NROM (iNES board class 0) is the most basic of NES cartridge logic boards. It consists of a 16 kilobyte or 32 kilobyte program ROM, a 4 kilobyte or 8 kilobyte graphics ROM, and an NES lockout chip. The address pins on the NES are wired directly to the ROM with no mapper hardware intervening. There is no support for extra work RAM.
Any ROM with a size of 40 KB or less is most likely an NROM. This includes most early NES games, up to and including the original Super Mario Brothers.
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