.SMS File Format – Mappers

Sega mapper

Used by nearly all games. Four registers sit at 0xFFFC–0xFFFF, in the top four bytes of the console’s own RAM mirror: a write goes to both the register and the RAM, a read returns only the RAM, so the last value written can be read back from there. Any ROM page can go into any slot, and up to two 16 KB pages of cartridge RAM can be mapped into the upper slots.

First kilobyte. 0x0000–0x03FF is never remapped, so it always shows the start of page 0 and the interrupt vectors stay in place; slot 0’s register applies to 0x0400–0x3FFF (it shows the last 15 KB of the chosen page).

Bank shift. Bits 1–0 of 0xFFFC add an offset to every page number written to 0xFFFD–0xFFFF: 00 adds 0, 01 adds 0x18, 10 adds 0x10, 11 adds 0x08. It takes effect only when a page register is next written, no known software uses it, and its behaviour with ROMs other than 512 KB is undefined.

Width and reset. The register is 3 bits wide on 128 KB cartridges, 5 on 512 KB and 6 on 1 MB. SMS Power says the reset state is undefined for some chips, and gives the 315-5235’s as 0xFFFC = 0, 0xFFFD = 0, 0xFFFE = 1, 0xFFFF = 2. The explorer’s code view starts from pages 0, 1 and 2 (JSSMS does the same, wrapping the number by the page count).

Codemasters mapper

Registers at 0x0000 (slot 0), 0x4000 (slot 1) and 0x8000 (slot 2); each answers a write anywhere in its own 16 KB slot. Nothing protects the first kilobyte, so the page in slot 0 replaces the interrupt vectors too, and a program has to switch it carefully. The start state is pages 0, 1 and 0 in slots 0, 1 and 2. Writes are not mirrored in RAM, so the last value cannot be read back. Ernie Els Golf has 8 KB of cartridge RAM: writing a byte with bit 7 set to 0x4000 maps the upper 8 KB of slot 2 (0xA000–0xBFFF) to it.

Korean mapper

Slots 0 and 1 are fixed to pages 0 and 1, and slot 2 (0x8000–0xBFFF) is chosen by a write to 0xA000. SMS Power’s example is Run! Dodgeball King.

Other mappers

Described by SMS Power but not modelled by the explorer, which offers the three above:

• MSX and Nemesis mappers. 8 KB pages, with registers at 0x0000 (maps 0x8000–0x9FFF), 0x0001 (0xA000–0xBFFF), 0x0002 (0x4000–0x5FFF) and 0x0003 (0x6000–0x7FFF). On the MSX variant the first 16 KB is fixed; on the Nemesis variant (only Nemesis uses it) the last 8 KB page is fixed at 0x0000–0x1FFF and the second to last at 0x2000–0x3FFF.
• Janggun mapper (Janggun-ui Adeul, Korea). 8 KB registers at 0x4000, 0x6000, 0x8000 and 0xA000, and 16 KB registers at 0xFFFE (0x4000–0x7FFF) and 0xFFFF (0x8000–0xBFFF); bits 0–5 choose the page and bit 6 reverses the bit order of reads.

What the explorer shows

The code view lists the three slots: for the Sega mapper the fixed first kilobyte and then slots 0, 1 and 2 with the pages chosen in the options bar (with page 0 in slot 0 the first two are one block); for the Codemasters mapper the three pages whole; for the Korean mapper pages 0 and 1 and the chosen page. It does not follow the program to see which pages it maps. In the disassembly the Sega mapper’s four registers are named MAP_CTRL, MAP_SLOT0, MAP_SLOT1 and MAP_SLOT2; the other mappers’ registers are ordinary addresses and are not named.

Compared with JSSMS (emulators.org), whose z80.js was run in a test beside this reader: the same page appears at the same address for any three page numbers written to 0xFFFD–0xFFFF, the first kilobyte stays on page 0, the registers read back, and the start state is pages 0, 1 and 2. JSSMS implements only the Sega mapper.

Sources

• Mappers – SMS Power
• Memory Map – SMS Power: the registers sit in the RAM mirror

Not described

Cartridge RAM size and battery, which the header does not give, and the mappers of the Korean and other unlicensed cartridges that SMS Power does not list on this page. Only the Sega mapper’s page registers are checked against a running emulator; the Codemasters and Korean mappers are given as SMS Power writes them.