| Assembling in MRI Compatibility Mode

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The '-M' or '--mri' option selects MRI compatibility mode. This changes the syntax and pseudo-op handling of `as` to make it compatible with the `ASM68K` assembler from Microtec Research. The exact nature of the MRI syntax will not be documented here; see the MRI manuals for more information. Note in particular that the handling of macros and macro arguments is somewhat different. The purpose of this option is to permit assembling existing MRI assembler code using `as`.
The MRI compatibility is not complete. Certain operations of the MRI assembler depend upon its object file format, and can not be supported using other object file formats. Supporting these would require enhancing each object file format individually. These are:

- global symbols in common section The m68k MRI assembler supports common sections which are merged by the linker. Other object file formats do not support this. `as` handles common sections by treating them as a single common symbol. It permits local symbols to be defined within a common section, but it can not support global symbols, since it has no way to describe them.
- complex relocations The MRI assemblers support relocations against a negated section address, and relocations which combine the start addresses of two or more sections. These are not support by other object file formats.
- `END` pseudo-op specifying start address The MRI `END` pseudo-op permits the specification of a start address. This is not supported by other object file formats. The start address may instead be specified using the '-e' option to the linker, or in a linker script.
- `IDNT`, `.ident` and `NAME` pseudo-ops The MRI `IDNT`, `.ident` and `NAME` pseudo-ops assign a module name to the output file. This is not supported by other object file formats.
- `ORG` pseudo-op The m68k MRI `ORG` pseudo-op begins an absolute section at a given address. This differs from the usual `as` `.org` pseudo-op, which changes the location within the current section. Absolute sections are not supported by other object file formats. The address of a section may be assigned within a linker script.

There are some other features of the MRI assembler which are not supported by `as`, typically either because they are difficult or because they seem of little consequence. Some of these may be supported in future releases.

- EBCDIC strings EBCDIC strings are not supported.
- packed binary coded decimal Packed binary coded decimal is not supported. This means that the `DC.P` and `DCB.P` pseudo-ops are not supported.
- `FEQU` pseudo-op The m68k `FEQU` pseudo-op is not supported.
- `NOOBJ` pseudo-op The m68k `NOOBJ` pseudo-op is not supported.
- `OPT` branch control options The m68k `OPT` branch control options - `B`, `BRS`, `BRB`, `BRL`, and `BRW` - are ignored. `as` automatically relaxes all branches, whether forward or backward, to an appropriate size, so these options serve no purpose.
- `OPT` list control options The following m68k `OPT` list control options are ignored: `C`, `CEX`, `CL`, `CRE`, `E`, `G`, `I`, `M`, `MEX`, `MC`, `MD`, `X`.
- other `OPT` options The following m68k `OPT` options are ignored: `NEST`, `O`, `OLD`, `OP`, `P`, `PCO`, `PCR`, `PCS`, `R`.
- `OPT` `D` option is default The m68k `OPT` `D` option is the default, unlike the MRI assembler. `OPT NOD` may be used to turn it off.
- `XREF` pseudo-op. The m68k `XREF` pseudo-op is ignored.
- `.debug` pseudo-op The i960 `.debug` pseudo-op is not supported.
- `.extended` pseudo-op The i960 `.extended` pseudo-op is not supported.
- `.list` pseudo-op. The various options of the i960 `.list` pseudo-op are not supported.
- `.optimize` pseudo-op The i960 `.optimize` pseudo-op is not supported.
- `.output` pseudo-op The i960 `.output` pseudo-op is not supported.
- `.setreal` pseudo-op The i960 `.setreal` pseudo-op is not supported.