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.

The following GxROM boards are known to exist:

Board	PRG ROM	CHR
GNROM	128 KB	8,16,32 KB ROM
MHROM	64 KB	8,16,32 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

In theory, it would be possible to implement the bank select register with a 74HC377 octal D latch, allowing up to 512 KB of PRG ROM and 128 KB of CHR ROM. Most official licensed games, however, used ASIC mappers instead for smaller bank sizes. There are a large number of other variants on GNROM, where the bits or the writeable address were moved around.

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.

Disch's Notes

 Here are Disch's original notes:
 ========================
 =  Mapper 066          =
 ========================
 
 aka
 --------------------------
 GxROM and compatible
 
 
 Example Games:
 --------------------------
 Doraemon
 Dragon Power
 Gumshoe
 Thunder & Lightning
 Super Mario Bros. + Duck Hunt
 
 
 Notes:
 ---------------------------
 I do not know whether or not this mapper suffers from bus conflicts.  Use caution!
 
 This mapper is INFAMOUS for having bad headers.  Probably 80% or more of these ROMs floating around out
 there have the wrong mirroring mode set in the header.
 
 Some games are marked as mapper 066 that are really mapper 140.  See mapper 140 for info.
 
 
 Registers:
 --------------------------
   $8000-FFFF:  [..PP ..CC]
     P = Selects 32k PRG @ $8000
     C = Selects 8k CHR @ $0000