- PRG ROM size: 128 KB for UNROM, 256 KB for UOROM (DIP-28/32 Nintendo pinout)
- PRG ROM bank size: 16 KB
- PRG RAM: None
- CHR capacity: 8 KB RAM
- CHR bank size: Not bankswitched
- Nametable mirroring: Solder pads select vertical or horizontal mirroring
- Subject to bus conflicts: Yes
- CPU $8000-$BFFF: 16 KB switchable PRG ROM bank
- CPU $C000-$FFFF: 16 KB PRG ROM bank, fixed to the last bank
Solder pad config
- Horizontal mirroring : 'H' disconnected, 'V' connected.
- Vertical mirroring : 'H' connected, 'V' disconnected.
Bank select ($8000-$FFFF)
7 bit 0 ---- ---- xxxx pPPP |||| ++++- Select 16 KB PRG ROM bank for CPU $8000-$BFFF (UNROM uses bits 2-0; UOROM uses bits 3-0)
/PRGSEL A14 A13-A0 | | | | | | | D3-D0-. ,------' | | | . | | | ,-----+--. |`+. | | |Register+--+0 `. | | `--------' | |_... | | | | | ||| | | R/W --' 7-+1 ,' ||| | | | ,' ||| | | |' ||| | | ||| | ,+--------------------+++------+-----. |/CE A1654 A13-A0 | | 128K by 8 bit ROM D7-D0+-- to 2A03 data bus | | `------------------------------------'
The quad OR gate here acts as a multiplexer. A 74HC02 quad NOR gate can be used instead if the banks are stored in reverse order in the ROM. If the program is 128 KiB or smaller, the 7402 way leaves one NOR gate free to invert R/W into /OE to avoid bus conflicts. Or if an actual multiplexer (74HC157 quad 2:1) is cheaper than an OR gate, a third-party UxROM-compatible board can use that instead of the 74HC32, as kyuusaku suggested.
Nintendo's HVC-UN1ROM board moves the bankswitching bits within the byte.
In theory, it would be possible to implement the bank select register with a 74HC377 octal D latch, allowing up to 4 megabytes of PRG ROM, but due to mask ROM cost in the NES era, no non-pirate NES cart used this much memory.The iNES format can't represent a ROM bigger than 2 MiB, so the biggest of these ROMs would have to be in NES 2.0 format.
There are two conceivable configurations. One uses an additional 74*32 on the high bits of the latch's output to force them high in the fixed bank, and this is what existing emulators appear to use for mapper 2.
The other leaves out the additional 74*32 and provides a separate fixed bank for each set of 16 banks in a way reminiscent of SUROM. For example, when bank 20 is swapped into $8000, bank 31 is swapped into $C000. As with SUROM, this configuration would be useful for making multicarts or other games with multiple distinct engines.
Crazy Climber replaces the 74HC32 quad-OR gate by a 74HC08 quad-AND gate, so that the first bank is fixed at $8000-$bfff and the switchable bank is present at $c000-$ffff. This configuration is assigned to iNES Mapper 180, which uses the same UNROM PCB.