Techniques & Technology

Memory Mapping

Addresses for everything

Memory-mapped I/O places hardware registers at specific addresses, allowing software to control graphics, sound, and input by reading and writing memory locations.

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Overview

Memory mapping assigns specific memory addresses to hardware registers, allowing programs to control peripherals through normal load/store operations rather than special I/O instructions. This approach defined how programmers interacted with 8-bit and 16-bit hardware.

Examples

Commodore 64:

  • $D000-$D3FF: VIC-II graphics chip (47 registers + mirrors)
  • $D400-$D7FF: SID sound chip (29 registers + mirrors)
  • $D800-$DBFF: colour RAM (1 KB, 4-bit-wide chip)
  • $DC00-$DCFF: CIA#1 (keyboard, joystick, IRQ source)
  • $DD00-$DDFF: CIA#2 (serial bus, RS-232, NMI source, VIC bank select)

NES:

  • $2000-$2007: PPU registers (mirrored every 8 bytes through $3FFF)
  • $4000-$4017: APU and I/O ($4014 = OAMDMA, $4016/$4017 = controllers)

Amiga:

  • $DFF000-$DFF1FE: custom chip registers (Agnus, Denise, Paula, Copper, Blitter)
  • $BFE001: CIA-A PRA (joystick fire buttons, parallel port)
  • $BFD000: CIA-B (serial port, disk control, mouse)

Memory-mapped vs port-mapped I/O

Most 8-bit and 16-bit systems use memory-mapped I/O — hardware registers live in the same address space as RAM, accessed by normal LDA/STA (or equivalent). The 6502, 6510, 6809, and 68000 have no separate I/O instructions, so memory mapping is the only option.

The Z80 is different: it has dedicated IN and OUT instructions that access a separate I/O port address space (256 ports on a basic Z80, 65,536 with the indirect form). The ZX Spectrum, Amstrad CPC, MSX, and Z80-based arcade boards all use port-mapped I/O for hardware control. Memory-mapped accesses to the same hardware on those systems would require dedicated address decoding and aren't typically used.

Hybrid systems (like Sega Master System with Z80 + memory-mapped VDP) place graphics in memory space but sound in port space, mixing both styles.

Programming Model

; C64: Set border colour to black
LDA #$00
STA $D020    ; Border colour register

; C64: Set volume
LDA #$0F
STA $D418    ; SID volume register

Advantages

  • Consistent programming model
  • Works with all CPU instructions
  • No special I/O instructions needed
  • Debuggers can monitor register access

See Also