GxROM

The designation GxROM refers to Nintendo cartridge boards labeled NES-GNROM and NES-MHROM (and their HVC counterparts), which use discrete logic to provide up to four 32 KB banks of PRG ROM and up to four 8 KB banks of CHR ROM. The iNES format assigns mapper 66 to these boards.

Board Types

The following GxROM boards are known to exist:

Board	PRG ROM	CHR
GNROM	128 KB	32 KB ROM
MHROM	64 KB	16 / 32 KB ROM

Overview

• PRG ROM size: Up to 128 KB
• PRG ROM bank size: 32 KB
• PRG RAM: None
• CHR capacity: Up to 32 KB ROM
• CHR bank size: 8 KB
• Nametable mirroring: Solder pads select vertical or horizontal mirroring
• Subject to bus conflicts: Yes

Banks

• CPU $8000-$FFFF: 32 KB switchable PRG ROM bank
• PPU $0000-$1FFF: 8 KB switchable CHR ROM bank

Registers

Bank select ($8000-$FFFF)

7  bit  0
---- ----
xxPP xxCC
  ||   ||
  ||   ++- Select 8 KB CHR ROM bank for PPU $0000-$1FFF
  ++------ Select 32 KB PRG ROM bank for CPU $8000-$FFFF

Bits 5 is not used on MHROM, which supports only 64 KB PRG.

Solder pad config

• Horizontal mirroring : 'H' disconnected, 'V' connected.
• Vertical mirroring : 'H' connected, 'V' disconnected.

Hardware

The GNROM board contains a 74HC161 binary counter used as a quad D latch (4-bit register) to select the current PRG and CHR banks. MHROM, on the other hand, was often a glop-top, as it was used for pack-in games, such as the Super Mario Bros./Duck Hunt multicart, and needed to be very inexpensive to produce in huge quantities.

Variants

Placing the bank register in $6000-$7FFF instead of $8000-$FFFF gives mapper 140. The Color Dreams board leaves the port at $8000-$FFFF, swaps the nibbles, expands CHR by two bits, and adds two bits for charge pump control.

Theoretically the bank select register could be implemented with a 74HC377 octal D latch, allowing up to 512 KB of PRG ROM and 128 KB of CHR ROM. There are a large number of other variants on GNROM, where the bits or the writeable address were moved around.