NES 2.0 submappers

Submapper is a term used in the NES 2.0 header for 4-bit codes designating functionally distinct variants of iNES mappers that cannot be distinguished by the memory size fields alone. Most emulators using iNES format distinguish these using CRC, SHA-1, or other hashes of the PRG ROM and CHR ROM, but this works only for games published prior to 1997, not for fan translations or ROM hacks, and not for new games on the same mapper.

Submapper 0 represents the default iNES behavour, so that backward compatibility is maintained.

This document is a living specification. Proposals for new submappers should be made at: Proposals.

001: MMC1

Most MMC1 boards wire all PRG ROM address lines A14-A18 to the mapper's output in a fairly predictable manner. Boards with CHR RAM wire PRG ROM A18 and the PRG RAM address lines to otherwise unused CHR bank outputs. But among boards designed to take a 32 KiB PRG ROM, some (SIROM) connect PRG ROM A14 to the MMC1's output, others (SEROM, SHROM, SH1ROM) directly to CPU A14.

0: Normal behavior. The PRG ROM size and PRG RAM size determine the function of bits.
1, 2, 4: As 0. Deprecated in favor of 0. (In the Kevtendo, 1 meant SUROM, 2 meant SOROM, and 4 meant SXROM. Since then, the community standardized on using size fields instead.)
3: Treat bit 4 of the PRG bank field as always 0 instead of using it to disable PRG RAM. Deprecated in favor of iNES Mapper 155 so that ROM images stay reasonably compatible with emulators that do not understand NES 2.0.
5: SEROM, SHROM, SH1ROM style: PRG ROM A14 connected directly to CPU A14 (and MMC1 A14 input) instead of MMC1 A14 output. Ignore MMC1 A14, ignore LSB of PRG bank, weird behavior of switchable PRG bank if PRG > 32KiB.

002, 003, 007: UxROM, CNROM, AxROM

Status: Draft

AxROM (mapper 7) is the only known licensed discrete logic mapper to unreliably come with bus conflict prevention circuitry. While no game documented in NesCartDB was released in one region on multiple board variants, several games did change boards when localized.

The following table is tentatively offered-
0: Normal (No advisory statement is made as to whether this game has bus conflicts) (uninvestigated AOROM)
1: Bus conflicts do not occur (ANROM)
2: Bus conflicts occur (AMROM)

Although all Nintendo-manufactured games using normal CNROM (mapper 3), normal UxROM (mapper 2), and inverted UxROM (mapper 180) had bus conflicts, apparently several unlicensed games require their absence, as does the updated version of Donkey Kong with the pie factory level.[1] This same table should be used for them, too.

CNROM with security diodes (mapper 185) has a different set of submapper definitions.

032: Irem G101

Status: Draft

Major League requires hardwired one-screen mirroring and entirely ignores writes to $9000.

0: Normal (H/V mapper-controlled mirroring)
1: CIRAM A10 is tied high (fixed one-screen mirroring) and PRG banking style is fixed as 8+8+16F

iNES Mapper 034

0: Normal (≤ 8KiB CHR implies BNROM, >8KiB CHR implies NINA-001)
1: Force NINA-001 behavior even with no CHR ROM

Source: KH's submappers

068 : Sunsoft 4

Status: Draft

In addition to its normal function, the Sunsoft 4 IC was used in Nantettatte!! Baseball which requires different PRG behavior:

The following table is tentatively offered-
0: Normal (max 256KiB PRG)
1: Sunsoft Dual Cartridge System a.k.a. NTB-ROM (max 128KiB PRG, licensing IC present, external option ROM of up to 128KiB should be selectable by a second menu)

iNES Mapper 071

Some games use this with 1-screen mirroring, where the mapper's mirroring control bit is wired directly to CIRAM A10. Others have hardwired horizontal or vertical mirroring.

0: Normal
1: Single-screen mirroring (Fire Hawk)

The Quattro multicarts have been moved to mapper 232.

Source: KH's submappers

iNES Mapper 078

One game uses this with 1-screen mirroring, where the mapper's mirroring control bit is wired directly to CIRAM A10. The other can switch between horizontal and vertical mirroring, using a multiplexer between PPU A10 and PPU A11 whose output is sent to CIRAM A10.

0: Not specified
1: Single-screen mirroring (nibble-swapped mapper 152)
2: Ignore mapper output and use fixed V/H/4 mirroring (effectively nibble-swapped mapper 70; not used in commercial games; deprecated)
3: Mapper-controlled H/V mirroring

Source: KH's submappers

iNES Mapper 185

Status: Draft

A few NROM-like games were released on CNROM boards where all four bits of the latch were solely used as an anti-piracy measure. While a documented heuristic exists for which values were used, we tentatively suggest that the submapper here indicate the value to be written to the latch for normal operation (submapper = (latch&0x30)/4+(latch&3))

3210  
||||
|||+- Bit 0 (bank number)
||+-- Bit 1 (bank number)
|+--- Bit 4 (diode config)
+---- Bit 5 (diode config)

In the case that any of the bits are "don't care", use 0.

210: Namco 175 and 340

Status: Draft

Mapper 210 doesn't distinguish between the 175's hardwired mirroring and 340's 1/H/V mirroring. Also, previous confusion and compatibility code used by Namco when they were developing games means that many 175- and 340- using games are incorrectly tagged as mapper 19.

Tentative suggestion:
Mapper 210:
0: No advisory statement is made (use runtime heuristics)
1: N175 (hardwired mirroring, no IRQ)
2: N340 (1/H/V mirroring, no IRQ, no internal or external RAM)

Source: KH's submappers

iNES Mapper 232

Similar to #71 above, with a separate register controlling which 64 KiB outer bank of the PRG ROM is used. This is used for the Quattro multicarts.

0: Normal
1: Swap the bits of the outer bank number (for the Aladdin Deck Enhancer versions)

References

• Original submapper list by kevtris
• Other definitions by kevtris