CNROM
| Company | Nintendo, others |
| Boards | CNROM |
| PRG ROM capacity | 32K |
| PRG ROM window | n/a |
| PRG RAM capacity | None |
| CHR capacity | 32K |
| CHR window | 8K |
| Nametable mirroring | Fixed H or V, controlled by solder pads |
| Bus conflicts | Yes |
| IRQ | No |
| Audio | No |
| iNES mappers | 003, 185 |
NES-CNROM (and its HVC counterpart) is a particular Nintendo cartridge board which uses uses discrete logic to provide up to four 8 KB banks of CHR ROM. The most common usage of this board, as well as other third-party compatible boards, is assigned to iNES mapper 3. (See iNES Mapper 003 for the suggested emulator implementation.) If the CNROM board mounts only 8 KiB of CHR-ROM, the 8 KiB CHR bank number becomes a Chip Select number for copy-protection purposes, described by iNES Mapper 185.
Banks
- CPU $8000-$FFFF: 16 KB PRG ROM, fixed (if 16 KB PRG ROM used, then this is the same as $C000-$FFFF)
- CPU $C000-$FFFF: 16 KB PRG ROM, fixed
- PPU $0000-$1FFF: 8 KB switchable CHR ROM bank
For 16 KB PRG ROM testing, Joust (NES) makes a worthwhile test subject.
Registers
Bank select ($8000-$FFFF)
7 bit 0 ---- ---- ..DC ..BA || || || ++- CHR A14..A13 (8 KiB bank) |+------ Output to Diode 2 (D2) +------- Output to Diode 1 (D1)
The CNROM board contains a 74HC161 binary counter used as a quad D latch (4-bit register) to select the current CHR bank.
Security diodes
As a (weak) copy protection mechanism, the CNROM circuit board has a spot for two diodes that produce additional bus conflicts to hinder cartridge dumping attempts. Diode 1 connects latch bit 'D' to CHR-ROM A10; Diode 2 connects latch bit 'C' to CHR-ROM A12. Each latch bit must be set such that the diode does not allow current to flow, because if it does, AND-type bus conflicts occur that may cause the wrong CHR-ROM A10 and A12 signals to be applied, depending on the relative output resistances of latch chip and console or dumping device. Both the NES PPU and modern Kazzo-like dumping devices have strong enough output drivers to always win these bus conflicts, but 1980s' dumping equipment will lose these bus conflicts and produce an unusable readout. Each diode can be mounted either with the anode or the cathode facing the latch output. As a diode will allow current to flow if the anode-side voltage is significantly higher than the cathode-side voltage, the latch bit must be 0 if it faces the anode, and 1 if it faces the cathode.
The security diodes were mounted by most Nintendo-manufactured Japanese CNROM games manufactured in 1986 as well as Bandai-manufactured CNROM games from 1986 and 1987. They were never used on North American or PAL CNROM games, even as the NES-CNROM board has an unpopulated spot for them until at least revision -02.
Solder Pad Config
- Horizontal mirroring : 'H' disconnected, 'V' connected.
- Vertical mirroring : 'H' connected, 'V' disconnected.
- 16 KB PRG ROM : 'SL' connected, 'CL' disconnected.
- 32 KB PRG ROM : 'SL' disconnected, 'CL' connected.
Variants
CNROM operates identically to a GNROM with one PRG bank.
The upper 2 bank select bits on the 74HC161 were connected to security diodes. If they were connected to CHR ROM address lines instead, this board could have theoretically supported up to 128 KB of CHR ROM. In fact, iNES Mapper 003 encompasses both CNROM and similar boards that used more CHR ROM, such as those made by Bandai and Panesian.
The Japanese version of Dance Aerobics adds a sound playback IC to a CNROM-like board. It adds a register mapped from $6000-$7FFF that can play one of eight voice recordings. However, the specifics of the compression are not yet known, and there is no standardized way to bundle audio data with iNES images.
Hayauchi Super Igo is a CNROM-like board with a 2KB SRAM mapped at $6000, using a 74HC10 as the address decoder.
Theoretically the bank select register could be implemented with a 74HC377 octal D latch, allowing up to 2 megabytes of CHR ROM.
See Also
- iNES Mapper 003 - The common emulator implementation used for this board, which includes some compatible additions.
- iNES Mapper 185 - Emulation for a subset of CNROM boards that implement a weak form of copy protection.
- Comprehensive NES Mapper Document by \Firebug\, information about mapper's initial state is inaccurate.
