.MD File Format – Vectors and ROM header

68000 vector table (0x000–0x0FF)

Sixty-four 32-bit addresses. On reset the processor loads its stack pointer and program counter from the first two, then jumps to the program counter. The others are where it goes on errors, traps and interrupts.

ROM header (0x100–0x1FF)

System type

The console itself checks only the first four characters. Later models, with the Trademark Security System (TMSS), look for SEGA there and refuse to start the cartridge otherwise. The other values matter only to flash cartridges and emulators.

Serial number

Fourteen characters, TT NNNNNNNN-RR: the software type, a space, the product number, a hyphen and the revision.

The product number is eight characters unique to the product. The revision is two digits, 00 for the first release, raised by one for each corrected version.

Checksum

Treat the ROM from 0x200 to its end as 16-bit big-endian words, add them, and keep the low 16 bits. The vectors and header are not included. Some games and emulators check it; otherwise it has no effect.

sum = 0
for offset = 0x200 to (ROM size − 2), step 2:
    sum = (sum + (rom[offset] × 256 + rom[offset + 1])) AND 0xFFFF
header[0x18E] = sum

Example: if the bytes from 0x200 were only 4E 71 4E 75 FF FF, the words would be 0x4E71 + 0x4E75 + 0xFFFF = 0x19CE5, and the checksum 0x9CE5.

Device codes

Letters in any order, padded with spaces. Most games give J, or J6 when they use the six-button pad’s extra buttons.

Extra memory

Spaces when the cartridge has no save memory. Otherwise:

Modem support

For games that used the Japanese Mega Modem, twelve characters MOxxxxyy,zww: MO, the four-character publisher code (as in the copyright field), a two-digit game number, a comma (some games use a full stop), a one-digit version (0 for the original), and two digits for region and microphone. Games with the same number can talk to each other, even from different publishers. Spaces otherwise.

Region codes

Three bytes, in one of two styles. The older style lists letters in any order, with spaces anywhere, so a reader should look at all three bytes:

The newer style is a single hexadecimal digit (0–9, A–F) at 0x1F0, whose bits name the regions:

So 4 is the Americas only, 5 Japan and the Americas, F everywhere. E is ambiguous: as a letter it means Europe; as a digit, everywhere except Japan. Plutiedev advises assuming Europe, and recommends the letters for new homebrew.

Worked example

0100  SEGA MEGA DRIVE                      system type
0110  (C)ABCD 2024.JAN                     publisher ABCD, January 2024
0120  MY GAME                         …    domestic title, spaces to 48 bytes
0150  MY GAME                         …    overseas title
0180  GM 00000001-00                       game, product 00000001, first release
018E  9C E5                                checksum 0x9CE5
0190  J6                                   3- and 6-button pads
01A0  00 00 00 00  00 0F FF FF             ROM 0x000000–0x0FFFFF (1 MiB)
01A8  00 FF 00 00  00 FF FF FF             work RAM
01B0  52 41 F8 20  00 20 00 01  00 20 3F FF   "RA", battery RAM on odd bytes,
                                           0x200001–0x203FFF (8 KiB)
01BC  (12 spaces)                          no modem
01C8  (40 spaces)                          reserved
01F0  JUE                                  all regions
01F3  (13 spaces)                          reserved

Sources

• ROM header reference – Plutiedev: every header field, the system types, serial format, checksum, device, extra memory, modem and region codes
• 68k vector table – NeoGeo Development Wiki: the 68000’s vector numbers and addresses
• Sega Mega Drive/Interrupts and VDP register notes – via search results: the Mega Drive uses only interrupt levels 2 (external), 4 (horizontal) and 6 (vertical)

The checksum example and the worked header are made up for this page.