CPU memory map: Difference between revisions
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(Clarifies that the "cartridge space" is really unmapped space cartridges can use. Adds the extremely common cartridge RAM and ROM mapping to the table to avoid users thinking ROM starts at $4020.) |
m (Fixes two references to the now-removed "cartridge space". Clarifies that the vectors in the unmapped space are supplied by the cartridge.) |
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| Line 29: | Line 29: | ||
** $6000–$7FFF: Battery-backed save or work RAM (usually referred to as WRAM or PRG-RAM) | ** $6000–$7FFF: Battery-backed save or work RAM (usually referred to as WRAM or PRG-RAM) | ||
** $8000–$FFFF: ROM and mapper registers (see [[MMC1]] and [[UxROM]] for examples) | ** $8000–$FFFF: ROM and mapper registers (see [[MMC1]] and [[UxROM]] for examples) | ||
: The cartridge is able to passively observe reads from and writes to any address in the CPU address space, even outside this | : The cartridge is able to passively observe reads from and writes to any address in the CPU address space, even outside this unmapped space, except for reads from $4015, the only readable register that is internal to the CPU. The cartridge can map writable registers anywhere, but its readable memory can only be placed where it does not interfere with other readable hardware, which would produce a [[bus conflict]]. While cartridges can map readable memory at $4000-$4014 and $4018-$401F, a quirk in the 2A03's register decoding can cause DMA to misbehave if the CPU is halted while reading from $4000-$401F, so it is recommended that cartridges only map readable memory from $4020-$FFFF. | ||
* If using [[APU DMC|DPCM playback]], samples are limited to the following practical range: | * If using [[APU DMC|DPCM playback]], samples are limited to the following practical range: | ||
| Line 35: | Line 35: | ||
: Sample playback wraps around from $FFFF to $8000. The highest sample starting address is $FFC0 and longest sample is $FF1 bytes, so the full DPCM range is $C000-$FFFF and $8000-$8FB0, but making use of the wraparound is challenging because of banking and the presence of the CPU vectors. | : Sample playback wraps around from $FFFF to $8000. The highest sample starting address is $FFC0 and longest sample is $FF1 bytes, so the full DPCM range is $C000-$FFFF and $8000-$8FB0, but making use of the wraparound is challenging because of banking and the presence of the CPU vectors. | ||
* The CPU expects interrupt vectors in a fixed place at the end of the | * The CPU expects interrupt vectors in a fixed place at the end of the unmapped space: | ||
** $FFFA–$FFFB: NMI vector, which points at an [[NMI]] handler | ** $FFFA–$FFFB: NMI vector, which points at an [[NMI]] handler | ||
** $FFFC–$FFFD: Reset vector, which points at [[init code|code to initialize the NES chipset]] | ** $FFFC–$FFFD: Reset vector, which points at [[init code|code to initialize the NES chipset]] | ||
** $FFFE–$FFFF: IRQ/BRK vector, which may point at a mapper's [[IRQ|interrupt]] handler (or, less often, a handler for APU interrupts) | ** $FFFE–$FFFF: IRQ/BRK vector, which may point at a mapper's [[IRQ|interrupt]] handler (or, less often, a handler for APU interrupts) | ||
: If a mapper doesn't fix $FFFA–$FFFF to some known bank (normally by fixing an entire bank-sized region at the top of the address space, such as $C000-$FFFF, to a specific bank) nor use some sort of reset detection, the vectors need to be stored in all banks. | : These vectors are supplied by the cartridge. If a mapper doesn't fix $FFFA–$FFFF to some known bank (normally by fixing an entire bank-sized region at the top of the address space, such as $C000-$FFFF, to a specific bank) nor use some sort of reset detection, the vectors need to be stored in all banks. | ||
* Reading from memory that is not mapped to anything normally returns [[Open bus behavior|open bus]]. The cartridge hardware may affect open bus behavior across the entire CPU address space, such as by pulling bits high or low. | * Reading from memory that is not mapped to anything normally returns [[Open bus behavior|open bus]]. The cartridge hardware may affect open bus behavior across the entire CPU address space, such as by pulling bits high or low. | ||
Revision as of 10:25, 18 March 2024
| Address range | Size | Device |
|---|---|---|
| $0000–$07FF | $0800 | 2 KB 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 • $6000–$7FFF • $8000–$FFFF |
$BFE0 $2000 $8000 |
Unmapped. Available for cartridge use. Usually cartridge RAM, when present. Usually cartridge ROM and mapper registers. |
- Some parts of the 2 KiB of internal RAM at $0000–$07FF have predefined purposes dictated by the 6502 architecture:
- $0000-$00FF: The zero page, which can be accessed with fewer bytes and cycles than other addresses
- $0100–$01FF: The page containing the stack, which can be located anywhere here, but typically starts at $01FF
- Games may divide up the rest however the programmer deems useful. See Sample RAM map for an example allocation strategy for this RAM. Most commonly, $0200-$02FF is used for the OAM buffer to be copied to PPU OAM during vblank.
- The unmapped space at $4020-$FFFF can be used by cartridges for any purpose, such as ROM, RAM, and registers. Many common mappers place ROM and save/work RAM in these locations:
- The cartridge is able to passively observe reads from and writes to any address in the CPU address space, even outside this unmapped space, except for reads from $4015, the only readable register that is internal to the CPU. The cartridge can map writable registers anywhere, but its readable memory can only be placed where it does not interfere with other readable hardware, which would produce a bus conflict. While cartridges can map readable memory at $4000-$4014 and $4018-$401F, a quirk in the 2A03's register decoding can cause DMA to misbehave if the CPU is halted while reading from $4000-$401F, so it is recommended that cartridges only map readable memory from $4020-$FFFF.
- If using DPCM playback, samples are limited to the following practical range:
- $C000–$FFF1: DPCM sample data
- Sample playback wraps around from $FFFF to $8000. The highest sample starting address is $FFC0 and longest sample is $FF1 bytes, so the full DPCM range is $C000-$FFFF and $8000-$8FB0, but making use of the wraparound is challenging because of banking and the presence of the CPU vectors.
- The CPU expects interrupt vectors in a fixed place at the end of the unmapped space:
- $FFFA–$FFFB: NMI vector, which points at an NMI handler
- $FFFC–$FFFD: Reset vector, which points at code to initialize the NES chipset
- $FFFE–$FFFF: IRQ/BRK vector, which may point at a mapper's interrupt handler (or, less often, a handler for APU interrupts)
- These vectors are supplied by the cartridge. If a mapper doesn't fix $FFFA–$FFFF to some known bank (normally by fixing an entire bank-sized region at the top of the address space, such as $C000-$FFFF, to a specific bank) nor use some sort of reset detection, the vectors need to be stored in all banks.
- Reading from memory that is not mapped to anything normally returns open bus. The cartridge hardware may affect open bus behavior across the entire CPU address space, such as by pulling bits high or low.
