GxROM: Difference between revisions
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[[Category:GNROM-like mappers]] | [[Category:GNROM-like mappers]][[Category:in NesCartDB]] | ||
The designation '''GxROM''' refers to Nintendo cartridge boards labeled NES-GNROM and NES-MHROM (and their [[Famicom|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 [[iNES Mapper 066|mapper 66]] to these boards. | The designation '''GxROM''' refers to Nintendo cartridge boards labeled NES-GNROM and NES-MHROM (and their [[Famicom|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 [[iNES Mapper 066|mapper 66]] to these boards. | ||
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== Variants == | == Variants == | ||
In theory, it would be possible to implement the bank select register with a [[74377|74HC377]] octal D latch, allowing up to 512 KB of PRG ROM and 128 KB of CHR ROM. The [[Color Dreams]] board, similar in function to GNROM, expanded CHR in much the same way. | In theory, it would be possible to implement the bank select register with a [[74377|74HC377]] octal D latch, allowing up to 512 KB of PRG ROM and 128 KB of CHR ROM. The [[Color Dreams]] board, similar in function to GNROM, expanded CHR in much the same way. There are a [[:Category:GNROM-like mappers|large number]] of other variants on GNROM, where the bits or the writeable address were moved around. Most official licensed games, however, used ASIC mappers instead for smaller bank sizes. | ||
Placing the bank register in $6000-$7FFF instead of $8000-$FFFF gives [[INES Mapper 140|mapper 140]]. | Placing the bank register in $6000-$7FFF instead of $8000-$FFFF gives [[INES Mapper 140|mapper 140]]. | ||
== 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 [[INES Mapper 140|mapper 140]]. See mapper 140 for info. | |||
Registers: | |||
-------------------------- | |||
$8000-FFFF: [..PP ..CC] | |||
P = Selects 32k PRG @ $8000 | |||
C = Selects 8k CHR @ $0000 | |||
Revision as of 19:56, 17 October 2012
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. The Color Dreams board, similar in function to GNROM, expanded CHR in much the same way. There are a large number of other variants on GNROM, where the bits or the writeable address were moved around. Most official licensed games, however, used ASIC mappers instead for smaller bank sizes.
Placing the bank register in $6000-$7FFF instead of $8000-$FFFF gives mapper 140.
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
