, you can use an explicit `a16` or `a32` prefix.

The segment register used to load from `[SI]` or `[ESI]` can be overridden by using a segment register name as a prefix (for example, `ES LODSB`).

`LODSW` and `LODSD` work in the same way, but they load a word or a doubleword instead of a byte, and increment or decrement the addressing registers by 2 or 4 instead of 1.

#### B.4.142 `LOOP`, `LOOPE`, `LOOPZ`, `LOOPNE`, `LOOPNZ`: Loop with Counter

    LOOP imm                      ; E2 rb                [8086]
    LOOP imm,CX                   ; a16 E2 rb            [8086]
    LOOP imm,ECX                  ; a32 E2 rb            [386]

    LOOPE imm                     ; E1 rb                [8086]
    LOOPE imm,CX                  ; a16 E1 rb            [8086]
    LOOPE imm,ECX                 ; a32 E1 rb            [386]
    LOOPZ imm                     ; E1 rb                [8086]
    LOOPZ imm,CX                  ; a16 E1 rb            [8086]
    LOOPZ imm,ECX                 ; a32 E1 rb            [386]

    LOOPNE imm                    ; E0 rb                [8086]
    LOOPNE imm,CX                 ; a16 E0 rb            [8086]
    LOOPNE imm,ECX                ; a32 E0 rb            [386]
    LOOPNZ imm                    ; E0 rb                [8086]
    LOOPNZ imm,CX                 ; a16 E0 rb            [8086]
    LOOPNZ imm,ECX                ; a32 E0 rb            [386]

`LOOP` decrements its counter register (either `CX` or `ECX` - if one is not specified explicitly, the `BITS` setting dictates which is used) by one, and if the counter does not become zero as a result of this operation, it jumps to the given label. The jump has a range of 128 bytes.

`LOOPE` (or its synonym `LOOPZ`) adds the additional condition that it only jumps if the counter is nonzero and the zero flag is set. Similarly, `LOOPNE` (and `LOOPNZ`) jumps only if the counter is nonzero and the zero flag is clear.

#### B.4.143 `LSL`: Load Segment Limit

    LSL reg16,r/m16               ; o16 0F 03 /r         [286,PRIV]
    LSL reg32,r/m32               ; o32 0F 03 /r         [286,PRIV]

`LSL` is given a segment selector in its source (second) operand; it computes the segment limit value by loading the segment limit field from the associated segment descriptor in the `GDT` or `LDT`. (This involves shifting left by 12 bits if the segment limit is page-granular, and not if it is byte-granular; so you end up with a byte limit in either case.) The segment limit obtained is then loaded into the destination (first) operand.

#### B.4.144 `LTR`: Load Task Register

    LTR r/m16                     ; 0F 00 /3             [286,PRIV]

`LTR` looks up the segment base and limit in the GDT or LDT descriptor specified by the segment selector given as its operand, and loads them into the Task Register.

#### B.4.145 `MASKMOVDQU`: Byte Mask Write

    MASKMOVDQU xmm1,xmm2          ; 66 0F F7 /r     [WILLAMETTE,SSE2]

`MASKMOVDQU` stores data from xmm1 to the location specified by `ES:(E)DI`. The size of the store depends on the address-size attribute. The most significant bit in each byte of the mask register xmm2 is used to selectively write the data (0 = no write, 1 = write) on a per-byte basis.

#### B.4.146 `MASKMOVQ`: Byte Mask Write

    MASKMOVQ mm1,mm2              ; 0F F7 /r        [KATMAI,MMX]

`MASKMOVQ` stores data from mm1 to the location specified by `ES:(E)DI`. The size of the store depends on the address-size attribute. The most significant bit in each byte of the mask register mm2 is used to selectively write the data (0 = no write, 1 = write) on a per-byte basis.

#### B.4.147 `MAXPD`: Return Packed Double-Precision FP Maximum

    MAXPD xmm1,xmm2/m128          ; 66 0F 5F /r     [WILLAMETTE,SSE2]

`MAXPD` performs a SIMD compare of the packed double-precision FP numbers from xmm1 and xmm2/mem, and stores the maximum values of each pair of values in xmm1. If the values being compared are both zeroes, source2 (xmm2/m128) would be returned. If source2 (xmm2/m128) is an SNaN, this SNaN is forwarded unchanged to the destination (i.e., a QNaN version of the SNaN is not returned).

#### B.4.148 `MAXPS`: Return Packed Single-Precision FP Maximum

    MAXPS xmm1,xmm2/m128          ; 0F 5F /r        [KATMAI,SSE]

`MAXPS` performs a SIMD compare of the packed single-precision FP numbers from xmm1 and xmm2/mem, and stores the maximum values of each pair of values in xmm1. If the values being compared are both zeroes, source2 (xmm2/m128) would be returned. If source2 (xmm2/m128) is an SNaN, this SNaN is forwarded unchanged to the destination (i.e., a QNaN version of the SNaN is not returned).

#### B.4.149 `MAXSD`: Return Scalar Double-Precision FP Maximum

    MAXSD xmm1,xmm2/m64           ; F2 0F 5F /r     [WILLAMETTE,SSE2]

`MAXSD` compares the low-order double-precision FP numbers from xmm1 and xmm2/mem, and stores the maximum value in xmm1. If the values being compared are both zeroes, source2 (xmm2/m64) would be returned. If source2 (xmm2/m64) is an SNaN, this SNaN is forwarded unchanged to the destination (i.e., a QNaN version of the SNaN is not returned). The high quadword of the destination is left unchanged.

#### B.4.150 `MAXSS`: Return Scalar Single-Precision FP Maximum

    MAXSS xmm1,xmm2/m32           ; F3 0F 5F /r     [KATMAI,SSE]

`MAXSS` compares the low-order single-precision FP numbers from xmm1 and xmm2/mem, and stores the maximum value in xmm1. If the values being compared are both zeroes, source2 (xmm2/m32) would be returned. If source2 (xmm2/m32) is an SNaN, this SNaN is forwarded unchanged to the destination (i.e., a QNaN version of the SNaN is not returned). The high three doublewords of the destination are left unchanged.

#### B.4.151 `MFENCE`: Memory Fence

    MFENCE                        ; 0F AE /6        [WILLAMETTE,SSE2]

`MFENCE` performs a serialising operation on all loads from memory and writes to memory that were issued before the `MFENCE` instruction. This guarantees that all memory reads and writes before the `MFENCE` instruction are completed before any reads and writes after the `MFENCE` instruction.

`MFENCE` is ordered respective to other `MFENCE` instructions, `LFENCE`, `SFENCE`, any memory read and any other serialising instruction (such as `CPUID`).

Weakly ordered memory types can be used to achieve higher processor performance through such techniques as out-of-order issue, speculative reads, write-combining, and write-collapsing. The degree to which a consumer of data recognizes or knows that the data is weakly ordered varies among applications and may be unknown to the producer of this data. The `MFENCE` instruction provides a performance-efficient way of ensuring load and store ordering between routines that produce weakly-ordered results and routines that consume that data.

`MFENCE` uses the following ModRM encoding:

              Mod (7:6)        = 11B
              Reg/Opcode (5:3) = 110B
              R/M (2:0)        = 000B

All other ModRM encodings are defined to be reserved, and use of these encodings risks incompatibility with future processors.

See also `LFENCE` (section B.4.137) and `SFENCE` (section B.4.288).

#### B.4.152 `MINPD`: Return Packed Double-Precision FP Minimum

    MINPD xmm1,xmm2/m128          ; 66 0F 5D /r     [WILLAMETTE,SSE2]

`MINPD` performs a SIMD compare of the packed double-precision FP numbers from xmm1 and xmm2/mem, and stores the minimum values of each pair of values in xmm1. If the values being compared are both zeroes, source2 (xmm2/m128) would be returned. If source2 (xmm2/m128) is an SNaN, this SNaN is forwarded unchanged to the destination (i.e., a QNaN version of the SNaN is not returned).

#### B.4.153 `MINPS`: Return Packed Single-Precision FP Minimum

    MINPS xmm1,xmm2/m128          ; 0F 5D /r        [KATMAI,SSE]

`MINPS` performs a SIMD compare of the packed single-precision FP numbers from xmm1 and xmm2/mem, and stores the minimum values of each pair of values in xmm1. If the values being compared are both zeroes, source2 (xmm2/m128) would be returned. If source2 (xmm2/m128) is an SNaN, this SNaN is forwarded unchanged to the destination (i.e., a QNaN version of the SNaN is not returned).

#### B.4.154 `MINSD`: Return Scalar Double-Precision FP Minimum

    MINSD xmm1,xmm2/m64           ; F2 0F 5D /r     [WILLAMETTE,SSE2]

`MINSD` compares the low-order double-precision FP numbers from xmm1 and xmm2/mem, and stores the minimum value in xmm1. If the values being compared are both zeroes, source2 (xmm2/m64) would be returned. If source2 (xmm2/m64) is an SNaN, this SNaN is forwarded unchanged to the destination (i.e., a QNaN version of the SNaN is not returned). The high quadword of the destination is left unchanged.

#### B.4.155 `MINSS`: Return Scalar Single-Precision FP Minimum

    MINSS xmm1,xmm2/m32           ; F3 0F 5D /r     [KATMAI,SSE]

`MINSS` compares the low-order single-precision FP numbers from xmm1 and xmm2/mem, and stores the minimum value in xmm1. If the values being compared are both zeroes, source2 (xmm2/m32) would be returned. If source2 (xmm2/m32) is an SNaN, this SNaN is forwarded unchanged to the destination (i.e., a QNaN version of the SNaN is not returned). The high three doublewords of the destination are left unchanged.

#### B.4.156 `MOV`: Move Data

    MOV r/m8,reg8                 ; 88 /r                [8086]
    MOV r/m16,reg16               ; o16 89 /r            [8086]
    MOV r/m32,reg32               ; o32 89 /r            [386]
    MOV reg8,r/m8                 ; 8A /r                [8086]
    MOV reg16,r/m16               ; o16 8B /r            [8086]
    MOV reg32,r/m32               ; o32 8B /r            [386]

    MOV reg8,imm8                 ; B0+r ib              [8086]
    MOV reg16,imm16               ; o16 B8+r iw          [8086]
    MOV reg32,imm32               ; o32 B8+r id          [386]
    MOV r/m8,imm8                 ; C6 /0 ib             [8086]
    MOV r/m16,imm16               ; o16 C7 /0 iw         [8086]
    MOV r/m32,imm32               ; o32 C7 /0 id         [386]

    MOV AL,memoffs8               ; A0 ow/od             [8086]
    MOV AX,memoffs16              ; o16 A1 ow/od         [8086]
    MOV EAX,memoffs32             ; o32 A1 ow/od         [386]
    MOV memoffs8,AL               ; A2 ow/od             [8086]
    MOV memoffs16,AX              ; o16 A3 ow/od         [8086]
    MOV memoffs32,EAX             ; o32 A3 ow/od         [386]

    MOV r/m16,segreg              ; o16 8C /r            [8086]
    MOV r/m32,segreg              ; o32 8C /r            [386]
    MOV segreg,r/m16              ; o16 8E /r            [8086]
    MOV segreg,r/m32              ; o32 8E /r            [386]

    MOV reg32,CR0/2/3/4           ; 0F 20 /r             [386]
    MOV reg32,DR0/1/2/3/6/7       ; 0F 21 /r             [386]
    MOV reg32,TR3/4/5/6/7         ; 0F 24 /r             [386]
    MOV CR0/2/3/4,reg32           ; 0F 22 /r             [386]
    MOV DR0/1/2/3/6/7,reg32       ; 0F 23 /r             [386]
    MOV TR3/4/5/6/7,reg32         ; 0F 26 /r             [386]

`MOV` copies the contents of its source (second) operand into its destination (first) operand.

In all forms of the `MOV` instruction, the two operands are the same size, except for moving between a segment register and an `r/m32` operand. These instructions are treated exactly like the corresponding 16-bit equivalent (so that, for example, `MOV DS,EAX` functions identically to `MOV DS,AX` but saves a prefix when in 32-bit mode), except that when a segment register is moved into a 32-bit destination, the top two bytes of the result are undefined.

`MOV` may not use `CS` as a destination.

`CR4` is only a supported register on the Pentium and above.

Test registers are supported on 386/486 processors and on some non-Intel Pentium class processors.

#### B.4.157 `MOVAPD`: Move Aligned Packed Double-Precision FP Values

    MOVAPD xmm1,xmm2/mem128       ; 66 0F 28 /r     [WILLAMETTE,SSE2]
    MOVAPD xmm1/mem128,xmm2       ; 66 0F 29 /r     [WILLAMETTE,SSE2]

`MOVAPD` moves a double quadword containing 2 packed double-precision FP values from the source operand to the destination. When the source or destination operand is a memory location, it must be aligned on a 16-byte boundary.

To move data in and out of memory locations that are not known to be on 16-byte boundaries, use the `MOVUPD` instruction (section B.4.182).

#### B.4.158 `MOVAPS`: Move Aligned Packed Single-Precision FP Values

    MOVAPS xmm1,xmm2/mem128       ; 0F 28 /r        [KATMAI,SSE]
    MOVAPS xmm1/mem128,xmm2       ; 0F 29 /r        [KATMAI,SSE]

`MOVAPS` moves a double quadword containing 4 packed single-precision FP values from the source operand to the destination. When the source or destination operand is a memory location, it must be aligned on a 16-byte boundary.

To move data in and out of memory locations that are not known to be on 16-byte boundaries, use the `MOVUPS` instruction (section B.4.183).

#### B.4.159 `MOVD`: Move Doubleword to/from MMX Register

    MOVD mm,r/m32                 ; 0F 6E /r             [PENT,MMX]
    MOVD r/m32,mm                 ; 0F 7E /r             [PENT,MMX]
    MOVD xmm,r/m32                ; 66 0F 6E /r     [WILLAMETTE,SSE2]
    MOVD r/m32,xmm                ; 66 0F 7E /r     [WILLAMETTE,SSE2]

`MOVD` copies 32 bits from its source (second) operand into its destination (first) operand. When the destination is a 64-bit `MMX` register or a 128-bit `XMM` register, the input value is zero-extended to fill the destination register.

#### B.4.160 `MOVDQ2Q`: Move Quadword from XMM to MMX register.

    MOVDQ2Q mm,xmm                ; F2 OF D6 /r     [WILLAMETTE,SSE2]

`MOVDQ2Q` moves the low quadword from the source operand to the destination operand.

#### B.4.161 `MOVDQA`: Move Aligned Double Quadword

    MOVDQA xmm1,xmm2/m128         ; 66 OF 6F /r     [WILLAMETTE,SSE2]
    MOVDQA xmm1/m128,xmm2         ; 66 OF 7F /r     [WILLAMETTE,SSE2]

`MOVDQA` moves a double quadword from the source operand to the destination operand. When the source or destination operand is a memory location, it must be aligned to a 16-byte boundary.

To move a double quadword to or from unaligned memory locations, use the `MOVDQU` instruction (section B.4.162).

#### B.4.162 `MOVDQU`: Move Unaligned Double Quadword

    MOVDQU xmm1,xmm2/m128         ; F3 OF 6F /r     [WILLAMETTE,SSE2]
    MOVDQU xmm1/m128,xmm2         ; F3 OF 7F /r     [WILLAMETTE,SSE2]

`MOVDQU` moves a double quadword from the source operand to the destination operand. When the source or destination operand is a memory location, the memory may be unaligned.

To move a double quadword to or from known aligned memory locations, use the `MOVDQA` instruction (section B.4.161).

#### B.4.163 `MOVHLPS`: Move Packed Single-Precision FP High to Low

    MOVHLPS xmm1,xmm2             ; OF 12 /r        [KATMAI,SSE]

`MOVHLPS` moves the two packed single-precision FP values from the high quadword of the source register xmm2 to the low quadword of the destination register, xmm2. The upper quadword of xmm1 is left unchanged.

The operation of this instruction is:

       dst[0-63]   := src[64-127],
       dst[64-127] remains unchanged.

#### B.4.164 `MOVHPD`: Move High Packed Double-Precision FP

    MOVHPD xmm,m64               ; 66 OF 16 /r      [WILLAMETTE,SSE2]
    MOVHPD m64,xmm               ; 66 OF 17 /r      [WILLAMETTE,SSE2]

`MOVHPD` moves a double-precision FP value between the source and destination operands. One of the operands is a 64-bit memory location, the other is the high quadword of an `XMM` register.

The operation of this instruction is:

       mem[0-63]   := xmm[64-127];

or

       xmm[0-63]   remains unchanged;
       xmm[64-127] := mem[0-63].

#### B.4.165 `MOVHPS`: Move High Packed Single-Precision FP

    MOVHPS xmm,m64               ; 0F 16 /r         [KATMAI,SSE]
    MOVHPS m64,xmm               ; 0F 17 /r         [KATMAI,SSE]

`MOVHPS` moves two packed single-precision FP values between the source and destination operands. One of the operands is a 64-bit memory location, the other is the high quadword of an `XMM` register.

The operation of this instruction is:

       mem[0-63]   := xmm[64-127];

or

       xmm[0-63]   remains unchanged;
       xmm[64-127] := mem[0-63].

#### B.4.166 `MOVLHPS`: Move Packed Single-Precision FP Low to High

    MOVLHPS xmm1,xmm2             ; OF 16 /r         [KATMAI,SSE]

`MOVLHPS` moves the two packed single-precision FP values from the low quadword of the source register xmm2 to the high quadword of the destination register, xmm2. The low quadword of xmm1 is left unchanged.

The operation of this instruction is:

       dst[0-63]   remains unchanged;
       dst[64-127] := src[0-63].

#### B.4.167 `MOVLPD`: Move Low Packed Double-Precision FP

    MOVLPD xmm,m64                ; 66 OF 12 /r     [WILLAMETTE,SSE2]
    MOVLPD m64,xmm                ; 66 OF 13 /r     [WILLAMETTE,SSE2]

`MOVLPD` moves a double-precision FP value between the source and destination operands. One of the operands is a 64-bit memory location, the other is the low quadword of an `XMM` register.

The operation of this instruction is:

       mem(0-63)   := xmm(0-63);

or

       xmm(0-63)   := mem(0-63);
       xmm(64-127) remains unchanged.

#### B.4.168 `MOVLPS`: Move Low Packed Single-Precision FP

    MOVLPS xmm,m64                ; OF 12 /r        [KATMAI,SSE]
    MOVLPS m64,xmm                ; OF 13 /r        [KATMAI,SSE]

`MOVLPS` moves two packed single-precision FP values between the source and destination operands. One of the operands is a 64-bit memory location, the other is the low quadword of an `XMM` register.

The operation of this instruction is:

       mem(0-63)   := xmm(0-63);

or

       xmm(0-63)   := mem(0-63);
       xmm(64-127) remains unchanged.

#### B.4.169 `MOVMSKPD`: Extract Packed Double-Precision FP Sign Mask

    MOVMSKPD reg32,xmm              ; 66 0F 50 /r   [WILLAMETTE,SSE2]

`MOVMSKPD` inserts a 2-bit mask in r32, formed of the most significant bits of each double-precision FP number of the source operand.

#### B.4.170 `MOVMSKPS`: Extract Packed Single-Precision FP Sign Mask

    MOVMSKPS reg32,xmm              ; 0F 50 /r      [KATMAI,SSE]

`MOVMSKPS` inserts a 4-bit mask in r32, formed of the most significant bits of each single-precision FP number of the source operand.

#### B.4.171 `MOVNTDQ`: Move Double Quadword Non Temporal

    MOVNTDQ m128,xmm              ; 66 0F E7 /r     [WILLAMETTE,SSE2]

`MOVNTDQ` moves the double quadword from the `XMM` source register to the destination memory location, using a non-temporal hint. This store instruction minimizes cache pollution.

#### B.4.172 `MOVNTI`: Move Doubleword Non Temporal

    MOVNTI m32,reg32              ; 0F C3 /r        [WILLAMETTE,SSE2]

`MOVNTI` moves the doubleword in the source register to the destination memory location, using a non-temporal hint. This store instruction minimizes cache pollution.

#### B.4.173 `MOVNTPD`: Move Aligned Four Packed Single-Precision FP Values Non Temporal

    MOVNTPD m128,xmm              ; 66 0F 2B /r     [WILLAMETTE,SSE2]

`MOVNTPD` moves the double quadword from the `XMM` source register to the destination memory location, using a non-temporal hint. This store instruction minimizes cache pollution. The memory location must be aligned to a 16-byte boundary.

#### B.4.174 `MOVNTPS`: Move Aligned Four Packed Single-Precision FP Values Non Temporal

    MOVNTPS m128,xmm              ; 0F 2B /r        [KATMAI,SSE]

`MOVNTPS` moves the double quadword from the `XMM` source register to the destination memory location, using a non-temporal hint. This store instruction minimizes cache pollution. The memory location must be aligned to a 16-byte boundary.

#### B.4.175 `MOVNTQ`: Move Quadword Non Temporal

    MOVNTQ m64,mm                 ; 0F E7 /r        [KATMAI,MMX]

`MOVNTQ` moves the quadword in the `MMX` source register to the destination memory location, using a non-temporal hint. This store instruction minimizes cache pollution.

#### B.4.176 `MOVQ`: Move Quadword to/from MMX Register

    MOVQ mm1,mm2/m64               ; 0F 6F /r             [PENT,MMX]
    MOVQ mm1/m64,mm2               ; 0F 7F /r             [PENT,MMX]

    MOVQ xmm1,xmm2/m64             ; F3 0F 7E /r    [WILLAMETTE,SSE2]
    MOVQ xmm1/m64,xmm2             ; 66 0F D6 /r    [WILLAMETTE,SSE2]

`MOVQ` copies 64 bits from its source (second) operand into its destination (first) operand. When the source is an `XMM` register, the low quadword is moved. When the destination is an `XMM` register, the destination is the low quadword, and the high quadword is cleared.

#### B.4.177 `MOVQ2DQ`: Move Quadword from MMX to XMM register.

    MOVQ2DQ xmm,mm                ; F3 OF D6 /r     [WILLAMETTE,SSE2]

`MOVQ2DQ` moves the quadword from the source operand to the low quadword of the destination operand, and clears the high quadword.

#### B.4.178 `MOVSB`, `MOVSW`, `MOVSD`: Move String

    MOVSB                         ; A4                   [8086]
    MOVSW                         ; o16 A5               [8086]
    MOVSD                         ; o32 A5               [386]

`MOVSB` copies the byte at `[DS:SI]` or `[DS:ESI]` to `[ES:DI]` or `[ES:EDI]`. It then increments or decrements (depending on the direction flag: increments if the flag is clear, decrements if it is set) `SI` and `DI` (or `ESI` and `EDI`).

The registers used are `SI` and `DI` if the address size is 16 bits, and `ESI` and `EDI` if it is 32 bits. If you need to use an address size not equal to the current `BITS` setting, you can use an explicit `a16` or `a32` prefix.

The segment register used to load from `[SI]` or `[ESI]` can be overridden by using a segment register name as a prefix (for example, `es movsb`). The use of `ES` for the store to `[DI]` or `[EDI]` cannot be overridden.

`MOVSW` and `MOVSD` work in the same way, but they copy a word or a doubleword instead of a byte, and increment or decrement the addressing registers by 2 or 4 instead of 1.

The `REP` prefix may be used to repeat the instruction `CX` (or `ECX` - again, the address size chooses which) times.

#### B.4.179 `MOVSD`: Move Scalar Double-Precision FP Value

    MOVSD xmm1,xmm2/m64           ; F2 0F 10 /r     [WILLAMETTE,SSE2]
    MOVSD xmm1/m64,xmm2           ; F2 0F 11 /r     [WILLAMETTE,SSE2]

`MOVSD` moves a double-precision FP value from the source operand to the destination operand. When the source or destination is a register, the low-order FP value is read or written.

#### B.4.180 `MOVSS`: Move Scalar Single-Precision FP Value

    MOVSS xmm1,xmm2/m32           ; F3 0F 10 /r     [KATMAI,SSE]
    MOVSS xmm1/m32,xmm2           ; F3 0F 11 /r     [KATMAI,SSE]

`MOVSS` moves a single-precision FP value from the source operand to the destination operand. When the source or destination is a register, the low-order FP value is read or written.

#### B.4.181 `MOVSX`, `MOVZX`: Move Data with Sign or Zero Extend

    MOVSX reg16,r/m8              ; o16 0F BE /r         [386]
    MOVSX reg32,r/m8              ; o32 0F BE /r         [386]
    MOVSX reg32,r/m16             ; o32 0F BF /r         [386]

    MOVZX reg16,r/m8              ; o16 0F B6 /r         [386]
    MOVZX reg32,r/m8              ; o32 0F B6 /r         [386]
    MOVZX reg32,r/m16             ; o32 0F B7 /r         [386]

`MOVSX` sign-extends its source (second) operand to the length of its destination (first) operand, and copies the result into the destination operand. `MOVZX` does the same, but zero-extends rather than sign-extending.

#### B.4.182 `MOVUPD`: Move Unaligned Packed Double-Precision FP Values

    MOVUPD xmm1,xmm2/mem128       ; 66 0F 10 /r     [WILLAMETTE,SSE2]
    MOVUPD xmm1/mem128,xmm2       ; 66 0F 11 /r     [WILLAMETTE,SSE2]

`MOVUPD` moves a double quadword containing 2 packed double-precision FP values from the source operand to the destination. This instruction makes no assumptions about alignment of memory operands.

To move data in and out of memory locations that are known to be on 16-byte boundaries, use the `MOVAPD` instruction (section B.4.157).

#### B.4.183 `MOVUPS`: Move Unaligned Packed Single-Precision FP Values

    MOVUPS xmm1,xmm2/mem128       ; 0F 10 /r        [KATMAI,SSE]
    MOVUPS xmm1/mem128,xmm2       ; 0F 11 /r        [KATMAI,SSE]

`MOVUPS` moves a double quadword containing 4 packed single-precision FP values from the source operand to the destination. This instruction makes no assumptions about alignment of memory operands.

To move data in and out of memory locations that are known to be on 16-byte boundaries, use the `MOVAPS` instruction (section B.4.158).

#### B.4.184 `MUL`: Unsigned Integer Multiply

    MUL r/m8                      ; F6 /4                [8086]
    MUL r/m16                     ; o16 F7 /4            [8086]
    MUL r/m32                     ; o32 F7 /4            [386]

`MUL` performs unsigned integer multiplication. The other operand to the multiplication, and the destination operand, are implicit, in the following way:

- For `MUL r/m8`, `AL` is multiplied by the given operand; the product is stored in `AX`.
- For `MUL r/m16`, `AX` is multiplied by the given operand; the product is stored in `DX:AX`.
- For `MUL r/m32`, `EAX` is multiplied by the given operand; the product is stored in `EDX:EAX`.

Signed integer multiplication is performed by the `IMUL` instruction: see section B.4.118.

#### B.4.185 `MULPD`: Packed Single-FP Multiply

    MULPD xmm1,xmm2/mem128        ; 66 0F 59 /r     [WILLAMETTE,SSE2]

`MULPD` performs a SIMD multiply of the packed double-precision FP values in both operands, and stores the results in the destination register.

#### B.4.186 `MULPS`: Packed Single-FP Multiply

    MULPS xmm1,xmm2/mem128        ; 0F 59 /r        [KATMAI,SSE]

`MULPS` performs a SIMD multiply of the packed single-precision FP values in both operands, and stores the results in the destination register.

#### B.4.187 `MULSD`: Scalar Single-FP Multiply

    MULSD xmm1,xmm2/mem32         ; F2 0F 59 /r     [WILLAMETTE,SSE2]

`MULSD` multiplies the lowest double-precision FP values of both operands, and stores the result in the low quadword of xmm1.

#### B.4.188 `MULSS`: Scalar Single-FP Multiply

    MULSS xmm1,xmm2/mem32         ; F3 0F 59 /r     [KATMAI,SSE]

`MULSS` multiplies the lowest single-precision FP values of both operands, and stores the result in the low doubleword of xmm1.

#### B.4.189 `NEG`, `NOT`: Two's and One's Complement

    NEG r/m8                      ; F6 /3                [8086]
    NEG r/m16                     ; o16 F7 /3            [8086]
    NEG r/m32                     ; o32 F7 /3            [386]

    NOT r/m8                      ; F6 /2                [8086]
    NOT r/m16                     ; o16 F7 /2            [8086]
    NOT r/m32                     ; o32 F7 /2            [386]

`NEG` replaces the contents of its operand by the two's complement negation (invert all the bits and then add one) of the original value. `NOT`, similarly, performs one's complement (inverts all the bits).

#### B.4.190 `NOP`: No Operation

    NOP                           ; 90                   [8086]

`NOP` performs no operation. Its opcode is the same as that generated by `XCHG AX,AX` or `XCHG EAX,EAX` (depending on the processor mode; see section B.4.333).

#### B.4.191 `OR`: Bitwise OR

    OR r/m8,reg8                  ; 08 /r                [8086]
    OR r/m16,reg16                ; o16 09 /r            [8086]
    OR r/m32,reg32                ; o32 09 /r            [386]

    OR reg8,r/m8                  ; 0A /r                [8086]
    OR reg16,r/m16                ; o16 0B /r            [8086]
    OR reg32,r/m32                ; o32 0B /r            [386]

    OR r/m8,imm8                  ; 80 /1 ib             [8086]
    OR r/m16,imm16                ; o16 81 /1 iw         [8086]
    OR r/m32,imm32                ; o32 81 /1 id         [386]

    OR r/m16,imm8                 ; o16 83 /1 ib         [8086]
    OR r/m32,imm8                 ; o32 83 /1 ib         [386]

    OR AL,imm8                    ; 0C ib                [8086]
    OR AX,imm16                   ; o16 0D iw            [8086]
    OR EAX,imm32                  ; o32 0D id            [386]

`OR` performs a bitwise OR operation between its two operands (i.e. each bit of the result is 1 if and only if at least one of the corresponding bits of the two inputs was 1), and stores the result in the destination (first) operand.

In the forms with an 8-bit immediate second operand and a longer first operand, the second operand is considered to be signed, and is sign-extended to the length of the first operand. In these cases, the `BYTE` qualifier is necessary to force NASM to generate this form of the instruction.

The MMX instruction `POR` (see section B.4.247) performs the same operation on the 64-bit MMX registers.

#### B.4.192 `ORPD`: Bit-wise Logical OR of Double-Precision FP Data

    ORPD xmm1,xmm2/m128           ; 66 0F 56 /r     [WILLAMETTE,SSE2]

`ORPD` return a bit-wise logical OR between xmm1 and xmm2/mem, and stores the result in xmm1. If the source operand is a memory location, it must be aligned to a 16-byte boundary.

#### B.4.193 `ORPS`: Bit-wise Logical OR of Single-Precision FP Data

    ORPS xmm1,xmm2/m128           ; 0F 56 /r        [KATMAI,SSE]

`ORPS` return a bit-wise logical OR between xmm1 and xmm2/mem, and stores the result in xmm1. If the source operand is a memory location, it must be aligned to a 16-byte boundary.

#### B.4.194 `OUT`: Output Data to I/O Port

    OUT imm8,AL                   ; E6 ib                [8086]
    OUT imm8,AX                   ; o16 E7 ib            [8086]
    OUT imm8,EAX                  ; o32 E7 ib            [386]
    OUT DX,AL                     ; EE                   [8086]
    OUT DX,AX                     ; o16 EF               [8086]
    OUT DX,EAX                    ; o32 EF               [386]

`OUT` writes the contents of the given source register to the specified I/O port. The port number may be specified as an immediate value if it is between 0 and 255, and otherwise must be stored in `DX`. See also `IN` (section B.4.119).

#### B.4.195 `OUTSB`, `OUTSW`, `OUTSD`: Output String to I/O Port

    OUTSB                         ; 6E                   [186]
    OUTSW                         ; o16 6F               [186]
    OUTSD                         ; o32 6F               [386]

`OUTSB` loads a byte from `[DS:SI]` or `[DS:ESI]` and writes it to the I/O port specified in `DX`. It then increments or decrements (depending on the direction flag: increments if the flag is clear, decrements if it is set) `SI` or `ESI`.

The register used is `SI` if the address size is 16 bits, and `ESI` if it is 32 bits. If you need to use an address size not equal to the current `BITS` setting, you can use an explicit `a16` or `a32` prefix.

The segment register used to load from `[SI]` or `[ESI]` can be overridden by using a segment register name as a prefix (for example, `es outsb`).

`OUTSW` and `OUTSD` work in the same way, but they output a word or a doubleword instead of a byte, and increment or decrement the addressing registers by 2 or 4 instead of 1.

The `REP` prefix may be used to repeat the instruction `CX` (or `ECX` - again, the address size chooses which) times.

#### B.4.196 `PACKSSDW`, `PACKSSWB`, `PACKUSWB`: Pack Data

    PACKSSDW mm1,mm2/m64          ; 0F 6B /r             [PENT,MMX]
    PACKSSWB mm1,mm2/m64          ; 0F 63 /r             [PENT,MMX]
    PACKUSWB mm1,mm2/m64          ; 0F 67 /r             [PENT,MMX]

    PACKSSDW xmm1,xmm2/m128       ; 66 0F 6B /r     [WILLAMETTE,SSE2]
    PACKSSWB xmm1,xmm2/m128       ; 66 0F 63 /r     [WILLAMETTE,SSE2]
    PACKUSWB xmm1,xmm2/m128       ; 66 0F 67 /r     [WILLAMETTE,SSE2]

All these instructions start by combining the source and destination operands, and then splitting the result in smaller sections which it then packs into the destination register. The `MMX` versions pack two 64-bit operands into one 64-bit register, while the `SSE` versions pack two 128-bit operands into one 128-bit register.

- `PACKSSWB` splits the combined value into words, and then reduces the words to bytes, using signed saturation. It then packs the bytes into the destination register in the same order the words were in.
- `PACKSSDW` performs the same operation as `PACKSSWB`, except that it reduces doublewords to words, then packs them into the destination register.
- `PACKUSWB` performs the same operation as `PACKSSWB`, except that it uses unsigned saturation when reducing the size of the elements.

To perform signed saturation on a number, it is replaced by the largest signed number (`7FFFh` or `7Fh`) that will fit, and if it is too small it is replaced by the smallest signed number (`8000h` or `80h`) that will fit. To perform unsigned saturation, the input is treated as unsigned, and the input is replaced by the largest unsigned number that will fit.

#### B.4.197 `PADDB`, `PADDW`, `PADDD`: Add Packed Integers

    PADDB mm1,mm2/m64             ; 0F FC /r             [PENT,MMX]
    PADDW mm1,mm2/m64             ; 0F FD /r             [PENT,MMX]
    PADDD mm1,mm2/m64             ; 0F FE /r             [PENT,MMX]

    PADDB xmm1,xmm2/m128          ; 66 0F FC /r     [WILLAMETTE,SSE2]
    PADDW xmm1,xmm2/m128          ; 66 0F FD /r     [WILLAMETTE,SSE2]
    PADDD xmm1,xmm2/m128          ; 66 0F FE /r     [WILLAMETTE,SSE2]

`PADDx` performs packed addition of the two operands, storing the result in the destination (first) operand.

- `PADDB` treats the operands as packed bytes, and adds each byte individually;
- `PADDW` treats the operands as packed words;
- `PADDD` treats its operands as packed doublewords.

When an individual result is too large to fit in its destination, it is wrapped around and the low bits are stored, with the carry bit discarded.

#### B.4.198 `PADDQ`: Add Packed Quadword Integers

    PADDQ mm1,mm2/m64             ; 0F D4 /r             [PENT,MMX]

    PADDQ xmm1,xmm2/m128          ; 66 0F D4 /r     [WILLAMETTE,SSE2]

`PADDQ` adds the quadwords in the source and destination operands, and stores the result in the destination register.

When an individual result is too large to fit in its destination, it is wrapped around and the low bits are stored, with the carry bit discarded.

#### B.4.199 `PADDSB`, `PADDSW`: Add Packed Signed Integers With Saturation

    PADDSB mm1,mm2/m64            ; 0F EC /r             [PENT,MMX]
    PADDSW mm1,mm2/m64            ; 0F ED /r             [PENT,MMX]

    PADDSB xmm1,xmm2/m128         ; 66 0F EC /r     [WILLAMETTE,SSE2]
    PADDSW xmm1,xmm2/m128         ; 66 0F ED /r     [WILLAMETTE,SSE2]

`PADDSx` performs packed addition of the two operands, storing the result in the destination (first) operand. `PADDSB` treats the operands as packed bytes, and adds each byte individually; and `PADDSW` treats the operands as packed words.

When an individual result is too large to fit in its destination, a saturated value is stored. The resulting value is the value with the largest magnitude of the same sign as the result which will fit in the available space.

#### B.4.200 `PADDSIW`: MMX Packed Addition to Implicit Destination

    PADDSIW mmxreg,r/m64          ; 0F 51 /r             [CYRIX,MMX]

`PADDSIW`, specific to the Cyrix extensions to the MMX instruction set, performs the same function as `PADDSW`, except that the result is placed in an implied register.

To work out the implied register, invert the lowest bit in the register number. So `PADDSIW MM0,MM2` would put the result in `MM1`, but `PADDSIW MM1,MM2` would put the result in `MM0`.

#### B.4.201 `PADDUSB`, `PADDUSW`: Add Packed Unsigned Integers With Saturation

    PADDUSB mm1,mm2/m64           ; 0F DC /r             [PENT,MMX]
    PADDUSW mm1,mm2/m64           ; 0F DD /r             [PENT,MMX]

    PADDUSB xmm1,xmm2/m128         ; 66 0F DC /r    [WILLAMETTE,SSE2]
    PADDUSW xmm1,xmm2/m128         ; 66 0F DD /r    [WILLAMETTE,SSE2]

`PADDUSx` performs packed addition of the two operands, storing the result in the destination (first) operand. `PADDUSB` treats the operands as packed bytes, and adds each byte individually; and `PADDUSW` treats the operands as packed words.

When an individual result is too large to fit in its destination, a saturated value is stored. The resulting value is the maximum value that will fit in the available space.

#### B.4.202 `PAND`, `PANDN`: MMX Bitwise AND and AND-NOT

    PAND mm1,mm2/m64              ; 0F DB /r             [PENT,MMX]
    PANDN mm1,mm2/m64             ; 0F DF /r             [PENT,MMX]

    PAND xmm1,xmm2/m128           ; 66 0F DB /r     [WILLAMETTE,SSE2]
    PANDN xmm1,xmm2/m128          ; 66 0F DF /r     [WILLAMETTE,SSE2]

`PAND` performs a bitwise AND operation between its two operands (i.e. each bit of the result is 1 if and only if the corresponding bits of the two inputs were both 1), and stores the result in the destination (first) operand.

`PANDN` performs the same operation, but performs a one's complement operation on the destination (first) operand first.

#### B.4.203 `PAUSE`: Spin Loop Hint

    PAUSE                         ; F3 90           [WILLAMETTE,SSE2]

`PAUSE` provides a hint to the processor that the following code is a spin loop. This improves processor performance by bypassing possible memory order violations. On older processors, this instruction operates as a `NOP`.

#### B.4.204 `PAVEB`: MMX Packed Average

    PAVEB mmxreg,r/m64            ; 0F 50 /r             [CYRIX,MMX]

`PAVEB`, specific to the Cyrix MMX extensions, treats its two operands as vectors of eight unsigned bytes, and calculates the average of the corresponding bytes in the operands. The resulting vector of eight averages is stored in the first operand.

This opcode maps to `MOVMSKPS r32, xmm` on processors that support the SSE instruction set.

#### B.4.205 `PAVGB` `PAVGW`: Average Packed Integers

    PAVGB mm1,mm2/m64             ; 0F E0 /r        [KATMAI,MMX]
    PAVGW mm1,mm2/m64             ; 0F E3 /r        [KATMAI,MMX,SM]

    PAVGB xmm1,xmm2/m128          ; 66 0F E0 /r     [WILLAMETTE,SSE2]
    PAVGW xmm1,xmm2/m128          ; 66 0F E3 /r     [WILLAMETTE,SSE2]

`PAVGB` and `PAVGW` add the unsigned data elements of the source operand to the unsigned data elements of the destination register, then adds 1 to the temporary results. The results of the add are then each independently right-shifted by one bit position. The high order bits of each element are filled with the carry bits of the corresponding sum.

- `PAVGB` operates on packed unsigned bytes, and
- `PAVGW` operates on packed unsigned words.

#### B.4.206 `PAVGUSB`: Average of unsigned packed 8-bit values

    PAVGUSB mm1,mm2/m64           ; 0F 0F /r BF          [PENT,3DNOW]

`PAVGUSB` adds the unsigned data elements of the source operand to the unsigned data elements of the destination register, then adds 1 to the temporary results. The results of the add are then each independently right-shifted by one bit position. The high order bits of each element are filled with the carry bits of the corresponding sum.

This instruction performs exactly the same operations as the `PAVGB` `MMX` instruction (section B.4.205).

#### B.4.207 `PCMPxx`: Compare Packed Integers.

    PCMPEQB mm1,mm2/m64           ; 0F 74 /r             [PENT,MMX]
    PCMPEQW mm1,mm2/m64           ; 0F 75 /r             [PENT,MMX]
    PCMPEQD mm1,mm2/m64           ; 0F 76 /r             [PENT,MMX]

    PCMPGTB mm1,mm2/m64           ; 0F 64 /r             [PENT,MMX]
    PCMPGTW mm1,mm2/m64           ; 0F 65 /r             [PENT,MMX]
    PCMPGTD mm1,mm2/m64           ; 0F 66 /r             [PENT,MMX]

    PCMPEQB xmm1,xmm2/m128        ; 66 0F 74 /r     [WILLAMETTE,SSE2]
    PCMPEQW xmm1,xmm2/m128        ; 66 0F 75 /r     [WILLAMETTE,SSE2]
    PCMPEQD xmm1,xmm2/m128        ; 66 0F 76 /r     [WILLAMETTE,SSE2]

    PCMPGTB xmm1,xmm2/m128        ; 66 0F 64 /r     [WILLAMETTE,SSE2]
    PCMPGTW xmm1,xmm2/m128        ; 66 0F 65 /r     [WILLAMETTE,SSE2]
    PCMPGTD xmm1,xmm2/m128        ; 66 0F 66 /r     [WILLAMETTE,SSE2]

The `PCMPxx` instructions all treat their operands as vectors of bytes, words, or doublewords; corresponding elements of the source and destination are compared, and the corresponding element of the destination (first) operand is set to all zeros or all ones depending on the result of the comparison.

- `PCMPxxB` treats the operands as vectors of bytes;
- `PCMPxxW` treats the operands as vectors of words;
- `PCMPxxD` treats the operands as vectors of doublewords;
- `PCMPEQx` sets the corresponding element of the destination operand to all ones if the two elements compared are equal;
- `PCMPGTx` sets the destination element to all ones if the element of the first (destination) operand is greater (treated as a signed integer) than that of the second (source) operand.

#### B.4.208 `PDISTIB`: MMX Packed Distance and Accumulate with Implied Register

    PDISTIB mm,m64                ; 0F 54 /r             [CYRIX,MMX]

`PDISTIB`, specific to the Cyrix MMX extensions, treats its two input operands as vectors of eight unsigned bytes. For each byte position, it finds the absolute difference between the bytes in that position in the two input operands, and adds that value to the byte in the same position in the implied output register. The addition is saturated to an unsigned byte in the same way as `PADDUSB`.

To work out the implied register, invert the lowest bit in the register number. So `PDISTIB MM0,M64` would put the result in `MM1`, but `PDISTIB MM1,M64` would put the result in `MM0`.

Note that `PDISTIB` cannot take a register as its second source operand.

Operation:

       dstI[0-7]     := dstI[0-7]   + ABS(src0[0-7] - src1[0-7]),
       dstI[8-15]    := dstI[8-15]  + ABS(src0[8-15] - src1[8-15]),
       .......
       .......
       dstI[56-63]   := dstI[56-63] + ABS(src0[56-63] - src1[56-63]).

#### B.4.209 `PEXTRW`: Extract Word

    PEXTRW reg32,mm,imm8          ; 0F C5 /r ib     [KATMAI,MMX]
    PEXTRW reg32,xmm,imm8         ; 66 0F C5 /r ib  [WILLAMETTE,SSE2]

`PEXTRW` moves the word in the source register (second operand) that is pointed to by the count operand (third operand), into the lower half of a 32-bit general purpose register. The upper half of the register is cleared to all 0s.

When the source operand is an `MMX` register, the two least significant bits of the count specify the source word. When it is an `SSE` register, the three least significant bits specify the word location.

#### B.4.210 `PF2ID`: Packed Single-Precision FP to Integer Convert

    PF2ID mm1,mm2/m64             ; 0F 0F /r 1D          [PENT,3DNOW]

`PF2ID` converts two single-precision FP values in the source operand to signed 32-bit integers, using truncation, and stores them in the destination operand. Source values that are outside the range supported by the destination are saturated to the largest absolute value of the same sign.

#### B.4.211 `PF2IW`: Packed Single-Precision FP to Integer Word Convert

    PF2IW mm1,mm2/m64             ; 0F 0F /r 1C          [PENT,3DNOW]

`PF2IW` converts two single-precision FP values in the source operand to signed 16-bit integers, using truncation, and stores them in the destination operand. Source values that are outside the range supported by the destination are saturated to the largest absolute value of the same sign.

- In the K6-2 and K6-III, the 16-bit value is zero-extended to 32-bits before storing.
- In the K6-2+, K6-III+ and Athlon processors, the value is sign-extended to 32-bits before storing.

#### B.4.212 `PFACC`: Packed Single-Precision FP Accumulate

    PFACC mm1,mm2/m64             ; 0F 0F /r AE          [PENT,3DNOW]

`PFACC` adds the two single-precision FP values from the destination operand together, then adds the two single-precision FP values from the source operand, and places the results in the low and high doublewords of the destination operand.

The operation is:

       dst[0-31]   := dst[0-31] + dst[32-63],
       dst[32-63]  := src[0-31] + src[32-63].

#### B.4.213 `PFADD`: Packed Single-Precision FP Addition

    PFADD mm1,mm2/m64             ; 0F 0F /r 9E          [PENT,3DNOW]

`PFADD` performs addition on each of two packed single-precision FP value pairs.

       dst[0-31]   := dst[0-31]  + src[0-31],
       dst[32-63]  := dst[32-63] + src[32-63].

#### B.4.214 `PFCMPxx`: Packed Single-Precision FP Compare

    PFCMPEQ mm1,mm2/m64           ; 0F 0F /r B0          [PENT,3DNOW]
    PFCMPGE mm1,mm2/m64           ; 0F 0F /r 90          [PENT,3DNOW]
    PFCMPGT mm1,mm2/m64           ; 0F 0F /r A0          [PENT,3DNOW]

The `PFCMPxx` instructions compare the packed single-point FP values in the source and destination operands, and set the destination according to the result. If the condition is true, the destination is set to all 1s, otherwise it's set to all 0s.

- `PFCMPEQ` tests whether dst == src;
- `PFCMPGE` tests whether dst >= src;
- `PFCMPGT` tests whether dst > src.

#### B.4.215 `PFMAX`: Packed Single-Precision FP Maximum

    PFMAX mm1,mm2/m64             ; 0F 0F /r A4          [PENT,3DNOW]

`PFMAX` returns the higher of each pair of single-precision FP values. If the higher value is zero, it is returned as positive zero.

#### B.4.216 `PFMIN`: Packed Single-Precision FP Minimum

    PFMIN mm1,mm2/m64             ; 0F 0F /r 94          [PENT,3DNOW]

`PFMIN` returns the lower of each pair of single-precision FP values. If the lower value is zero, it is returned as positive zero.

#### B.4.217 `PFMUL`: Packed Single-Precision FP Multiply

    PFMUL mm1,mm2/m64             ; 0F 0F /r B4          [PENT,3DNOW]

`PFMUL` returns the product of each pair of single-precision FP values.

       dst[0-31]  := dst[0-31]  * src[0-31],
       dst[32-63] := dst[32-63] * src[32-63].

#### B.4.218 `PFNACC`: Packed Single-Precision FP Negative Accumulate

    PFNACC mm1,mm2/m64            ; 0F 0F /r 8A          [PENT,3DNOW]

`PFNACC` performs a negative accumulate of the two single-precision FP values in the source and destination registers. The result of the accumulate from the destination register is stored in the low doubleword of the destination, and the result of the source accumulate is stored in the high doubleword of the destination register.

The operation is:

       dst[0-31]  := dst[0-31] - dst[32-63],
       dst[32-63] := src[0-31] - src[32-63].

#### B.4.219 `PFPNACC`: Packed Single-Precision FP Mixed Accumulate

    PFPNACC mm1,mm2/m64           ; 0F 0F /r 8E          [PENT,3DNOW]

`PFPNACC` performs a positive accumulate of the two single-precision FP values in the source register and a negative accumulate of the destination register. The result of the accumulate from the destination register is stored in the low doubleword of the destination, and the result of the source accumulate is stored in the high doubleword of the destination register.

The operation is:

       dst[0-31]  := dst[0-31] - dst[32-63],
       dst[32-63] := src[0-31] + src[32-63].

#### B.4.220 `PFRCP`: Packed Single-Precision FP Reciprocal Approximation

    PFRCP mm1,mm2/m64             ; 0F 0F /r 96          [PENT,3DNOW]

`PFRCP` performs a low precision estimate of the reciprocal of the low-order single-precision FP value in the source operand, storing the result in both halves of the destination register. The result is accurate to 14 bits.

For higher precision reciprocals, this instruction should be followed by two more instructions: `PFRCPIT1` (section B.4.221) and `PFRCPIT2` (section B.4.221). This will result in a 24-bit accuracy. For more details, see the AMD 3DNow! technology manual.

#### B.4.221 `PFRCPIT1`: Packed Single-Precision FP Reciprocal, First Iteration Step

    PFRCPIT1 mm1,mm2/m64          ; 0F 0F /r A6          [PENT,3DNOW]

`PFRCPIT1` performs the first intermediate step in the calculation of the reciprocal of a single-precision FP value. The first source value (`mm1` is the original value, and the second source value (`mm2/m64` is the result of a `PFRCP` instruction.

For the final step in a reciprocal, returning the full 24-bit accuracy of a single-precision FP value, see `PFRCPIT2` (section B.4.222). For more details, see the AMD 3DNow! technology manual.

#### B.4.222 `PFRCPIT2`: Packed Single-Precision FP Reciprocal/ Reciprocal Square Root, Second Iteration Step

    PFRCPIT2 mm1,mm2/m64          ; 0F 0F /r B6          [PENT,3DNOW]

`PFRCPIT2` performs the second and final intermediate step in the calculation of a reciprocal or reciprocal square root, refining the values returned by the `PFRCP` and `PFRSQRT` instructions, respectively.

The first source value (`mm1`) is the output of either a `PFRCPIT1` or a `PFRSQIT1` instruction, and the second source is the output of either the `PFRCP` or the `PFRSQRT` instruction. For more details, see the AMD 3DNow! technology manual.

#### B.4.223 `PFRSQIT1`: Packed Single-Precision FP Reciprocal Square Root, First Iteration Step

    PFRSQIT1 mm1,mm2/m64          ; 0F 0F /r A7          [PENT,3DNOW]

`PFRSQIT1` performs the first intermediate step in the calculation of the reciprocal square root of a single-precision FP value. The first source value (`mm1` is the square of the result of a `PFRSQRT` instruction, and the second source value (`mm2/m64` is the original value.

For the final step in a calculation, returning the full 24-bit accuracy of a single-precision FP value, see `PFRCPIT2` (section B.4.222). For more details, see the AMD 3DNow! technology manual.

#### B.4.224 `PFRSQRT`: Packed Single-Precision FP Reciprocal Square Root Approximation

    PFRSQRT mm1,mm2/m64           ; 0F 0F /r 97          [PENT,3DNOW]

`PFRSQRT` performs a low precision estimate of the reciprocal square root of the low-order single-precision FP value in the source operand, storing the result in both halves of the destination register. The result is accurate to 15 bits.

For higher precision reciprocals, this instruction should be followed by two more instructions: `PFRSQIT1` (section B.4.223) and `PFRCPIT2` (section B.4.221). This will result in a 24-bit accuracy. For more details, see the AMD 3DNow! technology manual.

#### B.4.225 `PFSUB`: Packed Single-Precision FP Subtract

    PFSUB mm1,mm2/m64             ; 0F 0F /r 9A          [PENT,3DNOW]

`PFSUB` subtracts the single-precision FP values in the source from those in the destination, and stores the result in the destination operand.

       dst[0-31]  := dst[0-31]  - src[0-31],
       dst[32-63] := dst[32-63] - src[32-63].

#### B.4.226 `PFSUBR`: Packed Single-Precision FP Reverse Subtract

    PFSUBR mm1,mm2/m64            ; 0F 0F /r AA          [PENT,3DNOW]

`PFSUBR` subtracts the single-precision FP values in the destination from those in the source, and stores the result in the destination operand.

       dst[0-31]  := src[0-31]  - dst[0-31],
       dst[32-63] := src[32-63] - dst[32-63].

#### B.4.227 `PI2FD`: Packed Doubleword Integer to Single-Precision FP Convert

    PI2FD mm1,mm2/m64             ; 0F 0F /r 0D          [PENT,3DNOW]

`PF2ID` converts two signed 32-bit integers in the source operand to single-precision FP values, using truncation of significant digits, and stores them in the destination operand.

#### B.4.228 `PF2IW`: Packed Word Integer to Single-Precision FP Convert

    PI2FW mm1,mm2/m64             ; 0F 0F /r 0C          [PENT,3DNOW]

`PF2IW` converts two signed 16-bit integers in the source operand to single-precision FP values, and stores them in the destination operand. The input values are in the low word of each doubleword.

#### B.4.229 `PINSRW`: Insert Word

    PINSRW mm,r16/r32/m16,imm8    ;0F C4 /r ib      [KATMAI,MMX]
    PINSRW xmm,r16/r32/m16,imm8   ;66 0F C4 /r ib   [WILLAMETTE,SSE2]

`PINSRW` loads a word from a 16-bit register (or the low half of a 32-bit register), or from memory, and loads it to the word position in the destination register, pointed at by the count operand (third operand). If the destination is an `MMX` register, the low two bits of the count byte are used, if it is an `XMM` register the low 3 bits are used. The insertion is done in such a way that the other words from the destination register are left untouched.

#### B.4.230 `PMACHRIW`: Packed Multiply and Accumulate with Rounding

    PMACHRIW mm,m64               ; 0F 5E /r             [CYRIX,MMX]

`PMACHRIW` takes two packed 16-bit integer inputs, multiplies the values in the inputs, rounds on bit 15 of each result, then adds bits 15-30 of each result to the corresponding position of the implied destination register.

The operation of this instruction is:

       dstI[0-15]  := dstI[0-15]  + (mm[0-15] *m64[0-15]
                                              + 0x00004000)[15-30],
       dstI[16-31] := dstI[16-31] + (mm[16-31]*m64[16-31]
                                              + 0x00004000)[15-30],
       dstI[32-47] := dstI[32-47] + (mm[32-47]*m64[32-47]
                                              + 0x00004000)[15-30],
       dstI[48-63] := dstI[48-63] + (mm[48-63]*m64[48-63]
                                              + 0x00004000)[15-30].

Note that `PMACHRIW` cannot take a register as its second source operand.

#### B.4.231 `PMADDWD`: MMX Packed Multiply and Add

    PMADDWD mm1,mm2/m64           ; 0F F5 /r             [PENT,MMX]
    PMADDWD xmm1,xmm2/m128        ; 66 0F F5 /r     [WILLAMETTE,SSE2]

`PMADDWD` treats its two inputs as vectors of signed words. It multiplies corresponding elements of the two operands, giving doubleword results. These are then added together in pairs and stored in the destination operand.

The operation of this instruction is:

       dst[0-31]   := (dst[0-15] * src[0-15])
                                   + (dst[16-31] * src[16-31]);
       dst[32-63]  := (dst[32-47] * src[32-47])
                                   + (dst[48-63] * src[48-63]);

The following apply to the `SSE` version of the instruction:

       dst[64-95]  := (dst[64-79] * src[64-79])
                                   + (dst[80-95] * src[80-95]);
       dst[96-127] := (dst[96-111] * src[96-111])
                                   + (dst[112-127] * src[112-127]).

#### B.4.232 `PMAGW`: MMX Packed Magnitude

    PMAGW mm1,mm2/m64             ; 0F 52 /r             [CYRIX,MMX]

`PMAGW`, specific to the Cyrix MMX extensions, treats both its operands as vectors of four signed words. It compares the absolute values of the words in corresponding positions, and sets each word of the destination (first) operand to whichever of the two words in that position had the larger absolute value.

#### B.4.233 `PMAXSW`: Packed Signed Integer Word Maximum

    PMAXSW mm1,mm2/m64            ; 0F EE /r        [KATMAI,MMX]
    PMAXSW xmm1,xmm2/m128         ; 66 0F EE /r     [WILLAMETTE,SSE2]

`PMAXSW` compares each pair of words in the two source operands, and for each pair it stores the maximum value in the destination register.

#### B.4.234 `PMAXUB`: Packed Unsigned Integer Byte Maximum

    PMAXUB mm1,mm2/m64            ; 0F DE /r        [KATMAI,MMX]
    PMAXUB xmm1,xmm2/m128         ; 66 0F DE /r     [WILLAMETTE,SSE2]

`PMAXUB` compares each pair of bytes in the two source operands, and for each pair it stores the maximum value in the destination register.

#### B.4.235 `PMINSW`: Packed Signed Integer Word Minimum

    PMINSW mm1,mm2/m64            ; 0F EA /r        [KATMAI,MMX]
    PMINSW xmm1,xmm2/m128         ; 66 0F EA /r     [WILLAMETTE,SSE2]

`PMINSW` compares each pair of words in the two source operands, and for each pair it stores the minimum value in the destination register.

#### B.4.236 `PMINUB`: Packed Unsigned Integer Byte Minimum

    PMINUB mm1,mm2/m64            ; 0F DA /r        [KATMAI,MMX]
    PMINUB xmm1,xmm2/m128         ; 66 0F DA /r     [WILLAMETTE,SSE2]

`PMINUB` compares each pair of bytes in the two source operands, and for each pair it stores the minimum value in the destination register.

#### B.4.237 `PMOVMSKB`: Move Byte Mask To Integer

    PMOVMSKB reg32,mm             ; 0F D7 /r        [KATMAI,MMX]
    PMOVMSKB reg32,xmm            ; 66 0F D7 /r     [WILLAMETTE,SSE2]

`PMOVMSKB` returns an 8-bit or 16-bit mask formed of the most significant bits of each byte of source operand (8-bits for an `MMX` register, 16-bits for an `XMM` register).

#### B.4.238 `PMULHRWC`, `PMULHRIW`: Multiply Packed 16-bit Integers With Rounding, and Store High Word

    PMULHRWC mm1,mm2/m64         ; 0F 59 /r              [CYRIX,MMX]
    PMULHRIW mm1,mm2/m64         ; 0F 5D /r              [CYRIX,MMX]

These instructions take two packed 16-bit integer inputs, multiply the values in the inputs, round on bit 15 of each result, then store bits 15-30 of each result to the corresponding position of the destination register.

- For `PMULHRWC`, the destination is the first source operand.
- For `PMULHRIW`, the destination is an implied register (worked out as described for `PADDSIW` (section B.4.200)).

The operation of this instruction is:

       dst[0-15]  := (src1[0-15] *src2[0-15]  + 0x00004000)[15-30]
       dst[16-31] := (src1[16-31]*src2[16-31] + 0x00004000)[15-30]
       dst[32-47] := (src1[32-47]*src2[32-47] + 0x00004000)[15-30]
       dst[48-63] := (src1[48-63]*src2[48-63] + 0x00004000)[15-30]

See also `PMULHRWA` (section B.4.239) for a 3DNow! version of this instruction.

#### B.4.239 `PMULHRWA`: Multiply Packed 16-bit Integers With Rounding, and Store High Word

    PMULHRWA mm1,mm2/m64          ; 0F 0F /r B7     [PENT,3DNOW]

`PMULHRWA` takes two packed 16-bit integer inputs, multiplies the values in the inputs, rounds on bit 16 of each result, then stores bits 16-31 of each result to the corresponding position of the destination register.

The operation of this instruction is:

       dst[0-15]  := (src1[0-15] *src2[0-15]  + 0x00008000)[16-31];
       dst[16-31] := (src1[16-31]*src2[16-31] + 0x00008000)[16-31];
       dst[32-47] := (src1[32-47]*src2[32-47] + 0x00008000)[16-31];
       dst[48-63] := (src1[48-63]*src2[48-63] + 0x00008000)[16-31].

See also `PMULHRWC` (section B.4.238) for a Cyrix version of this instruction.

#### B.4.240 `PMULHUW`: Multiply Packed 16-bit Integers, and Store High Word

    PMULHUW mm1,mm2/m64           ; 0F E4 /r        [KATMAI,MMX]
    PMULHUW xmm1,xmm2/m128        ; 66 0F E4 /r     [WILLAMETTE,SSE2]

`PMULHUW` takes two packed unsigned 16-bit integer inputs, multiplies the values in the inputs, then stores bits 16-31 of each result to the corresponding position of the destination register.

#### B.4.241 `PMULHW`, `PMULLW`: Multiply Packed 16-bit Integers, and Store

    PMULHW mm1,mm2/m64            ; 0F E5 /r             [PENT,MMX]
    PMULLW mm1,mm2/m64            ; 0F D5 /r             [PENT,MMX]

    PMULHW xmm1,xmm2/m128         ; 66 0F E5 /r     [WILLAMETTE,SSE2]
    PMULLW xmm1,xmm2/m128         ; 66 0F D5 /r     [WILLAMETTE,SSE2]

`PMULxW` takes two packed unsigned 16-bit integer inputs, and multiplies the values in the inputs, forming doubleword results.

- `PMULHW` then stores the top 16 bits of each doubleword in the destination (first) operand;
- `PMULLW` stores the bottom 16 bits of each doubleword in the destination operand.

#### B.4.242 `PMULUDQ`: Multiply Packed Unsigned 32-bit Integers, and Store.

    PMULUDQ mm1,mm2/m64           ; 0F F4 /r        [WILLAMETTE,SSE2]
    PMULUDQ xmm1,xmm2/m128        ; 66 0F F4 /r     [WILLAMETTE,SSE2]

`PMULUDQ` takes two packed unsigned 32-bit integer inputs, and multiplies the values in the inputs, forming quadword results. The source is either an unsigned doubleword in the low doubleword of a 64-bit operand, or it's two unsigned doublewords in the first and third doublewords of a 128-bit operand. This produces either one or two 64-bit results, which are stored in the respective quadword locations of the destination register.

The operation is:

       dst[0-63]   := dst[0-31]  * src[0-31];
       dst[64-127] := dst[64-95] * src[64-95].

#### B.4.243 `PMVccZB`: MMX Packed Conditional Move

    PMVZB mmxreg,mem64            ; 0F 58 /r             [CYRIX,MMX]
    PMVNZB mmxreg,mem64           ; 0F 5A /r             [CYRIX,MMX]
    PMVLZB mmxreg,mem64           ; 0F 5B /r             [CYRIX,MMX]
    PMVGEZB mmxreg,mem64          ; 0F 5C /r             [CYRIX,MMX]

These instructions, specific to the Cyrix MMX extensions, perform parallel conditional moves. The two input operands are treated as vectors of eight bytes. Each byte of the destination (first) operand is either written from the corresponding byte of the source (second) operand, or left alone, depending on the value of the byte in the implied operand (specified in the same way as `PADDSIW`, in section B.4.200).

- `PMVZB` performs each move if the corresponding byte in the implied operand is zero;
- `PMVNZB` moves if the byte is non-zero;
- `PMVLZB` moves if the byte is less than zero;
- `PMVGEZB` moves if the byte is greater than or equal to zero.

Note that these instructions cannot take a register as their second source operand.

#### B.4.244 `POP`: Pop Data from Stack

    POP reg16                     ; o16 58+r             [8086]
    POP reg32                     ; o32 58+r             [386]

    POP r/m16                     ; o16 8F /0            [8086]
    POP r/m32                     ; o32 8F /0            [386]

    POP CS                        ; 0F                   [8086,UNDOC]
    POP DS                        ; 1F                   [8086]
    POP ES                        ; 07                   [8086]
    POP SS                        ; 17                   [8086]
    POP FS                        ; 0F A1                [386]
    POP GS                        ; 0F A9                [386]

`POP` loads a value from the stack (from `[SS:SP]` or `[SS:ESP]`) and then increments the stack pointer.

The address-size attribute of the instruction determines whether `SP` or `ESP` is used as the stack pointer: to deliberately override the default given by the `BITS` setting, you can use an `a16` or `a32` prefix.

The operand-size attribute of the instruction determines whether the stack pointer is incremented by 2 or 4: this means that segment register pops in `BITS 32` mode will pop 4 bytes off the stack and discard the upper two of them. If you need to override that, you can use an `o16` or `o32` prefix.

The above opcode listings give two forms for general-purpose register pop instructions: for example, `POP BX` has the two forms `5B` and `8F C3`. NASM will always generate the shorter form when given `POP BX`. NDISASM will disassemble both.

`POP CS` is not a documented instruction, and is not supported on any processor above the 8086 (since they use `0Fh` as an opcode prefix for instruction set extensions). However, at least some 8086 processors do support it, and so NASM generates it for completeness.

#### B.4.245 `POPAx`: Pop All General-Purpose Registers

    POPA                          ; 61                   [186]
    POPAW                         ; o16 61               [186]
    POPAD                         ; o32 61               [386]

- `POPAW` pops a word from the stack into each of, successively, `DI`, `SI`, `BP`, nothing (it discards a word from the stack which was a placeholder for `SP`), `BX`, `DX`, `CX` and `AX`. It is intended to reverse the operation of `PUSHAW` (see section B.4.264), but it ignores the value for `SP` that was pushed on the stack by `PUSHAW`.
- `POPAD` pops twice as much data, and places the results in `EDI`, `ESI`, `EBP`, nothing (placeholder for `ESP`), `EBX`, `EDX`, `ECX` and `EAX`. It reverses the operation of `PUSHAD`.

`POPA` is an alias mnemonic for either `POPAW` or `POPAD`, depending on the current `BITS` setting.

Note that the registers are popped in reverse order of their numeric values in opcodes (see section B.2.1).

#### B.4.246 `POPFx`: Pop Flags Register

    POPF                          ; 9D                   [8086]
    POPFW                         ; o16 9D               [8086]
    POPFD                         ; o32 9D               [386]

- `POPFW` pops a word from the stack and stores it in the bottom 16 bits of the flags register (or the whole flags register, on processors below a 386).
- `POPFD` pops a doubleword and stores it in the entire flags register.

`POPF` is an alias mnemonic for either `POPFW` or `POPFD`, depending on the current `BITS` setting.

See also `PUSHF` (section B.4.265).

#### B.4.247 `POR`: MMX Bitwise OR

    POR mm1,mm2/m64               ; 0F EB /r             [PENT,MMX]
    POR xmm1,xmm2/m128            ; 66 0F EB /r     [WILLAMETTE,SSE2]

`POR` performs a bitwise OR operation between its two operands (i.e. each bit of the result is 1 if and only if at least one of the corresponding bits of the two inputs was 1), and stores the result in the destination (first) operand.

#### B.4.248 `PREFETCH`: Prefetch Data Into Caches

    PREFETCH mem8                 ; 0F 0D /0             [PENT,3DNOW]
    PREFETCHW mem8                ; 0F 0D /1             [PENT,3DNOW]

`PREFETCH` and `PREFETCHW` fetch the line of data from memory that contains the specified byte. `PREFETCHW` performs differently on the Athlon to earlier processors.

For more details, see the 3DNow! Technology Manual.

#### B.4.249 `PREFETCHh`: Prefetch Data Into Caches

    PREFETCHNTA m8                ; 0F 18 /0        [KATMAI]
    PREFETCHT0 m8                 ; 0F 18 /1        [KATMAI]
    PREFETCHT1 m8                 ; 0F 18 /2        [KATMAI]
    PREFETCHT2 m8                 ; 0F 18 /3        [KATMAI]

The `PREFETCHh` instructions fetch the line of data from memory that contains the specified byte. It is placed in the cache according to rules specified by locality hints `h`:

The hints are:

- `T0` (temporal data) - prefetch data into all levels of the cache hierarchy.
- `T1` (temporal data with respect to first level cache) - prefetch data into level 2 cache and higher.
- `T2` (temporal data with respect to second level cache) - prefetch data into level 2 cache and higher.
- `NTA` (non-temporal data with respect to all cache levels) - prefetch data into non-temporal cache structure and into a location close to the processor, minimizing cache pollution.

Note that this group of instructions doesn't provide a guarantee that the data will be in the cache when it is needed. For more details, see the Intel IA32 Software Developer Manual, Volume 2.

#### B.4.250 `PSADBW`: Packed Sum of Absolute Differences

    PSADBW mm1,mm2/m64            ; 0F F6 /r        [KATMAI,MMX]
    PSADBW xmm1,xmm2/m128         ; 66 0F F6 /r     [WILLAMETTE,SSE2]

`PSADBW` The PSADBW instruction computes the absolute value of the difference of the packed unsigned bytes in the two source operands. These differences are then summed to produce a word result in the lower 16-bit field of the destination register; the rest of the register is cleared. The destination operand is an `MMX` or an `XMM` register. The source operand can either be a register or a memory operand.

#### B.4.251 `PSHUFD`: Shuffle Packed Doublewords

    PSHUFD xmm1,xmm2/m128,imm8    ; 66 0F 70 /r ib  [WILLAMETTE,SSE2]

`PSHUFD` shuffles the doublewords in the source (second) operand according to the encoding specified by imm8, and stores the result in the destination (first) operand.

Bits 0 and 1 of imm8 encode the source position of the doubleword to be copied to position 0 in the destination operand. Bits 2 and 3 encode for position 1, bits 4 and 5 encode for position 2, and bits 6 and 7 encode for position 3. For example, an encoding of 10 in bits 0 and 1 of imm8 indicates that the doubleword at bits 64-95 of the source operand will be copied to bits 0-31 of the destination.

#### B.4.252 `PSHUFHW`: Shuffle Packed High Words

    PSHUFHW xmm1,xmm2/m128,imm8   ; F3 0F 70 /r ib  [WILLAMETTE,SSE2]

`PSHUFW` shuffles the words in the high quadword of the source (second) operand according to the encoding specified by imm8, and stores the result in the high quadword of the destination (first) operand.

The operation of this instruction is similar to the `PSHUFW` instruction, except that the source and destination are the top quadword of a 128-bit operand, instead of being 64-bit operands. The low quadword is copied from the source to the destination without any changes.

#### B.4.253 `PSHUFLW`: Shuffle Packed Low Words

    PSHUFLW xmm1,xmm2/m128,imm8   ; F2 0F 70 /r ib  [WILLAMETTE,SSE2]

`PSHUFLW` shuffles the words in the low quadword of the source (second) operand according to the encoding specified by imm8, and stores the result in the low quadword of the destination (first) operand.

The operation of this instruction is similar to the `PSHUFW` instruction, except that the source and destination are the low quadword of a 128-bit operand, instead of being 64-bit operands. The high quadword is copied from the source to the destination without any changes.

#### B.4.254 `PSHUFW`: Shuffle Packed Words

    PSHUFW mm1,mm2/m64,imm8       ; 0F 70 /r ib     [KATMAI,MMX]

`PSHUFW` shuffles the words in the source (second) operand according to the encoding specified by imm8, and stores the result in the destination (first) operand.

Bits 0 and 1 of imm8 encode the source position of the word to be copied to position 0 in the destination operand. Bits 2 and 3 encode for position 1, bits 4 and 5 encode for position 2, and bits 6 and 7 encode for position 3. For example, an encoding of 10 in bits 0 and 1 of imm8 indicates that the word at bits 32-47 of the source operand will be copied to bits 0-15 of the destination.

#### B.4.255 `PSLLx`: Packed Data Bit Shift Left Logical

    PSLLW mm1,mm2/m64             ; 0F F1 /r             [PENT,MMX]
    PSLLW mm,imm8                 ; 0F 71 /6 ib          [PENT,MMX]

    PSLLW xmm1,xmm2/m128          ; 66 0F F1 /r     [WILLAMETTE,SSE2]
    PSLLW xmm,imm8                ; 66 0F 71 /6 ib  [WILLAMETTE,SSE2]

    PSLLD mm1,mm2/m64             ; 0F F2 /r             [PENT,MMX]
    PSLLD mm,imm8                 ; 0F 72 /6 ib          [PENT,MMX]

    PSLLD xmm1,xmm2/m128          ; 66 0F F2 /r     [WILLAMETTE,SSE2]
    PSLLD xmm,imm8                ; 66 0F 72 /6 ib  [WILLAMETTE,SSE2]

    PSLLQ mm1,mm2/m64             ; 0F F3 /r             [PENT,MMX]
    PSLLQ mm,imm8                 ; 0F 73 /6 ib          [PENT,MMX]

    PSLLQ xmm1,xmm2/m128          ; 66 0F F3 /r     [WILLAMETTE,SSE2]
    PSLLQ xmm,imm8                ; 66 0F 73 /6 ib  [WILLAMETTE,SSE2]

    PSLLDQ xmm1,imm8              ; 66 0F 73 /7 ib  [PENT,MMX]

`PSLLx` performs logical left shifts of the data elements in the destination (first) operand, moving each bit in the separate elements left by the number of bits specified in the source (second) operand, clearing the low-order bits as they are vacated.

- `PSLLW` shifts word sized elements.
- `PSLLD` shifts doubleword sized elements.
- `PSLLQ` shifts quadword sized elements.
- `PSLLDQ` shifts double quadword sized elements.

#### B.4.256 `PSRAx`: Packed Data Bit Shift Right Arithmetic

    PSRAW mm1,mm2/m64             ; 0F E1 /r             [PENT,MMX]
    PSRAW mm,imm8                 ; 0F 71 /4 ib          [PENT,MMX]

    PSRAW xmm1,xmm2/m128          ; 66 0F E1 /r     [WILLAMETTE,SSE2]
    PSRAW xmm,imm8                ; 66 0F 71 /4 ib  [WILLAMETTE,SSE2]

    PSRAD mm1,mm2/m64             ; 0F E2 /r             [PENT,MMX]
    PSRAD mm,imm8                 ; 0F 72 /4 ib          [PENT,MMX]

    PSRAD xmm1,xmm2/m128          ; 66 0F E2 /r     [WILLAMETTE,SSE2]
    PSRAD xmm,imm8                ; 66 0F 72 /4 ib  [WILLAMETTE,SSE2]

`PSRAx` performs arithmetic right shifts of the data elements in the destination (first) operand, moving each bit in the separate elements right by the number of bits specified in the source (second) operand, setting the high-order bits to the value of the original sign bit.

- `PSRAW` shifts word sized elements.
- `PSRAD` shifts doubleword sized elements.

#### B.4.257 `PSRLx`: Packed Data Bit Shift Right Logical

    PSRLW mm1,mm2/m64             ; 0F D1 /r             [PENT,MMX]
    PSRLW mm,imm8                 ; 0F 71 /2 ib          [PENT,MMX]

    PSRLW xmm1,xmm2/m128          ; 66 0F D1 /r     [WILLAMETTE,SSE2]
    PSRLW xmm,imm8                ; 66 0F 71 /2 ib  [WILLAMETTE,SSE2]

    PSRLD mm1,mm2/m64             ; 0F D2 /r             [PENT,MMX]
    PSRLD mm,imm8                 ; 0F 72 /2 ib          [PENT,MMX]

    PSRLD xmm1,xmm2/m128          ; 66 0F D2 /r     [WILLAMETTE,SSE2]
    PSRLD xmm,imm8                ; 66 0F 72 /2 ib  [WILLAMETTE,SSE2]

    PSRLQ mm1,mm2/m64             ; 0F D3 /r             [PENT,MMX]
    PSRLQ mm,imm8                 ; 0F 73 /2 ib          [PENT,MMX]

    PSRLQ xmm1,xmm2/m128          ; 66 0F D3 /r     [WILLAMETTE,SSE2]
    PSRLQ xmm,imm8                ; 66 0F 73 /2 ib  [WILLAMETTE,SSE2]

    PSRLDQ xmm1,imm8              ; 66 0F 73 /3 ib  [WILLAMETTE,SSE2]

`PSRLx` performs logical right shifts of the data elements in the destination (first) operand, moving each bit in the separate elements right by the number of bits specified in the source (second) operand, clearing the high-order bits as they are vacated.

- `PSRLW` shifts word sized elements.
- `PSRLD` shifts doubleword sized elements.
- `PSRLQ` shifts quadword sized elements.
- `PSRLDQ` shifts double quadword sized elements.

#### B.4.258 `PSUBx`: Subtract Packed Integers

    PSUBB mm1,mm2/m64             ; 0F F8 /r             [PENT,MMX]
    PSUBW mm1,mm2/m64             ; 0F F9 /r             [PENT,MMX]
    PSUBD mm1,mm2/m64             ; 0F FA /r             [PENT,MMX]
    PSUBQ mm1,mm2/m64             ; 0F FB /r        [WILLAMETTE,SSE2]

    PSUBB xmm1,xmm2/m128          ; 66 0F F8 /r     [WILLAMETTE,SSE2]
    PSUBW xmm1,xmm2/m128          ; 66 0F F9 /r     [WILLAMETTE,SSE2]
    PSUBD xmm1,xmm2/m128          ; 66 0F FA /r     [WILLAMETTE,SSE2]
    PSUBQ xmm1,xmm2/m128          ; 66 0F FB /r     [WILLAMETTE,SSE2]

`PSUBx` subtracts packed integers in the source operand from those in the destination operand. It doesn't differentiate between signed and unsigned integers, and doesn't set any of the flags.

- `PSUBB` operates on byte sized elements.
- `PSUBW` operates on word sized elements.
- `PSUBD` operates on doubleword sized elements.
- `PSUBQ` operates on quadword sized elements.

#### B.4.259 `PSUBSxx`, `PSUBUSx`: Subtract Packed Integers With Saturation

    PSUBSB mm1,mm2/m64            ; 0F E8 /r             [PENT,MMX]
    PSUBSW mm1,mm2/m64            ; 0F E9 /r             [PENT,MMX]

    PSUBSB xmm1,xmm2/m128         ; 66 0F E8 /r     [WILLAMETTE,SSE2]
    PSUBSW xmm1,xmm2/m128         ; 66 0F E9 /r     [WILLAMETTE,SSE2]

    PSUBUSB mm1,mm2/m64           ; 0F D8 /r             [PENT,MMX]
    PSUBUSW mm1,mm2/m64           ; 0F D9 /r             [PENT,MMX]

    PSUBUSB xmm1,xmm2/m128        ; 66 0F D8 /r     [WILLAMETTE,SSE2]
    PSUBUSW xmm1,xmm2/m128        ; 66 0F D9 /r     [WILLAMETTE,SSE2]

`PSUBSx` and `PSUBUSx` subtracts packed integers in the source operand from those in the destination operand, and use saturation for results that are outside the range supported by the destination operand.

- `PSUBSB` operates on signed bytes, and uses signed saturation on the results.
- `PSUBSW` operates on signed words, and uses signed saturation on the results.
- `PSUBUSB` operates on unsigned bytes, and uses signed saturation on the results.
- `PSUBUSW` operates on unsigned words, and uses signed saturation on the results.

#### B.4.260 `PSUBSIW`: MMX Packed Subtract with Saturation to Implied Destination

    PSUBSIW mm1,mm2/m64           ; 0F 55 /r             [CYRIX,MMX]

`PSUBSIW`, specific to the Cyrix extensions to the MMX instruction set, performs the same function as `PSUBSW`, except that the result is not placed in the register specified by the first operand, but instead in the implied destination register, specified as for `PADDSIW` (section B.4.200).

#### B.4.261 `PSWAPD`: Swap Packed Data

    PSWAPD mm1,mm2/m64            ; 0F 0F /r BB     [PENT,3DNOW]

`PSWAPD` swaps the packed doublewords in the source operand, and stores the result in the destination operand.

In the `K6-2` and `K6-III` processors, this opcode uses the mnemonic `PSWAPW`, and it swaps the order of words when copying from the source to the destination.

The operation in the `K6-2` and `K6-III` processors is

       dst[0-15]  = src[48-63];
       dst[16-31] = src[32-47];
       dst[32-47] = src[16-31];
       dst[48-63] = src[0-15].

The operation in the `K6-x+`, `ATHLON` and later processors is:

       dst[0-31]  = src[32-63];
       dst[32-63] = src[0-31].

#### B.4.262 `PUNPCKxxx`: Unpack and Interleave Data

    PUNPCKHBW mm1,mm2/m64         ; 0F 68 /r             [PENT,MMX]
    PUNPCKHWD mm1,mm2/m64         ; 0F 69 /r             [PENT,MMX]
    PUNPCKHDQ mm1,mm2/m64         ; 0F 6A /r             [PENT,MMX]

    PUNPCKHBW xmm1,xmm2/m128      ; 66 0F 68 /r     [WILLAMETTE,SSE2]
    PUNPCKHWD xmm1,xmm2/m128      ; 66 0F 69 /r     [WILLAMETTE,SSE2]
    PUNPCKHDQ xmm1,xmm2/m128      ; 66 0F 6A /r     [WILLAMETTE,SSE2]
    PUNPCKHQDQ xmm1,xmm2/m128     ; 66 0F 6D /r     [WILLAMETTE,SSE2]

    PUNPCKLBW mm1,mm2/m32         ; 0F 60 /r             [PENT,MMX]
    PUNPCKLWD mm1,mm2/m32         ; 0F 61 /r             [PENT,MMX]
    PUNPCKLDQ mm1,mm2/m32         ; 0F 62 /r             [PENT,MMX]

    PUNPCKLBW xmm1,xmm2/m128      ; 66 0F 60 /r     [WILLAMETTE,SSE2]
    PUNPCKLWD xmm1,xmm2/m128      ; 66 0F 61 /r     [WILLAMETTE,SSE2]
    PUNPCKLDQ xmm1,xmm2/m128      ; 66 0F 62 /r     [WILLAMETTE,SSE2]
    PUNPCKLQDQ xmm1,xmm2/m128     ; 66 0F 6C /r     [WILLAMETTE,SSE2]

`PUNPCKxx` all treat their operands as vectors, and produce a new vector generated by interleaving elements from the two inputs. The `PUNPCKHxx` instructions start by throwing away the bottom half of each input operand, and the `PUNPCKLxx` instructions throw away the top half.

The remaining elements, are then interleaved into the destination, alternating elements from the second (source) operand and the first (destination) operand: so the leftmost part of each element in the result always comes from the second operand, and the rightmost from the destination.

- `PUNPCKxBW` works a byte at a time, producing word sized output elements.
- `PUNPCKxWD` works a word at a time, producing doubleword sized output elements.
- `PUNPCKxDQ` works a doubleword at a time, producing quadword sized output elements.
- `PUNPCKxQDQ` works a quadword at a time, producing double quadword sized output elements.

So, for example, for `MMX` operands, if the first operand held `0x7A6A5A4A3A2A1A0A` and the second held `0x7B6B5B4B3B2B1B0B`, then:

- `PUNPCKHBW` would return `0x7B7A6B6A5B5A4B4A`.
- `PUNPCKHWD` would return `0x7B6B7A6A5B4B5A4A`.
- `PUNPCKHDQ` would return `0x7B6B5B4B7A6A5A4A`.
- `PUNPCKLBW` would return `0x3B3A2B2A1B1A0B0A`.
- `PUNPCKLWD` would return `0x3B2B3A2A1B0B1A0A`.
- `PUNPCKLDQ` would return `0x3B2B1B0B3A2A1A0A`.

#### B.4.263 `PUSH`: Push Data on Stack

    PUSH reg16                    ; o16 50+r             [8086]
    PUSH reg32                    ; o32 50+r             [386]

    PUSH r/m16                    ; o16 FF /6            [8086]
    PUSH r/m32                    ; o32 FF /6            [386]

    PUSH CS                       ; 0E                   [8086]
    PUSH DS                       ; 1E                   [8086]
    PUSH ES                       ; 06                   [8086]
    PUSH SS                       ; 16                   [8086]
    PUSH FS                       ; 0F A0                [386]
    PUSH GS                       ; 0F A8                [386]

    PUSH imm8                     ; 6A ib                [186]
    PUSH imm16                    ; o16 68 iw            [186]
    PUSH imm32                    ; o32 68 id            [386]

`PUSH` decrements the stack pointer (`SP` or `ESP`) by 2 or 4, and then stores the given value at `[SS:SP]` or `[SS:ESP]`.

The address-size attribute of the instruction determines whether `SP` or `ESP` is used as the stack pointer: to deliberately override the default given by the `BITS` setting, you can use an `a16` or `a32` prefix.

The operand-size attribute of the instruction determines whether the stack pointer is decremented by 2 or 4: this means that segment register pushes in `BITS 32` mode will push 4 bytes on the stack, of which the upper two are undefined. If you need to override that, you can use an `o16` or `o32` prefix.

The above opcode listings give two forms for general-purpose register push instructions: for example, `PUSH BX` has the two forms `53` and `FF F3`. NASM will always generate the shorter form when given `PUSH BX`. NDISASM will disassemble both.

Unlike the undocumented and barely supported `POP CS`, `PUSH CS` is a perfectly valid and sensible instruction, supported on all processors.

The instruction `PUSH SP` may be used to distinguish an 8086 from later processors: on an 8086, the value of `SP` stored is the value it has after the push instruction, whereas on later processors it is the value before the push instruction.

#### B.4.264 `PUSHAx`: Push All General-Purpose Registers

    PUSHA                         ; 60                   [186]
    PUSHAD                        ; o32 60               [386]
    PUSHAW                        ; o16 60               [186]

`PUSHAW` pushes, in succession, `AX`, `CX`, `DX`, `BX`, `SP`, `BP`, `SI` and `DI` on the stack, decrementing the stack pointer by a total of 16.

`PUSHAD` pushes, in succession, `EAX`, `ECX`, `EDX`, `EBX`, `ESP`, `EBP`, `ESI` and `EDI` on the stack, decrementing the stack pointer by a total of 32.

In both cases, the value of `SP` or `ESP` pushed is its original value, as it had before the instruction was executed.

`PUSHA` is an alias mnemonic for either `PUSHAW` or `PUSHAD`, depending on the current `BITS` setting.

Note that the registers are pushed in order of their numeric values in opcodes (see section B.2.1).

See also `POPA` (section B.4.245).

#### B.4.265 `PUSHFx`: Push Flags Register

    PUSHF                         ; 9C                   [8086]
    PUSHFD                        ; o32 9C               [386]
    PUSHFW                        ; o16 9C               [8086]

- `PUSHFW` pops a word from the stack and stores it in the bottom 16 bits of the flags register (or the whole flags register, on processors below a 386).
- `PUSHFD` pops a doubleword and stores it in the entire flags register.

`PUSHF` is an alias mnemonic for either `PUSHFW` or `PUSHFD`, depending on the current `BITS` setting.

See also `POPF` (section B.4.246).

#### B.4.266 `PXOR`: MMX Bitwise XOR

    PXOR mm1,mm2/m64              ; 0F EF /r             [PENT,MMX]
    PXOR xmm1,xmm2/m128           ; 66 0F EF /r     [WILLAMETTE,SSE2]

`PXOR` performs a bitwise XOR operation between its two operands (i.e. each bit of the result is 1 if and only if exactly one of the corresponding bits of the two inputs was 1), and stores the result in the destination (first) operand.

#### B.4.267 `RCL`, `RCR`: Bitwise Rotate through Carry Bit

    RCL r/m8,1                    ; D0 /2                [8086]
    RCL r/m8,CL                   ; D2 /2                [8086]
    RCL r/m8,imm8                 ; C0 /2 ib             [186]
    RCL r/m16,1                   ; o16 D1 /2            [8086]
    RCL r/m16,CL                  ; o16 D3 /2            [8086]
    RCL r/m16,imm8                ; o16 C1 /2 ib         [186]
    RCL r/m32,1                   ; o32 D1 /2            [386]
    RCL r/m32,CL                  ; o32 D3 /2            [386]
    RCL r/m32,imm8                ; o32 C1 /2 ib         [386]

    RCR r/m8,1                    ; D0 /3                [8086]
    RCR r/m8,CL                   ; D2 /3                [8086]
    RCR r/m8,imm8                 ; C0 /3 ib             [186]
    RCR r/m16,1                   ; o16 D1 /3            [8086]
    RCR r/m16,CL                  ; o16 D3 /3            [8086]
    RCR r/m16,imm8                ; o16 C1 /3 ib         [186]
    RCR r/m32,1                   ; o32 D1 /3            [386]
    RCR r/m32,CL                  ; o32 D3 /3            [386]
    RCR r/m32,imm8                ; o32 C1 /3 ib         [386]

`RCL` and `RCR` perform a 9-bit, 17-bit or 33-bit bitwise rotation operation, involving the given source/destination (first) operand and the carry bit. Thus, for example, in the operation `RCL AL,1`, a 9-bit rotation is performed in which `AL` is shifted left by 1, the top bit of `AL` moves into the carry flag, and the original value of the carry flag is placed in the low bit of `AL`.

The number of bits to rotate by is given by the second operand. Only the bottom five bits of the rotation count are considered by processors above the 8086.

You can force the longer (286 and upwards, beginning with a `C1` byte) form of `RCL foo,1` by using a `BYTE` prefix: `RCL foo,BYTE 1`. Similarly with `RCR`.

#### B.4.268 `RCPPS`: Packed Single-Precision FP Reciprocal

    RCPPS xmm1,xmm2/m128          ; 0F 53 /r        [KATMAI,SSE]

`RCPPS` returns an approximation of the reciprocal of the packed single-precision FP values from xmm2/m128. The maximum error for this approximation is: |Error| <= 1.5 x 2^-12

#### B.4.269 `RCPSS`: Scalar Single-Precision FP Reciprocal

    RCPSS xmm1,xmm2/m128          ; F3 0F 53 /r     [KATMAI,SSE]

`RCPSS` returns an approximation of the reciprocal of the lower single-precision FP value from xmm2/m32; the upper three fields are passed through from xmm1. The maximum error for this approximation is: |Error| <= 1.5 x 2^-12

#### B.4.270 `RDMSR`: Read Model-Specific Registers

    RDMSR                         ; 0F 32                [PENT,PRIV]

`RDMSR` reads the processor Model-Specific Register (MSR) whose index is stored in `ECX`, and stores the result in `EDX:EAX`. See also `WRMSR` (section B.4.329).

#### B.4.271 `RDPMC`: Read Performance-Monitoring Counters

    RDPMC                         ; 0F 33                [P6]

`RDPMC` reads the processor performance-monitoring counter whose index is stored in `ECX`, and stores the result in `EDX:EAX`.

This instruction is available on P6 and later processors and on MMX class processors.

#### B.4.272 `RDSHR`: Read SMM Header Pointer Register

    RDSHR r/m32                   ; 0F 36 /0        [386,CYRIX,SMM]

`RDSHR` reads the contents of the SMM header pointer register and saves it to the destination operand, which can be either a 32 bit memory location or a 32 bit register.

See also `WRSHR` (section B.4.330).

#### B.4.273 `RDTSC`: Read Time-Stamp Counter

    RDTSC                         ; 0F 31                [PENT]

`RDTSC` reads the processor's time-stamp counter into `EDX:EAX`.

#### B.4.274 `RET`, `RETF`, `RETN`: Return from Procedure Call

    RET                           ; C3                   [8086]
    RET imm16                     ; C2 iw                [8086]

    RETF                          ; CB                   [8086]
    RETF imm16                    ; CA iw                [8086]

    RETN                          ; C3                   [8086]
    RETN imm16                    ; C2 iw                [8086]

- `RET`, and its exact synonym `RETN`, pop `IP` or `EIP` from the stack and transfer control to the new address. Optionally, if a numeric second operand is provided, they increment the stack pointer by a further `imm16` bytes after popping the return address.
- `RETF` executes a far return: after popping `IP`/`EIP`, it then pops `CS`, and then increments the stack pointer by the optional argument if present.

#### B.4.275 `ROL`, `ROR`: Bitwise Rotate

    ROL r/m8,1                    ; D0 /0                [8086]
    ROL r/m8,CL                   ; D2 /0                [8086]
    ROL r/m8,imm8                 ; C0 /0 ib             [186]
    ROL r/m16,1                   ; o16 D1 /0            [8086]
    ROL r/m16,CL                  ; o16 D3 /0            [8086]
    ROL r/m16,imm8                ; o16 C1 /0 ib         [186]
    ROL r/m32,1                   ; o32 D1 /0            [386]
    ROL r/m32,CL                  ; o32 D3 /0            [386]
    ROL r/m32,imm8                ; o32 C1 /0 ib         [386]

    ROR r/m8,1                    ; D0 /1                [8086]
    ROR r/m8,CL                   ; D2 /1                [8086]
    ROR r/m8,imm8                 ; C0 /1 ib             [186]
    ROR r/m16,1                   ; o16 D1 /1            [8086]
    ROR r/m16,CL                  ; o16 D3 /1            [8086]
    ROR r/m16,imm8                ; o16 C1 /1 ib         [186]
    ROR r/m32,1                   ; o32 D1 /1            [386]
    ROR r/m32,CL                  ; o32 D3 /1            [386]
    ROR r/m32,imm8                ; o32 C1 /1 ib         [386]

`ROL` and `ROR` perform a bitwise rotation operation on the given source/destination (first) operand. Thus, for example, in the operation `ROL AL,1`, an 8-bit rotation is performed in which `AL` is shifted left by 1 and the original top bit of `AL` moves round into the low bit.

The number of bits to rotate by is given by the second operand. Only the bottom five bits of the rotation count are considered by processors above the 8086.

You can force the longer (286 and upwards, beginning with a `C1` byte) form of `ROL foo,1` by using a `BYTE` prefix: `ROL foo,BYTE 1`. Similarly with `ROR`.

#### B.4.276 `RSDC`: Restore Segment Register and Descriptor

    RSDC segreg,m80               ; 0F 79 /r        [486,CYRIX,SMM]

`RSDC` restores a segment register (DS, ES, FS, GS, or SS) from mem80, and sets up its descriptor.

#### B.4.277 `RSLDT`: Restore Segment Register and Descriptor

    RSLDT m80                     ; 0F 7B /0        [486,CYRIX,SMM]

`RSLDT` restores the Local Descriptor Table (LDTR) from mem80.

#### B.4.278 `RSM`: Resume from System-Management Mode

    RSM                           ; 0F AA                [PENT]

`RSM` returns the processor to its normal operating mode when it was in System-Management Mode.

#### B.4.279 `RSQRTPS`: Packed Single-Precision FP Square Root Reciprocal

    RSQRTPS xmm1,xmm2/m128        ; 0F 52 /r        [KATMAI,SSE]

`RSQRTPS` computes the approximate reciprocals of the square roots of the packed single-precision floating-point values in the source and stores the results in xmm1. The maximum error for this approximation is: |Error| <= 1.5 x 2^-12

#### B.4.280 `RSQRTSS`: Scalar Single-Precision FP Square Root Reciprocal

    RSQRTSS xmm1,xmm2/m128        ; F3 0F 52 /r     [KATMAI,SSE]

`RSQRTSS` returns an approximation of the reciprocal of the square root of the lowest order single-precision FP value from the source, and stores it in the low doubleword of the destination register. The upper three fields of xmm1 are preserved. The maximum error for this approximation is: |Error| <= 1.5 x 2^-12

#### B.4.281 `RSTS`: Restore TSR and Descriptor

    RSTS m80                      ; 0F 7D /0        [486,CYRIX,SMM]

`RSTS` restores Task State Register (TSR) from mem80.

#### B.4.282 `SAHF`: Store AH to Flags

    SAHF                          ; 9E                   [8086]

`SAHF` sets the low byte of the flags word according to the contents of the `AH` register.

The operation of `SAHF` is:

     AH --> SF:ZF:0:AF:0:PF:1:CF

See also `LAHF` (section B.4.131).

#### B.4.283 `SAL`, `SAR`: Bitwise Arithmetic Shifts

    SAL r/m8,1                    ; D0 /4                [8086]
    SAL r/m8,CL                   ; D2 /4                [8086]
    SAL r/m8,imm8                 ; C0 /4 ib             [186]
    SAL r/m16,1                   ; o16 D1 /4            [8086]
    SAL r/m16,CL                  ; o16 D3 /4            [8086]
    SAL r/m16,imm8                ; o16 C1 /4 ib         [186]
    SAL r/m32,1                   ; o32 D1 /4            [386]
    SAL r/m32,CL                  ; o32 D3 /4            [386]
    SAL r/m32,imm8                ; o32 C1 /4 ib         [386]

    SAR r/m8,1                    ; D0 /7                [8086]
    SAR r/m8,CL                   ; D2 /7                [8086]
    SAR r/m8,imm8                 ; C0 /7 ib             [186]
    SAR r/m16,1                   ; o16 D1 /7            [8086]
    SAR r/m16,CL                  ; o16 D3 /7            [8086]
    SAR r/m16,imm8                ; o16 C1 /7 ib         [186]
    SAR r/m32,1                   ; o32 D1 /7            [386]
    SAR r/m32,CL                  ; o32 D3 /7            [386]
    SAR r/m32,imm8                ; o32 C1 /7 ib         [386]

`SAL` and `SAR` perform an arithmetic shift operation on the given source/destination (first) operand. The vacated bits are filled with zero for `SAL`, and with copies of the original high bit of the source operand for `SAR`.

`SAL` is a synonym for `SHL` (see section B.4.290). NASM will assemble either one to the same code, but NDISASM will always disassemble that code as `SHL`.

The number of bits to shift by is given by the second operand. Only the bottom five bits of the shift count are considered by processors above the 8086.

You can force the longer (286 and upwards, beginning with a `C1` byte) form of `SAL foo,1` by using a `BYTE` prefix: `SAL foo,BYTE 1`. Similarly with `SAR`.

#### B.4.284 `SALC`: Set AL from Carry Flag

    SALC                          ; D6                  [8086,UNDOC]

`SALC` is an early undocumented instruction similar in concept to `SETcc` (section B.4.287). Its function is to set `AL` to zero if the carry flag is clear, or to `0xFF` if it is set.

#### B.4.285 `SBB`: Subtract with Borrow

    SBB r/m8,reg8                 ; 18 /r                [8086]
    SBB r/m16,reg16               ; o16 19 /r            [8086]
    SBB r/m32,reg32               ; o32 19 /r            [386]

    SBB reg8,r/m8                 ; 1A /r                [8086]
    SBB reg16,r/m16               ; o16 1B /r            [8086]
    SBB reg32,r/m32               ; o32 1B /r            [386]

    SBB r/m8,imm8                 ; 80 /3 ib             [8086]
    SBB r/m16,imm16               ; o16 81 /3 iw         [8086]
    SBB r/m32,imm32               ; o32 81 /3 id         [386]

    SBB r/m16,imm8                ; o16 83 /3 ib         [8086]
    SBB r/m32,imm8                ; o32 83 /3 ib         [386]

    SBB AL,imm8                   ; 1C ib                [8086]
    SBB AX,imm16                  ; o16 1D iw            [8086]
    SBB EAX,imm32                 ; o32 1D id            [386]

`SBB` performs integer subtraction: it subtracts its second operand, plus the value of the carry flag, from its first, and leaves the result in its destination (first) operand. The flags are set according to the result of the operation: in particular, the carry flag is affected and can be used by a subsequent `SBB` instruction.

In the forms with an 8-bit immediate second operand and a longer first operand, the second operand is considered to be signed, and is sign-extended to the length of the first operand. In these cases, the `BYTE` qualifier is necessary to force NASM to generate this form of the instruction.

To subtract one number from another without also subtracting the contents of the carry flag, use `SUB` (section B.4.305).

#### B.4.286 `SCASB`, `SCASW`, `SCASD`: Scan String

    SCASB                         ; AE                   [8086]
    SCASW                         ; o16 AF               [8086]
    SCASD                         ; o32 AF               [386]

`SCASB` compares the byte in `AL` with the byte at `[ES:DI]` or `[ES:EDI]`, and sets the flags accordingly. It then increments or decrements (depending on the direction flag: increments if the flag is clear, decrements if it is set) `DI` (or `EDI`).

The register used is `DI` if the address size is 16 bits, and `EDI` if it is 32 bits. If you need to use an address size not equal to the current `BITS` setting, you can use an explicit `a16` or `a32` prefix.

Segment override prefixes have no effect for this instruction: the use of `ES` for the load from `[DI]` or `[EDI]` cannot be overridden.

`SCASW` and `SCASD` work in the same way, but they compare a word to `AX` or a doubleword to `EAX` instead of a byte to `AL`, and increment or decrement the addressing registers by 2 or 4 instead of 1.

The `REPE` and `REPNE` prefixes (equivalently, `REPZ` and `REPNZ`) may be used to repeat the instruction up to `CX` (or `ECX` - again, the address size chooses which) times until the first unequal or equal byte is found.

#### B.4.287 `SETcc`: Set Register from Condition

    SETcc r/m8                    ; 0F 90+cc /2          [386]

`SETcc` sets the given 8-bit operand to zero if its condition is not satisfied, and to 1 if it is.

#### B.4.288 `SFENCE`: Store Fence

    SFENCE                 ; 0F AE /7               [KATMAI]

`SFENCE` performs a serialising operation on all writes to memory that were issued before the `SFENCE` instruction. This guarantees that all memory writes before the `SFENCE` instruction are visible before any writes after the `SFENCE` instruction.

`SFENCE` is ordered respective to other `SFENCE` instruction, `MFENCE`, any memory write and any other serialising instruction (such as `CPUID`).

Weakly ordered memory types can be used to achieve higher processor performance through such techniques as out-of-order issue, write-combining, and write-collapsing. The degree to which a consumer of data recognizes or knows that the data is weakly ordered varies among applications and may be unknown to the producer of this data. The `SFENCE` instruction provides a performance-efficient way of insuring store ordering between routines that produce weakly-ordered results and routines that consume this data.

`SFENCE` uses the following ModRM encoding:

              Mod (7:6)        = 11B
              Reg/Opcode (5:3) = 111B
              R/M (2:0)        = 000B

All other ModRM encodings are defined to be reserved, and use of these encodings risks incompatibility with future processors.

See also `LFENCE` (section B.4.137) and `MFENCE` (section B.4.151).

#### B.4.289 `SGDT`, `SIDT`, `SLDT`: Store Descriptor Table Pointers

    SGDT mem                      ; 0F 01 /0             [286,PRIV]
    SIDT mem                      ; 0F 01 /1             [286,PRIV]
    SLDT r/m16                    ; 0F 00 /0             [286,PRIV]

`SGDT` and `SIDT` both take a 6-byte memory area as an operand: they store the contents of the GDTR (global descriptor table register) or IDTR (interrupt descriptor table register) into that area as a 32-bit linear address and a 16-bit size limit from that area (in that order). These are the only instructions which directly use linear addresses, rather than segment/offset pairs.

`SLDT` stores the segment selector corresponding to the LDT (local descriptor table) into the given operand.

See also `LGDT`, `LIDT` and `LLDT` (section B.4.138).

#### B.4.290 `SHL`, `SHR`: Bitwise Logical Shifts

    SHL r/m8,1                    ; D0 /4                [8086]
    SHL r/m8,CL                   ; D2 /4                [8086]
    SHL r/m8,imm8                 ; C0 /4 ib             [186]
    SHL r/m16,1                   ; o16 D1 /4            [8086]
    SHL r/m16,CL                  ; o16 D3 /4            [8086]
    SHL r/m16,imm8                ; o16 C1 /4 ib         [186]
    SHL r/m32,1                   ; o32 D1 /4            [386]
    SHL r/m32,CL                  ; o32 D3 /4            [386]
    SHL r/m32,imm8                ; o32 C1 /4 ib         [386]

    SHR r/m8,1                    ; D0 /5                [8086]
    SHR r/m8,CL                   ; D2 /5                [8086]
    SHR r/m8,imm8                 ; C0 /5 ib             [186]
    SHR r/m16,1                   ; o16 D1 /5            [8086]
    SHR r/m16,CL                  ; o16 D3 /5            [8086]
    SHR r/m16,imm8                ; o16 C1 /5 ib         [186]
    SHR r/m32,1                   ; o32 D1 /5            [386]
    SHR r/m32,CL                  ; o32 D3 /5            [386]
    SHR r/m32,imm8                ; o32 C1 /5 ib         [386]

`SHL` and `SHR` perform a logical shift operation on the given source/destination (first) operand. The vacated bits are filled with zero.

A synonym for `SHL` is `SAL` (see section B.4.283). NASM will assemble either one to the same code, but NDISASM will always disassemble that code as `SHL`.

The number of bits to shift by is given by the second operand. Only the bottom five bits of the shift count are considered by processors above the 8086.

You can force the longer (286 and upwards, beginning with a `C1` byte) form of `SHL foo,1` by using a `BYTE` prefix: `SHL foo,BYTE 1`. Similarly with `SHR`.

#### B.4.291 `SHLD`, `SHRD`: Bitwise Double-Precision Shifts

    SHLD r/m16,reg16,imm8         ; o16 0F A4 /r ib      [386]
    SHLD r/m16,reg32,imm8         ; o32 0F A4 /r ib      [386]
    SHLD r/m16,reg16,CL           ; o16 0F A5 /r         [386]
    SHLD r/m16,reg32,CL           ; o32 0F A5 /r         [386]

    SHRD r/m16,reg16,imm8         ; o16 0F AC /r ib      [386]
    SHRD r/m32,reg32,imm8         ; o32 0F AC /r ib      [386]
    SHRD r/m16,reg16,CL           ; o16 0F AD /r         [386]
    SHRD r/m32,reg32,CL           ; o32 0F AD /r         [386]

- `SHLD` performs a double-precision left shift. It notionally places its second operand to the right of its first, then shifts the entire bit string thus generated to the left by a number of bits specified in the third operand. It then updates only the first operand according to the result of this. The second operand is not modified.
- `SHRD` performs the corresponding right shift: it notionally places the second operand to the left of the first, shifts the whole bit string right, and updates only the first operand.

For example, if `EAX` holds `0x01234567` and `EBX` holds `0x89ABCDEF`, then the instruction `SHLD EAX,EBX,4` would update `EAX` to hold `0x12345678`. Under the same conditions, `SHRD EAX,EBX,4` would update `EAX` to hold `0xF0123456`.

The number of bits to shift by is given by the third operand. Only the bottom five bits of the shift count are considered.

#### B.4.292 `SHUFPD`: Shuffle Packed Double-Precision FP Values

    SHUFPD xmm1,xmm2/m128,imm8    ; 66 0F C6 /r ib  [WILLAMETTE,SSE2]

`SHUFPD` moves one of the packed double-precision FP values from the destination operand into the low quadword of the destination operand; the upper quadword is generated by moving one of the double-precision FP values from the source operand into the destination. The select (third) operand selects which of the values are moved to the destination register.

The select operand is an 8-bit immediate: bit 0 selects which value is moved from the destination operand to the result (where 0 selects the low quadword and 1 selects the high quadword) and bit 1 selects which value is moved from the source operand to the result. Bits 2 through 7 of the shuffle operand are reserved.

#### B.4.293 `SHUFPS`: Shuffle Packed Single-Precision FP Values

    SHUFPS xmm1,xmm2/m128,imm8    ; 0F C6 /r ib     [KATMAI,SSE]

`SHUFPS` moves two of the packed single-precision FP values from the destination operand into the low quadword of the destination operand; the upper quadword is generated by moving two of the single-precision FP values from the source operand into the destination. The select (third) operand selects which of the values are moved to the destination register.

The select operand is an 8-bit immediate: bits 0 and 1 select the value to be moved from the destination operand the low doubleword of the result, bits 2 and 3 select the value to be moved from the destination operand the second doubleword of the result, bits 4 and 5 select the value to be moved from the source operand the third doubleword of the result, and bits 6 and 7 select the value to be moved from the source operand to the high doubleword of the result.

#### B.4.294 `SMI`: System Management Interrupt

    SMI                           ; F1                   [386,UNDOC]

`SMI` puts some AMD processors into SMM mode. It is available on some 386 and 486 processors, and is only available when DR7 bit 12 is set, otherwise it generates an Int 1.

#### B.4.295 `SMINT`, `SMINTOLD`: Software SMM Entry (CYRIX)

    SMINT                         ; 0F 38                [PENT,CYRIX]
    SMINTOLD                      ; 0F 7E                [486,CYRIX]

`SMINT` puts the processor into SMM mode. The CPU state information is saved in the SMM memory header, and then execution begins at the SMM base address.

`SMINTOLD` is the same as `SMINT`, but was the opcode used on the 486.

This pair of opcodes are specific to the Cyrix and compatible range of processors (Cyrix, IBM, Via).

#### B.4.296 `SMSW`: Store Machine Status Word

    SMSW r/m16                    ; 0F 01 /4             [286,PRIV]

`SMSW` stores the bottom half of the `CR0` control register (or the Machine Status Word, on 286 processors) into the destination operand. See also `LMSW` (section B.4.139).

For 32-bit code, this would use the low 16-bits of the specified register (or a 16bit memory location), without needing an operand size override byte.

#### B.4.297 `SQRTPD`: Packed Double-Precision FP Square Root

    SQRTPD xmm1,xmm2/m128         ; 66 0F 51 /r     [WILLAMETTE,SSE2]

`SQRTPD` calculates the square root of the packed double-precision FP value from the source operand, and stores the double-precision results in the destination register.

#### B.4.298 `SQRTPS`: Packed Single-Precision FP Square Root

    SQRTPS xmm1,xmm2/m128         ; 0F 51 /r        [KATMAI,SSE]

`SQRTPS` calculates the square root of the packed single-precision FP value from the source operand, and stores the single-precision results in the destination register.

#### B.4.299 `SQRTSD`: Scalar Double-Precision FP Square Root

    SQRTSD xmm1,xmm2/m128         ; F2 0F 51 /r     [WILLAMETTE,SSE2]

`SQRTSD` calculates the square root of the low-order double-precision FP value from the source operand, and stores the double-precision result in the destination register. The high-quadword remains unchanged.

#### B.4.300 `SQRTSS`: Scalar Single-Precision FP Square Root

    SQRTSS xmm1,xmm2/m128         ; F3 0F 51 /r     [KATMAI,SSE]

`SQRTSS` calculates the square root of the low-order single-precision FP value from the source operand, and stores the single-precision result in the destination register. The three high doublewords remain unchanged.

#### B.4.301 `STC`, `STD`, `STI`: Set Flags

    STC                           ; F9                   [8086]
    STD                           ; FD                   [8086]
    STI                           ; FB                   [8086]

These instructions set various flags. `STC` sets the carry flag; `STD` sets the direction flag; and `STI` sets the interrupt flag (thus enabling interrupts).

To clear the carry, direction, or interrupt flags, use the `CLC`, `CLD` and `CLI` instructions (section B.4.20). To invert the carry flag, use `CMC` (section B.4.22).

#### B.4.302 `STMXCSR`: Store Streaming SIMD Extension Control/Status

    STMXCSR m32                   ; 0F AE /3        [KATMAI,SSE]

`STMXCSR` stores the contents of the `MXCSR` control/status register to the specified memory location. `MXCSR` is used to enable masked/unmasked exception handling, to set rounding modes, to set flush-to-zero mode, and to view exception status flags. The reserved bits in the `MXCSR` register are stored as 0s.

For details of the `MXCSR` register, see the Intel processor docs.

See also `LDMXCSR` (section B.4.133).

#### B.4.303 `STOSB`, `STOSW`, `STOSD`: Store Byte to String

    STOSB                         ; AA                   [8086]
    STOSW                         ; o16 AB               [8086]
    STOSD                         ; o32 AB               [386]

`STOSB` stores the byte in `AL` at `[ES:DI]` or `[ES:EDI]`, and sets the flags accordingly. It then increments or decrements (depending on the direction flag: increments if the flag is clear, decrements if it is set) `DI` (or `EDI`).

The register used is `DI` if the address size is 16 bits, and `EDI` if it is 32 bits. If you need to use an address size not equal to the current `BITS` setting, you can use an explicit `a16` or `a32` prefix.

Segment override prefixes have no effect for this instruction: the use of `ES` for the store to `[DI]` or `[EDI]` cannot be overridden.

`STOSW` and `STOSD` work in the same way, but they store the word in `AX` or the doubleword in `EAX` instead of the byte in `AL`, and increment or decrement the addressing registers by 2 or 4 instead of 1.

The `REP` prefix may be used to repeat the instruction `CX` (or `ECX` - again, the address size chooses which) times.

#### B.4.304 `STR`: Store Task Register

    STR r/m16                     ; 0F 00 /1             [286,PRIV]

`STR` stores the segment selector corresponding to the contents of the Task Register into its operand. When the operand size is a 16-bit register, the upper 16-bits are cleared to 0s. When the destination operand is a memory location, 16 bits are written regardless of the operand size.

#### B.4.305 `SUB`: Subtract Integers

    SUB r/m8,reg8                 ; 28 /r                [8086]
    SUB r/m16,reg16               ; o16 29 /r            [8086]
    SUB r/m32,reg32               ; o32 29 /r            [386]

    SUB reg8,r/m8                 ; 2A /r                [8086]
    SUB reg16,r/m16               ; o16 2B /r            [8086]
    SUB reg32,r/m32               ; o32 2B /r            [386]

    SUB r/m8,imm8                 ; 80 /5 ib             [8086]
    SUB r/m16,imm16               ; o16 81 /5 iw         [8086]
    SUB r/m32,imm32               ; o32 81 /5 id         [386]

    SUB r/m16,imm8                ; o16 83 /5 ib         [8086]
    SUB r/m32,imm8                ; o32 83 /5 ib         [386]

    SUB AL,imm8                   ; 2C ib                [8086]
    SUB AX,imm16                  ; o16 2D iw            [8086]
    SUB EAX,imm32                 ; o32 2D id            [386]

`SUB` performs integer subtraction: it subtracts its second operand from its first, and leaves the result in its destination (first) operand. The flags are set according to the result of the operation: in particular, the carry flag is affected and can be used by a subsequent `SBB` instruction (section B.4.285).

In the forms with an 8-bit immediate second operand and a longer first operand, the second operand is considered to be signed, and is sign-extended to the length of the first operand. In these cases, the `BYTE` qualifier is necessary to force NASM to generate this form of the instruction.

#### B.4.306 `SUBPD`: Packed Double-Precision FP Subtract

    SUBPD xmm1,xmm2/m128          ; 66 0F 5C /r     [WILLAMETTE,SSE2]

`SUBPD` subtracts the packed double-precision FP values of the source operand from those of the destination operand, and stores the result in the destination operation.

#### B.4.307 `SUBPS`: Packed Single-Precision FP Subtract

    SUBPS xmm1,xmm2/m128          ; 0F 5C /r        [KATMAI,SSE]

`SUBPS` subtracts the packed single-precision FP values of the source operand from those of the destination operand, and stores the result in the destination operation.

#### B.4.308 `SUBSD`: Scalar Single-FP Subtract

    SUBSD xmm1,xmm2/m128          ; F2 0F 5C /r     [WILLAMETTE,SSE2]

`SUBSD` subtracts the low-order double-precision FP value of the source operand from that of the destination operand, and stores the result in the destination operation. The high quadword is unchanged.

#### B.4.309 `SUBSS`: Scalar Single-FP Subtract

    SUBSS xmm1,xmm2/m128          ; F3 0F 5C /r     [KATMAI,SSE]

`SUBSS` subtracts the low-order single-precision FP value of the source operand from that of the destination operand, and stores the result in the destination operation. The three high doublewords are unchanged.

#### B.4.310 `SVDC`: Save Segment Register and Descriptor

    SVDC m80,segreg               ; 0F 78 /r        [486,CYRIX,SMM]

`SVDC` saves a segment register (DS, ES, FS, GS, or SS) and its descriptor to mem80.

#### B.4.311 `SVLDT`: Save LDTR and Descriptor

    SVLDT m80                     ; 0F 7A /0        [486,CYRIX,SMM]

`SVLDT` saves the Local Descriptor Table (LDTR) to mem80.

#### B.4.312 `SVTS`: Save TSR and Descriptor

    SVTS m80                      ; 0F 7C /0        [486,CYRIX,SMM]

`SVTS` saves the Task State Register (TSR) to mem80.

#### B.4.313 `SYSCALL`: Call Operating System

    SYSCALL                       ; 0F 05                [P6,AMD]

`SYSCALL` provides a fast method of transferring control to a fixed entry point in an operating system.

- The `EIP` register is copied into the `ECX` register.
- Bits [31-0] of the 64-bit SYSCALL/SYSRET Target Address Register (`STAR`) are copied into the `EIP` register.
- Bits [47-32] of the `STAR` register specify the selector that is copied into the `CS` register.
- Bits [47-32]+1000b of the `STAR` register specify the selector that is copied into the SS register.

The `CS` and `SS` registers should not be modified by the operating system between the execution of the `SYSCALL` instruction and its corresponding `SYSRET` instruction.

For more information, see the `SYSCALL and SYSRET Instruction Specification` (AMD document number 21086.pdf).

#### B.4.314 `SYSENTER`: Fast System Call

    SYSENTER                      ; 0F 34                [P6]

`SYSENTER` executes a fast call to a level 0 system procedure or routine. Before using this instruction, various MSRs need to be set up:

- `SYSENTER_CS_MSR` contains the 32-bit segment selector for the privilege level 0 code segment. (This value is also used to compute the segment selector of the privilege level 0 stack segment.)
- `SYSENTER_EIP_MSR` contains the 32-bit offset into the privilege level 0 code segment to the first instruction of the selected operating procedure or routine.
- `SYSENTER_ESP_MSR` contains the 32-bit stack pointer for the privilege level 0 stack.

`SYSENTER` performs the following sequence of operations:

- Loads the segment selector from the `SYSENTER_CS_MSR` into the `CS` register.
- Loads the instruction pointer from the `SYSENTER_EIP_MSR` into the `EIP` register.
- Adds 8 to the value in `SYSENTER_CS_MSR` and loads it into the `SS` register.
- Loads the stack pointer from the `SYSENTER_ESP_MSR` into the `ESP` register.
- Switches to privilege level 0.
- Clears the `VM` flag in the `EFLAGS` register, if the flag is set.
- Begins executing the selected system procedure.

In particular, note that this instruction des not save the values of `CS` or `(E)IP`. If you need to return to the calling code, you need to write your code to cater for this.

For more information, see the Intel Architecture Software Developer's Manual, Volume 2.

#### B.4.315 `SYSEXIT`: Fast Return From System Call

    SYSEXIT                       ; 0F 35                [P6,PRIV]

`SYSEXIT` executes a fast return to privilege level 3 user code. This instruction is a companion instruction to the `SYSENTER` instruction, and can only be executed by privilege level 0 code. Various registers need to be set up before calling this instruction:

- `SYSENTER_CS_MSR` contains the 32-bit segment selector for the privilege level 0 code segment in which the processor is currently executing. (This value is used to compute the segment selectors for the privilege level 3 code and stack segments.)
- `EDX` contains the 32-bit offset into the privilege level 3 code segment to the first instruction to be executed in the user code.
- `ECX` contains the 32-bit stack pointer for the privilege level 3 stack.

`SYSEXIT` performs the following sequence of operations:

- Adds 16 to the value in `SYSENTER_CS_MSR` and loads the sum into the `CS` selector register.
- Loads the instruction pointer from the `EDX` register into the `EIP` register.
- Adds 24 to the value in `SYSENTER_CS_MSR` and loads the sum into the `SS` selector register.
- Loads the stack pointer from the `ECX` register into the `ESP` register.
- Switches to privilege level 3.
- Begins executing the user code at the `EIP` address.

For more information on the use of the `SYSENTER` and `SYSEXIT` instructions, see the Intel Architecture Software Developer's Manual, Volume 2.

#### B.4.316 `SYSRET`: Return From Operating System

    SYSRET                        ; 0F 07                [P6,AMD,PRIV]

`SYSRET` is the return instruction used in conjunction with the `SYSCALL` instruction to provide fast entry/exit to an operating system.

- The `ECX` register, which points to the next sequential instruction after the corresponding `SYSCALL` instruction, is copied into the `EIP` register.
- Bits [63-48] of the `STAR` register specify the selector that is copied into the `CS` register.
- Bits [63-48]+1000b of the `STAR` register specify the selector that is copied into the `SS` register.
- Bits [1-0] of the `SS` register are set to 11b (RPL of 3) regardless of the value of bits [49-48] of the `STAR` register.

The `CS` and `SS` registers should not be modified by the operating system between the execution of the `SYSCALL` instruction and its corresponding `SYSRET` instruction.

For more information, see the `SYSCALL and SYSRET Instruction Specification` (AMD document number 21086.pdf).

#### B.4.317 `TEST`: Test Bits (notional bitwise AND)

    TEST r/m8,reg8                ; 84 /r                [8086]
    TEST r/m16,reg16              ; o16 85 /r            [8086]
    TEST r/m32,reg32              ; o32 85 /r            [386]

    TEST r/m8,imm8                ; F6 /0 ib             [8086]
    TEST r/m16,imm16              ; o16 F7 /0 iw         [8086]
    TEST r/m32,imm32              ; o32 F7 /0 id         [386]

    TEST AL,imm8                  ; A8 ib                [8086]
    TEST AX,imm16                 ; o16 A9 iw            [8086]
    TEST EAX,imm32                ; o32 A9 id            [386]

`TEST` performs a `mental' bitwise AND of its two operands, and affects the flags as if the operation had taken place, but does not store the result of the operation anywhere.

#### B.4.318 `UCOMISD`: Unordered Scalar Double-Precision FP compare and set EFLAGS

    UCOMISD xmm1,xmm2/m128        ; 66 0F 2E /r     [WILLAMETTE,SSE2]

`UCOMISD` compares the low-order double-precision FP numbers in the two operands, and sets the `ZF`, `PF` and `CF` bits in the `EFLAGS` register. In addition, the `OF`, `SF` and `AF` bits in the `EFLAGS` register are zeroed out. The unordered predicate (`ZF`, `PF` and `CF` all set) is returned if either source operand is a `NaN` (`qNaN` or `sNaN`).

#### B.4.319 `UCOMISS`: Unordered Scalar Single-Precision FP compare and set EFLAGS

    UCOMISS xmm1,xmm2/m128        ; 0F 2E /r        [KATMAI,SSE]

`UCOMISS` compares the low-order single-precision FP numbers in the two operands, and sets the `ZF`, `PF` and `CF` bits in the `EFLAGS` register. In addition, the `OF`, `SF` and `AF` bits in the `EFLAGS` register are zeroed out. The unordered predicate (`ZF`, `PF` and `CF` all set) is returned if either source operand is a `NaN` (`qNaN` or `sNaN`).

#### B.4.320 `UD0`, `UD1`, `UD2`: Undefined Instruction

    UD0                           ; 0F FF                [186,UNDOC]
    UD1                           ; 0F B9                [186,UNDOC]
    UD2                           ; 0F 0B                [186]

`UDx` can be used to generate an invalid opcode exception, for testing purposes.

`UD0` is specifically documented by AMD as being reserved for this purpose.

`UD1` is documented by Intel as being available for this purpose.

`UD2` is specifically documented by Intel as being reserved for this purpose. Intel document this as the preferred method of generating an invalid opcode exception.

All these opcodes can be used to generate invalid opcode exceptions on all currently available processors.

#### B.4.321 `UMOV`: User Move Data

    UMOV r/m8,reg8                ; 0F 10 /r             [386,UNDOC]
    UMOV r/m16,reg16              ; o16 0F 11 /r         [386,UNDOC]
    UMOV r/m32,reg32              ; o32 0F 11 /r         [386,UNDOC]

    UMOV reg8,r/m8                ; 0F 12 /r             [386,UNDOC]
    UMOV reg16,r/m16              ; o16 0F 13 /r         [386,UNDOC]
    UMOV reg32,r/m32              ; o32 0F 13 /r         [386,UNDOC]

This undocumented instruction is used by in-circuit emulators to access user memory (as opposed to host memory). It is used just like an ordinary memory/register or register/register `MOV` instruction, but accesses user space.

This instruction is only available on some AMD and IBM 386 and 486 processors.

#### B.4.322 `UNPCKHPD`: Unpack and Interleave High Packed Double-Precision FP Values

    UNPCKHPD xmm1,xmm2/m128       ; 66 0F 15 /r     [WILLAMETTE,SSE2]

`UNPCKHPD` performs an interleaved unpack of the high-order data elements of the source and destination operands, saving the result in `xmm1`. It ignores the lower half of the sources.

The operation of this instruction is:

       dst[63-0]   := dst[127-64];
       dst[127-64] := src[127-64].

#### B.4.323 `UNPCKHPS`: Unpack and Interleave High Packed Single-Precision FP Values

    UNPCKHPS xmm1,xmm2/m128       ; 0F 15 /r        [KATMAI,SSE]

`UNPCKHPS` performs an interleaved unpack of the high-order data elements of the source and destination operands, saving the result in `xmm1`. It ignores the lower half of the sources.

The operation of this instruction is:

       dst[31-0]   := dst[95-64];
       dst[63-32]  := src[95-64];
       dst[95-64]  := dst[127-96];
       dst[127-96] := src[127-96].

#### B.4.324 `UNPCKLPD`: Unpack and Interleave Low Packed Double-Precision FP Data

    UNPCKLPD xmm1,xmm2/m128       ; 66 0F 14 /r     [WILLAMETTE,SSE2]

`UNPCKLPD` performs an interleaved unpack of the low-order data elements of the source and destination operands, saving the result in `xmm1`. It ignores the lower half of the sources.

The operation of this instruction is:

       dst[63-0]   := dst[63-0];
       dst[127-64] := src[63-0].

#### B.4.325 `UNPCKLPS`: Unpack and Interleave Low Packed Single-Precision FP Data

    UNPCKLPS xmm1,xmm2/m128       ; 0F 14 /r        [KATMAI,SSE]

`UNPCKLPS` performs an interleaved unpack of the low-order data elements of the source and destination operands, saving the result in `xmm1`. It ignores the lower half of the sources.

The operation of this instruction is:

       dst[31-0]   := dst[31-0];
       dst[63-32]  := src[31-0];
       dst[95-64]  := dst[63-32];
       dst[127-96] := src[63-32].

#### B.4.326 `VERR`, `VERW`: Verify Segment Readability/Writability

    VERR r/m16                    ; 0F 00 /4             [286,PRIV]

    VERW r/m16                    ; 0F 00 /5             [286,PRIV]

- `VERR` sets the zero flag if the segment specified by the selector in its operand can be read from at the current privilege level. Otherwise it is cleared.
- `VERW` sets the zero flag if the segment can be written.

#### B.4.327 `WAIT`: Wait for Floating-Point Processor

    WAIT                          ; 9B                   [8086]
    FWAIT                         ; 9B                   [8086]

`WAIT`, on 8086 systems with a separate 8087 FPU, waits for the FPU to have finished any operation it is engaged in before continuing main processor operations, so that (for example) an FPU store to main memory can be guaranteed to have completed before the CPU tries to read the result back out.

On higher processors, `WAIT` is unnecessary for this purpose, and it has the alternative purpose of ensuring that any pending unmasked FPU exceptions have happened before execution continues.

#### B.4.328 `WBINVD`: Write Back and Invalidate Cache

    WBINVD                        ; 0F 09                [486]

`WBINVD` invalidates and empties the processor's internal caches, and causes the processor to instruct external caches to do the same. It writes the contents of the caches back to memory first, so no data is lost. To flush the caches quickly without bothering to write the data back first, use `INVD` (section B.4.125).

#### B.4.329 `WRMSR`: Write Model-Specific Registers

    WRMSR                         ; 0F 30                [PENT]

`WRMSR` writes the value in `EDX:EAX` to the processor Model-Specific Register (MSR) whose index is stored in `ECX`. See also `RDMSR` (section B.4.270).

#### B.4.330 `WRSHR`: Write SMM Header Pointer Register

    WRSHR r/m32                   ; 0F 37 /0        [386,CYRIX,SMM]

`WRSHR` loads the contents of either a 32-bit memory location or a 32-bit register into the SMM header pointer register.

See also `RDSHR` (section B.4.272).

#### B.4.331 `XADD`: Exchange and Add

    XADD r/m8,reg8                ; 0F C0 /r             [486]
    XADD r/m16,reg16              ; o16 0F C1 /r         [486]
    XADD r/m32,reg32              ; o32 0F C1 /r         [486]

`XADD` exchanges the values in its two operands, and then adds them together and writes the result into the destination (first) operand. This instruction can be used with a `LOCK` prefix for multi-processor synchronisation purposes.

#### B.4.332 `XBTS`: Extract Bit String

    XBTS reg16,r/m16              ; o16 0F A6 /r         [386,UNDOC]
    XBTS reg32,r/m32              ; o32 0F A6 /r         [386,UNDOC]

The implied operation of this instruction is:

    XBTS r/m16,reg16,AX,CL
    XBTS r/m32,reg32,EAX,CL

Writes a bit string from the source operand to the destination. `CL` indicates the number of bits to be copied, and `(E)AX` indicates the low order bit offset in the source. The bits are written to the low order bits of the destination register. For example, if `CL` is set to 4 and `AX` (for 16-bit code) is set to 5, bits 5-8 of `src` will be copied to bits 0-3 of `dst`. This instruction is very poorly documented, and I have been unable to find any official source of documentation on it.

`XBTS` is supported only on the early Intel 386s, and conflicts with the opcodes for `CMPXCHG486` (on early Intel 486s). NASM supports it only for completeness. Its counterpart is `IBTS` (see section B.4.116).

#### B.4.333 `XCHG`: Exchange

    XCHG reg8,r/m8                ; 86 /r                [8086]
    XCHG reg16,r/m8               ; o16 87 /r            [8086]
    XCHG reg32,r/m32              ; o32 87 /r            [386]

    XCHG r/m8,reg8                ; 86 /r                [8086]
    XCHG r/m16,reg16              ; o16 87 /r            [8086]
    XCHG r/m32,reg32              ; o32 87 /r            [386]

    XCHG AX,reg16                 ; o16 90+r             [8086]
    XCHG EAX,reg32                ; o32 90+r             [386]
    XCHG reg16,AX                 ; o16 90+r             [8086]
    XCHG reg32,EAX                ; o32 90+r             [386]

`XCHG` exchanges the values in its two operands. It can be used with a `LOCK` prefix for purposes of multi-processor synchronisation.

`XCHG AX,AX` or `XCHG EAX,EAX` (depending on the `BITS` setting) generates the opcode `90h`, and so is a synonym for `NOP` (section B.4.190).

#### B.4.334 `XLATB`: Translate Byte in Lookup Table

    XLAT                          ; D7                   [8086]
    XLATB                         ; D7                   [8086]

`XLATB` adds the value in `AL`, treated as an unsigned byte, to `BX` or `EBX`, and loads the byte from the resulting address (in the segment specified by `DS`) back into `AL`.

The base register used is `BX` if the address size is 16 bits, and `EBX` if it is 32 bits. If you need to use an address size not equal to the current `BITS` setting, you can use an explicit `a16` or `a32` prefix.

The segment register used to load from `[BX+AL]` or `[EBX+AL]` can be overridden by using a segment register name as a prefix (for example, `es xlatb`).

#### B.4.335 `XOR`: Bitwise Exclusive OR

    XOR r/m8,reg8                 ; 30 /r                [8086]
    XOR r/m16,reg16               ; o16 31 /r            [8086]
    XOR r/m32,reg32               ; o32 31 /r            [386]

    XOR reg8,r/m8                 ; 32 /r                [8086]
    XOR reg16,r/m16               ; o16 33 /r            [8086]
    XOR reg32,r/m32               ; o32 33 /r            [386]

    XOR r/m8,imm8                 ; 80 /6 ib             [8086]
    XOR r/m16,imm16               ; o16 81 /6 iw         [8086]
    XOR r/m32,imm32               ; o32 81 /6 id         [386]

    XOR r/m16,imm8                ; o16 83 /6 ib         [8086]
    XOR r/m32,imm8                ; o32 83 /6 ib         [386]

    XOR AL,imm8                   ; 34 ib                [8086]
    XOR AX,imm16                  ; o16 35 iw            [8086]
    XOR EAX,imm32                 ; o32 35 id            [386]

`XOR` performs a bitwise XOR operation between its two operands (i.e. each bit of the result is 1 if and only if exactly one of the corresponding bits of the two inputs was 1), and stores the result in the destination (first) operand.

In the forms with an 8-bit immediate second operand and a longer first operand, the second operand is considered to be signed, and is sign-extended to the length of the first operand. In these cases, the `BYTE` qualifier is necessary to force NASM to generate this form of the instruction.

The `MMX` instruction `PXOR` (see section B.4.266) performs the same operation on the 64-bit `MMX` registers.

#### B.4.336 `XORPD`: Bitwise Logical XOR of Double-Precision FP Values

    XORPD xmm1,xmm2/m128          ; 66 0F 57 /r     [WILLAMETTE,SSE2]

`XORPD` returns a bit-wise logical XOR between the source and destination operands, storing the result in the destination operand.

#### B.4.337 `XORPS`: Bitwise Logical XOR of Single-Precision FP Values

    XORPS xmm1,xmm2/m128          ; 0F 57 /r        [KATMAI,SSE]

`XORPS` returns a bit-wise logical XOR between the source and destination operands, storing the result in the destination operand.

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