.MD File Format – SMD copier files

Sega Mega Drive ROM image

Many early Mega Drive dumps are in the form written by the Super Magic Drive, a game copier, usually with the extension .smd. No specification of the copier’s format was found. What follows is what emulator loaders (DGen’s) and the file command’s magic rules rely on, which is enough to convert such a file to a plain ROM; the meaning of most header bytes is unknown.

Layout

  1. 0header512 bytes
  2. 0x200block 0: ROM bytes 0x0000–0x3FFF, interleaved16 KiB
  3. 0x4200block 1: ROM bytes 0x4000–0x7FFF16 KiB
  4. …one block per 16 KiB of ROM16 KiB each
An SMD file. The data size is always a whole number of 16 KiB blocks.

Interleaving

The copier stored each 16 KiB block as two halves: the first 8 KiB holds the bytes at odd addresses and the second 8 KiB the bytes at even addresses. For byte j (0 to 8191) of block n:

rom[n × 16384 + 2j + 1] = block[j]          first half → odd addresses
rom[n × 16384 + 2j]     = block[8192 + j]   second half → even addresses
Turning an SMD block back into ROM order.

This is why the text SEGA GENESIS at ROM address 0x100 shows up in an SMD file as EAGN at offset 0x280 (512 for the header plus 0x80, the odd bytes 0x101, 0x103, 0x105, 0x107), and SEGA MEGA DRIVE as EAMG. Together with AA BB at offset 8, that is how the file command recognises an SMD image.

Converting to a plain ROM

  1. Check for AA BB at offset 8 (and, if wanted, EAGN or EAMG at 0x280).
  2. Drop the first 512 bytes.
  3. For each 16 KiB block, interleave its two halves as above.
  4. The result is an ordinary .md / .bin image whose header is at 0x100.

A plain ROM is told apart by finding SEGA at 0x100, where an SMD file has header bytes.

Sources

  • romload.c – DGen’s ROM loader: the AA BB test at offset 8, the block count and the de-interleaving loop
  • Magdir/console – the file command’s magic rules: block count, split flag, EAGN/EAMG at 0x280
  • Super Magic Drive BIOS notes – Spritesmind: the multi-file bit
  • SWC header – Archaic Ruins’ notes on the Super Wild Card header, which list the image types of the Magic Drive and related copiers