CPU memory map: Difference between revisions

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(This is as short as I can make the explanation of $4018-$401F, per http://forums.nesdev.com/viewtopic.php?f=9&t=14421)
(linking CPU Test Mode, changing wording (discarded doesn't seem right, they do exist, don't they?))
Line 16: Line 16:
| $4000-$4017 || $0018 || [[APU|NES APU]] and [[2A03|I/O registers]]
| $4000-$4017 || $0018 || [[APU|NES APU]] and [[2A03|I/O registers]]
|-
|-
| $4018-$401F || $0008 || APU and I/O functionality that was planned but later discarded
| $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)
| $4020-$FFFF || $BFE0 || Cartridge space: PRG ROM, PRG RAM, and [[mapper]] registers (See Note)

Revision as of 18:03, 8 July 2016

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.