| name | 6502-memory-map |
| description | Memory maps, I/O register layouts, ROM entry points, and zero-page conventions for the main 6502 home computers: the Apple II family, the Commodore 64, and the Atari 400/800/XL/XE. Covers Apple II soft switches ($C000-$C0FF), language card, Monitor ROM and Applesoft FP; C64 6510 $00/$01 banking, VIC-II/SID/CIA and the KERNAL table ($FFD2 CHROUT); and Atari ANTIC/GTIA/POKEY chips, shadow registers, CIO/SIO and the $E450 OS vectors. Use this skill WHENEVER you need to know what a memory address or hardware register means on an Apple II, C64, or Atari, what a soft switch or ROM routine does, where the screen/zero page/stack are, or how banking works. Trigger on addresses like $C050, $FDED, $D020, $FFD2, $D40A, $E456, on names like COUT/CHROUT/CIOV/POKEY/FAC, or any "what is at address X" / "what does this poke/STA do to hardware" question. Pairs with the 6502-instruction-set, 6502-merlin-assembler, 6502-sweet16, and 6502-to-rust skills. |
Apple II, Commodore 64 & Atari 8-bit Memory Maps
A 6502 program is its memory map: there are no system calls, just stores and
loads to magic addresses. To understand a routine you must know what each
address means on the target machine — a STA $D020 is "set the C64 border
color", a STA $C050 is "switch the Apple II to graphics", a STA $D40A is
"wait for horizontal blank on the Atari". The same address means different
things on different machines, so identify the platform first.
Identify the platform
The C64 and Atari both map custom-chip registers around $D000-$D7FF, so
disambiguate by which registers and ROM calls appear:
| Clue | Platform |
|---|
$C000–$C0FF soft-switch stores/loads, JSR $FDED, $0400 text screen, language card $C08x | Apple II |
STA $0001 banking, JSR $FFD2, $D800 color RAM, $D020/$D021 border/bg, SID at $D400 | Commodore 64 |
JSR $E456/$E459 (CIOV/SIOV), STA $D40A (WSYNC), display list at $D402, POKEY $D2xx, page-2 shadow regs ($022F, $0230), page 6 $0600 | Atari 8-bit |
JSR $F689 (SWEET16) | Apple II (Integer BASIC ROM) — see the sweet16 skill |
16-bit code, REP/SEP, toolbox calls | Apple IIgs — see 65816 notes in the 6502-instruction-set skill |
All three put the stack at $0100-$01FF and use a precious 256-byte zero page,
but almost everything else differs.
How to read the references
Read only the file for the platform and topic at hand:
Apple II
references/apple2-memory.md — full 64K layout, zero-page usage by
DOS/ProDOS/Monitor/Applesoft, the $C000-$C0FF soft switches (video, keyboard,
speaker, IIe aux-memory, game I/O), the $C080-$C08F language-card bank
switching, text/lo-res/hi-res page addresses, and ROM sizes per model.references/apple2-rom-routines.md — Monitor ROM entry points (character
I/O, screen, input, hex print) and the Applesoft floating-point package
(FAC/ARG layout and FADD/FMULT/FDIV/FOUT/… entry points) plus Applesoft
runtime zero-page (CHRGET, TXTPTR, the variable pointers).
Commodore 64
references/c64-memory.md — the 6510 $00/$01 processor port and the
ROM/RAM/IO banking it controls, the full memory layout, and the VIC-II
($D000), SID ($D400), color RAM ($D800), CIA1 ($DC00) and CIA2
($DD00) register maps.references/c64-kernal.md — the KERNAL jump table ($FF81-$FFF3:
CHROUT, CHRIN, GETIN, SETLFS, SETNAM, LOAD, SAVE, …), important KERNAL/BASIC
zero-page locations, and BASIC ROM notes.
Atari 400/800/XL/XE
references/atari8-memory.md — the memory layout, the GTIA ($D000),
POKEY ($D200), PIA ($D300) and ANTIC ($D400) register maps, the
display-list model, and the page-2 shadow registers the OS copies to
hardware each frame.references/atari8-os.md — the $E450 OS jump-vector table (CIOV, SIOV,
SETVBV…), the CIO device-independent I/O system and IOCBs, SIO, and the
(BCD) floating-point package at $D800.
Why this matters for porting
When porting (see the 6502-to-rust skill), every access to one of these
addresses is an external effect that the intermediate language must model
explicitly: a hardware register read/write, a ROM service call, or a
documented OS variable. A plain RAM access becomes an array index; a $D020
write becomes "set border color"; a JSR $FFD2 becomes "output character in A".
Mislabeling a hardware access as ordinary memory is the most common way a port
silently loses behavior. Use these references to classify every non-RAM address
the code touches.
