SUROM
SUROM (NES-SUROM and HVC-SUROM) is one of the less common boards within the SxROM subset. Like other SxROM boards, SUROM uses the Nintendo MMC1 ASIC, but because it always uses CHR RAM, it uses a spare CHR address line to control a PRG address bit, letting the program switch entire 256 KB banks (including the normally "fixed" bank). The normal PRG and control bits control only switching within a 256 KB bank.
Overview
- PRG ROM size: 512 KB (DIP-32 Nintendo pinout)
- PRG ROM bank size: 16 KB or 32 KB (inner); 256 KB (outer)
- PRG RAM: 8 KB + battery
- CHR capacity: 8 KB RAM
- CHR bank size: 8 KB or 4 KB
- Nametable mirroring: Controlled by mapper
- Subject to bus conflicts: No
Banks
Some modes of the MMC1 use a fixed PRG bank. However, in SUROM, the fixed bank is the first or last 16 KB of the currently selected 256 KB outer bank.
Solder pad config
- PRG RAM retaining data : 'SL' disconnected, Battery, D1, D2 and R2 present.
- PRG RAM not retaining data : 'SL' connected, leave slots for Battery, D1, D2 and R2 free.
Registers
The behavior of this board differs from that of a typical MMC1 board in the use of the upper CHR address line:
CHR bank 0 (internal, $A000-$BFFF)
4bit0 ----- PxxxC | | | +- Select 4 KB CHR RAM bank at PPU $0000 (ignored in 8 KB mode) +----- Select 256 KB PRG bank
CHR bank 1 (internal, $C000-$DFFF)
4bit0 ----- PxxxC | | | +- Select 4 KB CHR RAM bank at PPU $1000 (ignored in 8 KB mode) +----- Select 256 KB PRG bank (ignored in 8 KB mode)
Note: In 4KB CHR bank mode, the P bits in both CHR bank registers must be set to the same value, or the PRG ROM will be bankswitched as the PPU renders, causing disastrous results. As there is not much of a reason to use 4 KB bankswitching with CHR RAM, it is wise for programs to just set 8 KB bankswitching mode in the Control register.
Variants
In theory, wiring up more CHR bank bits to PRG address lines would allow a cart to use up to 4096 KB of PRG ROM, or possibly 1024 to 2048 KB of PRG ROM with a larger CHR RAM. But because of mask ROM prices during the NES's commercial life, no official game did this, not even Dragon Warrior. Any 1 MB dumps of Dragon Warrior III or Dragon Warrior IV floating around are overdumps created before SUROM was fully understood.
