AxROM

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Revision as of 18:37, 14 November 2010 by Bregalad (talk | contribs) (Summed up info about all boards of the family, board specific pages can be deleted)
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The generic designation AxROM refers to Nintendo cartridge boards NES-AMROM, NES-ANROM, NES-AN1ROM, NES-AOROM, their HVC counterparts, and clone boards. Most games developed by Rare Ltd. used an AxROM series board. The iNES format assigns mapper 7 to AxROM.

The following AxROM boards are known to exist:

Board PRG ROM Bus conflicts CHR
AMROM 128 KB Yes 16,32,64 KB ROM
ANROM 128 KB No 16,32,64 KB ROM
AN1ROM 64 KB No 128 KB ROM
AOROM 128, 256 KB Yes (by default) 16,32,64 KB ROM


Overview

  • 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

Banks

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

Solder pad config

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

Registers

Bank select ($8000-$FFFF)

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

Hardware

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.

Various notes

On the AOROM board, special mask ROMs with an additional positive CE on pin 2 (which is connected to PRG R/W) can be used to prevent bus conflicts without an additional chip. A few 128 KB games used AOROM instead of ANROM because this might sometimes be cheaper than a 74HC02 depending on production quantities.

Variants

The only difference between AMROM and BNROM is the mirroring configuration.