AOROM

AOROM refers to the NES-AOROM and HVC-AOROM boards of the AxROM category. Most NES games developed by Rare use this board. As Battletoads and other Rare games tend to depend on edge cases of NES PPU timing, they make good benchmarks for emulator accuracy.

Overview

• PRG ROM size: 128 or 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: Yes, unless custom mask ROMs with internal logic to prevent bus conflicts are used (see below)

Banks

• CPU $8000-$ffff : 32 KB Switchable PRG ROM bank.

The AOROM board can have either 4 or 8 PRG ROM banks.

Solder pad config

No solder pad config is needed on the AOROM board.

Chips and pinout

• PRG ROM - 2 MBit (2 KB x 8) (DIP-32) :

                       ---U---
                A17 - |01   32| - +5V
      /CE (/ROMSEL) - |02   31| - CE (R/W)
                A15 - |03   30| - +5V
                A12 - |04   29| - A14
                A7  - |05   28| - A13
                A6  - |06   27| - A8
                A5  - |07   26| - A9
                A4  - |08   25| - A11
                A3  - |09   24| - A16
                A2  - |10   23| - A10
                A1  - |11   22| - /CE (/ROMSEL)
                A0  - |12   21| - D7
                D0  - |13   20| - D6
                D1  - |14   19| - D5
                D2  - |15   18| - D4
            GND     - |16   17| - D3
                       -------

This pinout is not compatible with standard 27C020 EPROMs, so to insert them in the board manual rewiring is needed. The positive CE on pin 31 allow the chip to be disabled when the CPU writes to the mapper without an extra logic chip if this pin is implemented in the custom mask ROM, disabling bus conflicts. For that reason, the AOROM board was used sometimes as a substitution for ANROM on 128kb games because it avoids placing a 74HC02 in the game.

• CHR-RAM - 8KB / 64 kBit (DIP-28) : Standard 6264 pinout.

• 74HC161 (DIP-16)