INES Mapper 001

iNES Mapper 001 is used to designate the SxROM boardset, all of which use the Nintendo MMC1.

This has proven to be problematic for boards (such as SOROM, SUROM and SXROM) which use the upper CHR bank select lines to select additional PRG ROM or PRG RAM data; games which use SOROM or SXROM often must be handled individually based on the ROM checksum.

In the absence of data beyond basic iNES header data, the following procedure may work to guess a board type useful for emulation:

1. CHR ROM present on cartridge : MMC1 registers acts "normal".
2. CHR ROM not-present on cartridge : MMC1 CHR's registers bankswitches SRAM banks. If the battery bit is present, only banks which are accessed are saved to the disk when the game is quit. When loading a game with the battery bit set, if a 8KB .sav file is present, it is repeated equally across all banks. This will lead to data being saved when it wasn't supposed to for SOROM games, but 8KB of hard disk space isn't a problem.
3. PRG ROM size equal to 512 KiB: Higher CHR line switches 256 KB banks.
4. PRG ROM size equal to 256 KiB or less and CHR-ROM not present on the cartidge : Higher CHR line disables SRAM.