Bregalad wrote:
Battery backed SRAM ??
Not only normal NES boards doesn't have a slot for battery, but also all games that want to write to the mapper will ERASE rom data.
There are chips like the Dallas DS1245 that are SRAM with an internal battery and power monitor. But I think it'd be almost absurd to use it, unless you want NVRAM to write to. Those chips are physically huge and fairly expensive.
It's easy to prevent a write though, just don't hook memory up to the /write line.
I'd say to go with FlashROM though. You can get up to 512kB, 29F040 or 29C040. And 29C256 if you want 32kB (no rewiring needed for that one).