.SMS File Format

Introduction

A .sms file is a copy of the ROM in a cartridge or card for Sega’s Master System (and its Japanese forerunner, the Mark III), for playing in an emulator or writing to a flash cartridge. Game Gear games are the same kind of file, named .gg, and SG-1000 games, from the console before the Master System, are .sg. There is no header added by the dumping tool: the file is the ROM, in 16 KB pages, and the program it holds runs on a Zilog Z80. Some copiers put 512 bytes in front of the ROM, which readers skip.

Many games also carry a 16-byte Sega header, which this site reads. The export Master System and later Game Gear BIOSes will not start a cartridge without it, but Japanese consoles and many homebrew programs have none, and neither does the SG-1000.

Structure

The Z80 sees the ROM through three 16 KB slots at 0x0000, 0x4000 and 0x8000; a ROM larger than 48 KB is switched in by a mapper chip on the cartridge (see mappers). Execution starts at 0x0000, and the maskable interrupt and the non-maskable interrupt go to 0x0038 and 0x0066, so these always sit in the first page (memory map). The Sega header is the last 16 bytes of the first 8, 16 or 32 KB: the BIOS looks for it at 0x7FF0, then 0x3FF0, then 0x1FF0, and only 0x7FF0 is seen in known software. It holds the text TMR SEGA, a checksum, the product code, the version, the region and the ROM size, all described on the Sega header page.

Game Gear

A Game Gear ROM is the same format: the same Z80, the same header and checksum, the same Sega mapper. The header’s region code says which console it is for (5 Game Gear Japan, 6 Export, 7 International, against 3 and 4 for the Master System), and the Game Gear BIOS requires the TMR SEGA text but, as far as SMS Power says, does not check the region or the checksum. The differences that matter to a reader are the ports: ports 0x00 to 0x06 are Game Gear specific (the Start button, the serial port and the stereo control), and the Game Gear screen is 160 × 144 pixels where the Master System’s is 256 × 192 (the figures JSSMS uses). See ports. For that reason Game Gear ROMs share these pages with the Master System instead of having their own, as .gbc shares the Game Boy pages.

SG-1000 and SC-3000

The SG-1000 (and the SC-3000 computer built on it) has the same Z80, VDP and sound chip but no Sega header and no standard mapper in the common case. A .sg file is read as plain ROM from 0x0000. SMS Power’s pages give its port map only in part and no RAM size, and the SC-3000’s keyboard ports not at all, so these pages do not describe them. .sc files (SC-3000) are not read here: the extension is used by other formats.

Not described

The Sega header, the mappers, the memory map and the ports and VDP registers are written. What remains:

• Viewer: the explorer opens .sms, .gg and .sg files: the header found where the BIOS looks, its fields, the checksum computed the BIOS’s way, the 16 KB pages, the vectors, the mapper registers, the ports, and the code of the three slots disassembled for the Z80 with the mapper registers and the ports named. The mapper (Sega, Codemasters or Korean) and the page in each slot are your choice in the options bar; the file does not say, and nothing is guessed. It does not follow code from the entry point (all bytes are decoded as instructions), does not separate code from data, and shows one page per slot, not the pages a running program would map. It was tested on built ROMs and on six homebrew files (one with a real header), not on a commercial ROM, and has not been run in a browser by its author.
• Reference: the bits of the Game Gear ports 0x00 to 0x05, the SG-1000 and SC-3000 RAM and keyboard ports, the PSG and the VDP’s drawing, the 3-D glasses registers, light phaser and the other mappers (the MSX and Nemesis, the Janggun mapper are named on the mapper page but not modelled in the viewer), and the 512-byte copier header, whose rule here is the common convention and is not taken from a specification.

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