SxROM

The generic designation SxROM refers to cartridge boards made by Nintendo that use the Nintendo MMC1 mapper.

Board types

The following SxROM boards are known to exist:

Board	PRG ROM	PRG RAM	CHR	Comments
SAROM	64 KB	8 KB	16 / 32 / 64 KB ROM	NES only
SBROM	64 KB		16 / 32 / 64 KB ROM	NES only
SCROM	64 KB		128 KB ROM	NES only
SC1ROM	64 KB		128 KB ROM	Uses 7432 for 28-pin CHR ROM
SEROM	32 KB		16 / 32 / 64 KB ROM
SFROM	128 / 256 KB		16 / 32 / 64 KB ROM
SF1ROM	256 KB		64 KB ROM	PRG uses standard 32-pin EPROM pinout
SFEXPROM	256 KB		64 KB ROM	Patches PRG at runtime to correct a bad mask ROM run.
SGROM	128 / 256 KB		8 KB RAM/ROM
SHROM	32 KB		128 KB ROM	NES only
SH1ROM	32 KB		128 KB ROM	Uses 7432 for 28-pin CHR ROM
SIROM	32 KB	8 KB	16 / 32 / 64 KB ROM	Japan Only
SJROM	128 / 256 KB	8 KB	16 / 32 / 64 KB ROM
SKROM	128 / 256 KB	8 KB	128 KB ROM
SLROM	128 / 256 KB		128 KB ROM
SL1ROM	64 / 128 / 256 KB		128 KB ROM	Uses 7432 for 28-pin CHR ROM
SL2ROM	128 / 256 KB		128 KB ROM	CHR uses standard 32-pin EPROM pinout
SL3ROM	256 KB		128 KB ROM	Uses 7432 for 28-pin CHR ROM
SLRROM	128 / 256 KB		128 KB ROM	Difference from SLROM unknown
SMROM	256 KB		8 KB RAM	Japan Only
SNROM	128 / 256 KB	8 KB	8 KB RAM/ROM
SOROM	128 / 256 KB	16 KB	8 KB RAM/ROM
STROM	Not MMC1, actually NROM
SUROM	512 KB	8 KB	8 KB RAM/ROM
SXROM	128 / 256 / 512 KB	32 KB	8 KB RAM/ROM	Japan Only
SZROM	128 / 256 KB	16 KB	16-64 KB ROM	Japan Only

Solder pad config

Battery data retention

The below pertains to SAROM, SJROM, SKROM, SNROM, SUROM, and SXROM boards only:

• PRG RAM retaining data : 'SL' disconnected, Battery, D1, D2, R1 R2 and R3 present.
• PRG RAM not retaining data : 'SL' connected, leave slots for Battery, D1, D2, R1, R2 and R3 free.

Even if the SOROM and SZROM boards utilizes a battery, it is connected to only one PRG RAM chip. The first RAM chip will not retain its data, but the second one will.

Higher CHR lines

The SUROM, SOROM, and SXROM boards are extensions of SNROM, which has CHR RAM and PRG RAM. Because CHR RAM doesn't need bankswitching, these boards use the CHR bank select lines to switch different things:

• SNROM uses the upper CHR bank select line coming out of the mapper (CHR A16; bit 4 of bank number) as an additional chip enable for the PRG RAM.[1] All other boards with PRG RAM appear to tie /CE to ground.
• SUROM uses CHR A16 to control the upper address line (PRG A18) of its 512KB PRG ROM.
• SOROM uses a similar method, using the second-highest CHR bank select line to choose between two 8KB PRG RAM chips. Only the RAM chip selected by driving CHR A15 high can be battery backed.
• SXROM is a combination of SOROM and SUROM, addressing both 512KB of PRG ROM and 32KB of PRG RAM.

• SZROM retains multiple address lines for CHR ROM banking, but also has 16 KiB of PRG RAM. It uses CHR A16, instead of SOROM's CHR A15, to select between two 8 KiB PRG RAM chips. Similar to SOROM, only the RAM chip selected by driving CHR A16 high can be battery backed.

In these scenarios, however, both CHR bank registers must be set to the same value (or the CHR bank size must be set to 8KB), or the PRG ROM/RAM will be bankswitched as the PPU renders, causing disastrous results.

Various notes

• The SEROM, SHROM, and SH1ROM boards, all used for games with 32 KiB PRG such as Dr. Mario, do not connect the PRG ROM's A14 to the MMC1; instead A14 is connected directly to the NES. This means the PRG ROM bank register and the PRG ROM bank mode bits have no effect.[2] SIROM does not seem to share this property, despite having 32 KiB PRG.
• SLxROM boards are functionally identical to SLROM, but with different chip pinouts. Some of them have an additional 74HC32 chip to combine PPU /RD and PPU /A13 into a single enable signal for the CHR ROM chip that has only 28 pins.
• SMROM is functionally identical to SGROM, but features two 128 KB PRG ROM chips instead of one 256 KB. Only a very early MMC1 game in Japan, Hokkaidou Rensa Satsujin: Ohoutsuku ni Shouyu, is known to have used this board, and one of very few Nintendo-made boards which combine smaller ROM chips to get a bigger ROM.
• One SNROM game for Famicom, Morita Shogi, uses an 8 KiB CHR ROM instead of CHR RAM. The 6264 pinout is nearly identical to the pinout of an 8 KiB mask ROM, except for pins 26 and 27. On the 6264, these are a positive chip enable (CS2) and negative write enable (/WE) respectively; on the mask ROM, they may be additional positive chip enables. Either way, they're high during reads.
• Even boards which different usage of upper CHR lines are all assigned to NES Mapper 1. Emulators can distinguish SOROM and SXROM from SNROM using the new PRG RAM size fields in NES 2.0 or using a PRG hash for legacy iNES ROMs. Therefore, it is recommended that ROM images of SOROM and SXROM games be stored in NES 2.0 format to allow an emulator to distinguish them from SNROM or SUROM.