AxROM

The generic designation AxROM refers to Nintendo cartridge boards NES-AMROM, NES-ANROM, NES-AN1ROM, NES-AOROM, their HVC counterparts, and clone boards. AxROM and compatible boards are implemented in iNES format with iNES Mapper 7.

• Submapper number determines the bus conflict behavior of the game.
  • Submapper 0: Bus conflict behavior undefined, but emulators shouldn’t emulate bus conflicts
  • Submapper 1: No bus conflicts
  • Submapper 2: AND-type bus conflicts

The following AxROM boards are known to exist:

• PRG ROM size: up to 256 KB
• PRG ROM bank size: 32 KB
• PRG RAM: None
• CHR capacity: 8 KB RAM
• CHR bank size: Not bankswitched
• Nametable mirroring: Single-screen, mapper-selectable
• Subject to bus conflicts: AMROM/AOROM only

• CPU $8000-$FFFF: 32 KB switchable PRG ROM bank

No solder pad config is needed on the AxROM board family.

7  bit  0
---- ----
xxxM xPPP
   |  |||
   |  +++- Select 32 KB PRG ROM bank for CPU $8000-$FFFF
   +------ Select 1 KB VRAM page for all 4 nametables

The AxROM boards contain a 74HC161 binary counter used as a quad D latch (4-bit register). The ANROM and AN1ROM boards also contains a 74HC02 which is used to disable the PRG ROM during writes, thus avoiding bus conflicts.

On the AOROM board, special mask ROMs with an additional positive CE on pin 31 (which is connected to PRG R/W) can be used to prevent bus conflicts without an additional chip. Some 128 KB games were made with AOROM to save the cost of a 74HC02. It seems that only Double Dare and Wheel of Fortune employ this trick noticeably–that is, if emulated with bus conflicts enabled, the games will glitch. The list of AxROM games in NesCartDB lacks quality coverage of the PCB backs for the AOROM games, so it is hard to determine yet which games may be wired this way.

It is likely that every retail AOROM game could be emulated correctly without emulating bus conflicts.

• BNROM is the same as AMROM except it uses a fixed horizontal or vertical mirroring.
• NES 2.0 Mapper 470 adds an outer bank register to AOROM.
• Some emulators allow bit 3 to be used to select up to 512 KB of PRG ROM for an oversized AxROM. In hardware this could be implemented by using an octal latch in place of the quad latch (74HC377).
  • The pirate multicart and unlicensed music game Hot Dance 2000 uses a 512 KB AxROM variant.
  • A ROM Hack of Battletoads expanded the ROM size to 512 KB.

• Comprehensive NES Mapper Document by \Firebug\ information about mapper’s initial state is inaccurate.

Edit this page on GitHub