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Accessing the SRAM chip on the SNES cartridge

After quite some trouble trying to access the SRAM on the cartridge, I thought I would share the results here.

Unfortunately, I could not get the suggestions from DiskDude’s otherwise ultra-helpful SNES Kart document to work, so I had to do a bit of my own reverse engineering. The main issue is getting the SRAM chip’s !CS line to low state. This is accomplished by shredding a few control and address lines through a pair of 2-to-4 address decoders. Only when the combination of ones and zeros is right, the !CS input on the SRAM will go low. For Super Mario World (LoROM) and Super Mario Kart (HiROM), I reconstructed the following wirings: (click to magnify)

SRAM control for LoROM game (SMW)

SRAM control for LoROM game (SMW)

SRAM control for HiROM game (SMK)

SRAM control for HiROM game (SMK)

The correct values for accessing the SRAM appear to be as follows:

  • HiROM: A13, A14, B5 high; A15, B6 low
  • LoROM: RESET, B4, B5, B6 high; !CS low

As for reading and writing, any datasheet of a pin compatible SRAM chip, such as this one, should provide more than enough information.

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