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c64-vicii-audit

Validate Commodore 64 VIC-II emulation in EMULATORS, including $D000-$D3FF register behavior, PAL/NTSC timing, raster counters, badlines, BA/AEC bus stealing, video matrix and bitmap fetches, sprites, collisions, borders, IRQs, color RAM usage, and display modes. Use when Codex reviews or edits VICII, Screen, Memory/PLA, raster, sprite, video, or cycle-level behavior for C64 classes.

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Commnets/EMULATORS
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Files included (3)

  • SKILL.md3.1 KB
  • agents/openai.yaml222 B
  • references/vicii.md6.5 KB

SKILL.md (original)

Review the instructions the agent will receive before you install.

C64 VIC-II Audit

When compiling, testing, or executing on Windows, use $emulators-windows-build-runtime.

Core Workflow

Use this skill to audit whether EMULATORS simulates the VIC-II as a timing-sensitive bus participant, not only as a frame renderer.

  1. Read references/vicii.md before judging VIC-II behavior.
  2. Inspect the local implementation paths first:
    • include/COMMODORE/VICII/VICII.hpp, src/COMMODORE/VICII/VICII.cpp
    • include/COMMODORE/VICII/VICIIRegisters.hpp, src/COMMODORE/VICII/VICIIRegisters.cpp
    • include/C64/Screen.hpp, src/C64/Screen.cpp
    • include/C64/Memory.hpp, src/C64/Memory.cpp
    • include/C64/ColorMemory.hpp, src/C64/ColorMemory.cpp
    • CIA2 bank-selection code under include/C64/CIA2* and src/C64/CIA2*
  3. When changing C++ code, also use $emulators-framework-cpp-style.
  4. Decide whether the code path is PAL, NTSC, or model-agnostic before asserting exact line/cycle counts.
  5. Treat cycle placement, register side effects, and bus ownership as user-visible behavior. Demos and tests depend on these details.
  6. When changing the VICIIEVENTS overlay, its event meanings, or its colors, update the ;CVICIIEVENTS legend in the canonical projects/COMMODORE/Commodore.fmt. Do not edit distributed formatter copies directly; regenerate them through the normal build/distribution workflow when required.

Audit Priorities

Prioritize correctness in this order:

  • Raster counter and $D011/$D012 high-bit interaction.
  • Badline condition, display enable latch, VC/VCBASE/RC updates, and c-access timing.
  • BA/AEC bus stealing and CPU-visible wait behavior.
  • $D018 video matrix/character/bitmap pointer calculation inside the selected VIC bank.
  • Text, multicolor text, bitmap, multicolor bitmap, ECM, invalid modes, and idle state.
  • Sprite DMA, pointer fetches, expansion, multicolor mode, priority, and collision registers.
  • IRQ flags/masks, collision flag clearing, lightpen, border flip-flops, and open/unconnected read bits.

Validation Hints

Useful local programs and visual tests:

  • emulators/C64Programs/Code/Tests/TESTGRAPHMODES.asm
  • emulators/C64Programs/Code/Tests/TESTHRESMODES.asm
  • emulators/C64Programs/Code/Tests/TESTSPRITESIRQ.asm
  • emulators/C64Programs/Code/Tests/SPRITESPRIORITY.asm
  • emulators/C64Programs/Code/Tests/SP4.asm
  • emulators/C64Programs/Code/Tests/demos/RASTERBARS0.asm
  • emulators/C64Programs/Code/Tests/demos/RASTERBARS1.asm
  • emulators/C64Programs/Code/Tests/demos/RASTERBARS2.asm
  • emulators/C64Programs/Code/Tests/demos/BORDER1.asm
  • emulators/C64Programs/Code/Tests/demos/Sprite.asm

When a visual symptom is involved, prefer a narrow raster/sprite test over a whole-game comparison.

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