CPU memory map

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Address range Size Device
$0000-$07FF $0800 2KB internal RAM
$0800-$0FFF $0800 Mirrors of $0000-$07FF
$1000-$17FF $0800
$1800-$1FFF $0800
$2000-$2007 $0008 NES PPU registers
$2008-$3FFF $1FF8 Mirrors of $2000-2007 (repeats every 8 bytes)
$4000-$4017 $0018 NES APU and I/O registers
$4018-$401F $0008 APU and I/O functionality that is normally disabled. See CPU Test Mode.
$4020-$FFFF $BFE0 Cartridge space: PRG ROM, PRG RAM, and mapper registers (See Note)

See Sample RAM map for an example allocation strategy for the 2KB of internal RAM at $0000-$0800.

Note: Most common boards and iNES mappers address ROM and Save/Work RAM in this format:

  • $6000-$7FFF = Battery Backed Save or Work RAM
  • $8000-$FFFF = Usual ROM, commonly with Mapper Registers (see MMC1 and UxROM for example)

The CPU expects interrupt vectors in a fixed place at the end of the cartridge space:

  • $FFFA-$FFFB = NMI vector
  • $FFFC-$FFFD = Reset vector
  • $FFFE-$FFFF = IRQ/BRK vector

If a mapper doesn't fix $FFFA-$FFFF to some known bank (typically, along with the rest of the bank containing them, e.g. $C000-$FFFF for a 16KiB banking mapper) or use some sort of reset detection, the vectors need to be stored in all banks.