NES-SOROM

SOROM Board

Tech Specs:

Definately not standard. :-)
Max. 256K PRG ROM.
VRAM. Note that VRAM is banked through the MMC1, so you can have 1 8K bank or 2 4K banks.
16K of WRAM, 8K of which is battery backed.

Description:

One complex MMC1 board. My sample is from a Nobunga's Ambition cart.
The WRAM on here is banked using one of the CHR bank bits. At least one other MMC1 board uses this method as well, however I do not own it.

One of the CHR output select bits is used to control which 8K bank of WRAM is in use. Depending on which mode you have your CHR in, it will be bit 3 (8K CHR banks) or bit 4 (4K CHR banks). Note too that if you are in 4K mode, you MUST set both bits to the desired WRAM page or else your WRAM will be switching every time the PPU changes which bank it is using!!

So, to recap. For 4K CHR pages:

A CHR register.

7  bit  0
---------
xxxW xxxR

W: WRAM bank.
0 - bank 0
1 - bank 1
Note:  the "W" bit must be set THE SAME on BOTH CHR select
     registers, or else Bad things will happen!

R: VRAM 4K page select bit


For 8K CHR pages:
Note: only the first CHR register is used.

7  bit  0
---------
xxxx Wxxx

W: WRAM bank.
0 - bank 0
1 - bank 1


WRAM banks:

Bank 0 is not battery backed, while Bank 1 is.