c64-sid-audit

name

c64-sid-audit

description

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.

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