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c64-memory-map-audit

Validate Commodore 64 memory map, PLA/bank switching, 6510 port $0000/$0001 behavior, VIC bank selection, color RAM, ROM/RAM overlays, cartridge mapping, and mirrored I/O in the EMULATORS C64 namespace. Use when Codex reviews or edits C64 memory, PLA, CPU-visible address decoding, VIC-visible memory, color memory, char ROM, cartridge, expansion I/O, or mapped chip register behavior.

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Commnets/EMULATORS
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Files included (3)

  • SKILL.md3.1 KB
  • agents/openai.yaml214 B
  • references/memory-map.md5.0 KB

SKILL.md (original)

Review the instructions the agent will receive before you install.

C64 Memory Map Audit

Core Workflow

Use this skill to audit whether EMULATORS presents the same memory view as real C64 hardware.

  1. Read references/memory-map.md before making claims or edits.
  2. Inspect the relevant implementation paths first:
    • include/C64/Memory.hpp, src/C64/Memory.cpp
    • include/C64/PLA.hpp, src/C64/PLA.cpp
    • include/C64/IO6510PortRegisters.hpp, src/C64/IO6510PortRegisters.cpp
    • include/C64/ColorMemory.hpp, src/C64/ColorMemory.cpp
    • include/C64/Cartridge.hpp, src/C64/Cartridge.cpp
    • include/C64/IOExpansionMemory.hpp, src/C64/IOExpansionMemory.cpp
    • chip register classes under include/C64, src/C64, include/COMMODORE, and src/COMMODORE
  3. When changing C++ code, also use $emulators-framework-cpp-style.
  4. Separate CPU-visible mapping from VIC-visible mapping. The CPU sees PLA-selected RAM/ROM/I/O overlays; the VIC sees a selected 16 KB bank plus its own color/char-ROM behavior.
  5. For findings, cite the address range, controlling signal/bit, expected visibility, and the class or method that violates it.
  6. Audit every MCHEmul::Address construction that uses byte lists. Never use the ambiguous form MCHEmul::Address ({ xx, xx }): overload resolution can select Address (size_t, unsigned int) instead of a byte-container constructor. Always pass the endian flag explicitly, for example MCHEmul::Address ({ 0x00, 0xd0 }, false) for the framework's little-endian address notation, and verify the resulting address size and numeric value.

Audit Priorities

Prioritize correctness in this order:

  • $0000/$0001 6510 port DDR/data behavior and whether LORAM/HIRAM/CHAREN only act when configured as outputs.
  • PLA decoding for RAM, BASIC ROM, KERNAL ROM, character ROM, I/O, color RAM, cartridge ROML/ROMH, GAME, EXROM, and Ultimax-like open regions.
  • Writes through visible ROM to underlying RAM.
  • I/O range decoding and mirroring: VIC-II, SID, color RAM, CIA1, CIA2, IO1, IO2.
  • VIC memory bank selection through CIA2 PA0/PA1, including inverted bank encoding and char ROM visibility in VIC banks 0 and 2.
  • Color RAM as 1 KB x 4-bit storage with open/high-nibble read behavior.

Validation Hints

Prefer small address-decoder tests over broad emulator smoke tests. Useful existing assets include:

  • emulators/C64Programs/Code/Macros/MVICII.asm
  • emulators/C64Programs/Code/Macros/MCIAS.asm
  • emulators/C64Programs/Code/Tests/TESTGRAPHMODES.asm
  • emulators/C64Programs/Code/Tests/TESTHRESMODES.asm
  • emulators/C64Programs/Code/Tests/TESTCIA1.asm
  • emulators/C64Programs/Code/Tests/TESTCIA2NMI.asm

When proposing a correction, include at least one read path and one write path if the change touches mapped memory.

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