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c64-sid-audit

Validate Commodore 64 SID 6581/8580 sound emulation in EMULATORS, including $D400-$D7FF register decoding, voices, frequency, pulse width, control bits, ADSR/envelope behavior, oscillator sync/ring/noise/test, filters, volume DAC/sample clicks, paddles, read-only registers, mirrors, and reSID integration. Use when Codex reviews or edits SID, Sound, audio wrapper, register, filter, waveform, envelope, or mapped SID behavior.

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Commnets/EMULATORS
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Files included (3)

  • SKILL.md2.5 KB
  • agents/openai.yaml213 B
  • references/sid.md4.8 KB

SKILL.md (original)

Review the instructions the agent will receive before you install.

C64 SID Audit

Core Workflow

Use this skill to audit SID behavior while respecting that a large part of the chip is mixed-signal and model-dependent.

  1. Read references/sid.md before making claims or edits.
  2. Inspect the local implementation paths first:
    • include/COMMODORE/SID/SID.hpp, src/COMMODORE/SID/SID.cpp
    • include/COMMODORE/SID/SIDRegisters.hpp, src/COMMODORE/SID/SIDRegisters.cpp
    • include/COMMODORE/SID/SIDLibWrapper.hpp, src/COMMODORE/SID/SIDLibWrapper.cpp
    • include/C64/Sound.hpp, src/C64/Sound.cpp
    • thirdparties/resid/*
  3. When changing C++ code, also use $emulators-framework-cpp-style.
  4. Distinguish digital register semantics from analog sound character. Register behavior can be tested exactly; filter tone, distortion, and DAC nonlinearity are fidelity targets.
  5. Prefer delegating low-level waveform/envelope/filter generation to reSID unless the existing architecture intentionally wraps or constrains it.

Audit Priorities

Prioritize correctness in this order:

  • Address decoding and mirrors across $D400-$D7FF.
  • Voice register layout for all three voices and correct write-only/read-only behavior.
  • Control register side effects: gate, sync, ring modulation, test, and waveform selection.
  • ADSR state transitions, envelope readback, and known envelope corner cases.
  • 24-bit oscillator phase, noise LFSR behavior, pulse width, combined waveforms, and voice 3 oscillator readback.
  • Filter cutoff/resonance/routing, $D418 mode/volume, 6581 versus 8580 differences, and sample-click/digifix behavior.
  • Paddle reads through $D419/$D41A and interaction with CIA/control port selection.

Validation Hints

For register-level bugs, create small write/read tests. For audible fidelity, compare generated samples against reSID behavior or a known-good C64 emulator with the same SID model and sample rate.

Useful local anchors:

  • thirdparties/resid/sid.cc
  • thirdparties/resid/voice.cc
  • thirdparties/resid/wave.cc
  • thirdparties/resid/envelope.cc
  • thirdparties/resid/filter.cc

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