GxROM: Difference between revisions
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== See also == | == See also == | ||
*[http://www.romhacking.net/documents/362/ NES Mapper List] by | *[http://www.romhacking.net/documents/362/ NES Mapper List] by Disch | ||
*[http://nesdev.org/mappers.zip Comprehensive NES Mapper Document] by \Firebug\, information about mapper's initial state is inaccurate. | *[http://nesdev.org/mappers.zip Comprehensive NES Mapper Document] by \Firebug\, information about mapper's initial state is inaccurate. | ||
Revision as of 21:34, 22 April 2015
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.
See also
- NES Mapper List by Disch
- Comprehensive NES Mapper Document by \Firebug\, information about mapper's initial state is inaccurate.
