μMC1 - Porting
This section documents some tips for porting from common mappers to the μMC1.
MMC1
The MMC1 is the closest official mapper, and thus it is an obvious target for porting games from.
Differences with a MMC1
First, it is probably useful to briefly discuss the differences between a MMC1 and the μMC1.
While the μMC1 is based on the MMC1, some features had to be dropped to fit a useable mapper into the GreenPAK:
| MMC1 | μMC1 |
|---|---|
| Supports multiple banking modes | Supports only the UxROM-like mode (mode 3) |
| 4-bit PRG-ROM bank register (up to 256 KiB addressable) | 5-bit PRG-ROM bank register (up to 512 KiB addressable) |
| Supports CHR-ROM banking | No CHR-ROM banking supported |
| Built-in support for PRG-RAM | PRG-RAM is not supported without external logic |
| Register writes are buffered, and changes apply only when all 5 bits are written | Registers are implemented by shift registers, and changes are effective immediately |
| Ignores all writes but first during an opcode execution | Processes all CPU writes to its registers |
Therefore, a game that you want to port has to:
- Use CHR-RAM, which roughly 30% of the official NES/Famicom licensed titles do.
- Have no PRG-RAM.
- Use banking mode of the game needs to be 3 (which all games I have tested do).
These limitations means only 78 entries of commercial games on bootgod's database are suitable for porting.