User:Andlabs/68000 ROM guide

This should go in Guides but I'm not sure where

Most, if not all, game systems that use the MC68000 as a main CPU run the game as a 68000 boot ROM, meaning you'll need to understand how a 68000 boot ROM is laid out. After the boot ROM, games will usually have a console-specific header that could also be interpreted (for instance, in the Neo-Geo's case, the four entry points to the game program are here).

The 68000 Vector Table

The first $100 bytes of a ROM (at location $0) are split into 64 longword pointers to code. These are usually the target the CPU goes to in the case of an error.

$0         initial stack pointer
$4         entry point
$8         bus error handler
$C         address error handler
...........|
...........spurious exception handler
...........IRQ 0 handler; called when an IRQ level 0 is triggered
$  ...$7F      1..7
$80        trap #0 handler; called when trap #0 is executed
$84...$BC       #1...#$F
$C0...$FC  reserved for future use

Genesis/32X/Pico ROM header

This ROM header, which starts at $100, is purely for documentation purposes (though game code does look here for the checksum and region information). Each field is padded with spaces (ASCII $20).

$100-$10F  the console ID as a string; "SEGA MEGA DRIVE ", for instance.
           The "SEGA" at the start is the only mandatory part of the header.
$110-$11F  copyright information; it usually consists of the publisher code and either the build or release date
$120-$14F  the game's domestic (Japanese) name, usually as a Shift-JIS string
$150-$17F  the game's international name
           Some games (e.g. Sonic the Hedgehog) split these two into three 16-byte tuples
$180-$18D  the game's type code, serial number, and version (usually)
$18E word  the game's checksum
$190-$19F  peripheral support, one character per peripheral
$1A0 long  the ROM start address (0)
$1A4 long  the ROM end address (the byte where it ends, so $FFFFF for a 1MB ROM)
$1A8 long  the RAM start address ($FF0000)
$1AC long  the RAM end address ($FFFFFF)
$1B0 long  save RAM flags, or $20202020 (four ASCII spaces) if no save RAM
$1B4 long  save RAM start, or $20202020 (four ASCII spaces) if no save RAM
$1B8 long  save RAM end, or $20202020 (four ASCII spaces) if no save RAM
$1BC-$1C8  modem information (if the game uses the Mega Modem)
$1C8-$1EF  "notes"? typically blank
$1F0-$1FF  allowed regions; most games use a string of J, U, or E, but later games use an ASCII-encoded hex bitfield (so F -> 1111 -> all regions)

Only on exception is there anything really notable in this header (for instance, Super Hang-On).

Neo-Geo ROM header

This ROM header, which also starts at $100, is important, because it contains the four entry points that the Neo-Geo BIOS uses to jump into the game. In fact, the location at $4 will usually be the BIOS's own entry point, making it useless for reverse-engineering.

$100-$10x  the string
...