==============================================================================
  GRAPHICS ROM AND RAM      
==============================================================================

The Intellivision contains a 2K-byte Graphics ROM (GROM) and a 512-byte
Graphics RAM.  This file describes their contents.

------------------------------------------------------------------------------
  ACCESSING GROM AND GRAM
------------------------------------------------------------------------------

The Graphics ROM is mapped at $3000 - $37FF in the Intellivision's
memory map.  The Graphics RAM is mapped at $3800 - $3AFF.

GRAM and GROM are visible to the CPU during the VBLANK period (see 
"interrupts.txt" for details), and also when the display is blanked.  
GRAM and GROM are not available to the CPU during active display, as the
STIC requires these resources for generating the display.

------------------------------------------------------------------------------
  LAYOUT
------------------------------------------------------------------------------

The GROM and GRAM hold the 8x8 and 8x16 pictures that the STIC uses
for displaying background cards and MOBs onscreen.  The GROM holds 256
8x8 pictures, and the GRAM holds 64 8x8 pictures.  8x16 pictures are
constructed by using two consecutive 8x8 pictures. 

Each 8x8 picture is assigned a "card #".  Card #0 corresponds to the 
first picture in GROM.  Each card # occupies 8 bytes and starts on an
8-byte boundary.  Thus, card #0 occupies $3000 - $3007, card #1 occupies
$3008 - $300F, and so on.  Card numbers are converted to addresses
by the following formula:

        address = $3000 + card_number * 8

Altogether, the GROM and GRAM provide a total of 320 cards to programs.
Card #0 through card #255 come from GROM, and card #256 through card #319
come from GRAM.  Card #s 256 through #319 are sometimes referred to
as "GRAM card #0" through "GRAM card #63", to distinguish cards in GRAM
from cards in GROM.  The following formula converts a GRAM card number
to a GRAM address:

        address = $3800 + GRAM_card_number * 8

------------------------------------------------------------------------------
  PICTURE ENCODING IN GRAPHICS MEMORY
------------------------------------------------------------------------------

Each card stores a single 8x8 picture in a sequence of 8 bytes.  The first 
byte of the sequence holds the topmost row of pixels.  The last byte holds
the bottom row of pixels.  Bit 7 in each byte holds the leftmost column
of pixels, and bit 0 in each byte holds the rightmost column.  Pictorally:

                                    Bit #
                               7 6 5 4 3 2 1 0  
                              +---------------+
                           0  |               |
                        B  1  |               |
                        y  2  |               |
                        t  3  |               |
                        e  4  |               |
                           5  |               |
                        #  6  |               |
                           7  |               |
                              +---------------+

Suppose, for instance, we want to define a picture for the letter R.  That
picture might look like so:  ('#' indicates pixels that are "on", and '.'
indicates pixels that are "off")

                                    Bit #
                               7 6 5 4 3 2 1 0  
                              +---------------+
                           0  |# # # # # # . .|
                        B  1  |. # # . . # # .|
                        y  2  |. # # . . # # .|
                        t  3  |. # # # # # . .|
                        e  4  |. # # . . # # .|
                           5  |. # # . . # # .|
                        #  6  |. # # . . # # .|
                           7  |# # # . . # # #|
                              +---------------+

All of the "on" pixels are represented with 1 bits, and all of the "off"
pixels are represented with 0 bits.  The binary pattern for this character 
would be:

                             DECLE   %11111100
                             DECLE   %01100110
                             DECLE   %01100110
                             DECLE   %01111100
                             DECLE   %01100110
                             DECLE   %01100110
                             DECLE   %01100110
                             DECLE   %11100111

You could copy this exact bit pattern into one of the card slots in GRAM, 
and voila!  You have a stylized R.

------------------------------------------------------------------------------
  GROM PICTURE CATALOG
------------------------------------------------------------------------------

As much as I'd like to place a visual reference to all the GROM pictures 
here, I cannot.  However, if you have a copy of GROM.BIN, you can use the 
utility "show_grom" (included in SDK-1600) to generate such a reference 
yourself.  See the documentation for show_grom for details on how to 
operate it.

Although I can't provide a visual reference, I can describe its contents.
GROM card #0 through #94 correspond directly to ASCII character codes
#32 through #126.  Thus, you can convert an ASCII code to a GROM card #
by simply subtracting 32.  The reason for this is that ASCII codes below
32 are "control characters" -- these charactes have no pictures associated
with them.  Therefore, the GROM starts with the first printable ASCII 
character.

The following table illustrates the ASCII character set, and how it
corresponds to the first portion of the GROM.  Note that ASCII code #32
is the "space character".

        ASCII CODES    GROM CARDS   ASCII CHARCTERS
        -----------    ----------   -------------------------------
          32 - 47        0 - 15       ! " # $ % & ' ( ) * + , - . / 
          48 - 63       16 - 31     0 1 2 3 4 5 6 7 8 9 : ; < = > ? 
          64 - 79       32 - 47     @ A B C D E F G H I J K L M N O 
          80 - 95       48 - 63     P Q R S T U V W X Y Z [ \ ] ^ _ 
          96 - 111      64 - 79     ` a b c d e f g h i j k l m n o 
         112 - 126      80 - 95     p q r s t u v w x y z { | } ~   

The next table maps hexadecimal GROM card numbers to ASCII characters.  
It is the same information as the above table, in a slightly different 
form.  The column at the left gives the first hexadecimal digit, and
the row across the top gives the second hexadecimal digit.  For instance,
the letter A would be GROM card #$21.
 

        +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
        |x0 |x1 |x2 |x3 |x4 |x5 |x6 |x7 |x8 |x9 |xA |xB |xC |xD |xE |xF |
   +----+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
   | 0x |   | ! | " | # | $ | % | & | ' | ( | ) | * | + | , | - | . | / | 
   +----+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
   | 1x | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | : | ; | < | = | > | ? | 
   +----+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
   | 2x | @ | A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | 
   +----+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
   | 3x | P | Q | R | S | T | U | V | W | X | Y | Z | [ | \ | ] | ^ | _ | 
   +----+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
   | 4x | ` | a | b | c | d | e | f | g | h | i | j | k | l | m | n | o | 
   +----+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
   | 5x | p | q | r | s | t | u | v | w | x | y | z | { | | | } | ~ |
   +----+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+


GROM card #s above #94 contain a variety of graphic pictures (mostly 
various lines and filled areas), and also some pieces of EXEC ROM code (!).
To see what these look like, use the show_grom utility mentioned above.


------------------------------------------------------------------------------
  RESTRICTIONS
------------------------------------------------------------------------------

VBLANK 

As noted above, GROM and GRAM are only accessible during VBLANK.  The
exact timing requirements here are described in "interrupts.txt".


FOREGROUND/BACKGROUND MODE

Card #s 64 through 255 are unavailable when the STIC is in Foreground/
Background mode.  These cards are unavailable to both background cards
and MOBs.


DOUBLE-RESOLUTION MOBS

MOBs can either be single-resolution (8x8 picture) or double-resolution
(8x16 picture).  Double-resolution MOBs work by concatenating two
consecutive pictures from GRAM or GROM to make the taller 8x16 bitmap.
In this mode, the picture for the upper half must always come from an
even card #, and the picture for the lower half must always come from
an odd card #.

If an odd card # is specified for a double-resolution MOB, the LSB of
the card # is ignored.  That is, the hardware will round the card #
down to the next lower even card #.  Effectively, this makes the LSB
of the card # available for program storage when the MOB is a double-
resolution MOB.
