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c64-cia-audit

Validate Commodore 64 CIA 6526 emulation in EMULATORS, including CIA1/CIA2 $DC00/$DD00 register behavior, ports, DDRs, keyboard/joystick matrix, IEC/user port, VIC bank selection, timers A/B, underflow timing, PB toggle/pulse output, TOD clock/alarm, serial shift register, FLAG, ICR masks and IRQ/NMI lines. Use when Codex reviews or edits C64 CIA, timers, ports, keyboard, joystick, serial bus, interrupts, TOD, or VIC bank control.

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Commnets/EMULATORS
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Files included (3)

  • SKILL.md2.8 KB
  • agents/openai.yaml211 B
  • references/cia.md4.7 KB

SKILL.md (original)

Review the instructions the agent will receive before you install.

C64 CIA Audit

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

Core Workflow

Use this skill to audit whether the two C64 CIA chips behave like 6526 devices in their C64 wiring context.

  1. Read references/cia.md before making claims or edits.
  2. Inspect the local implementation paths first:
    • include/COMMODORE/CIA/CIA.hpp, src/COMMODORE/CIA/CIA.cpp
    • include/COMMODORE/CIA/CIARegisters.hpp, src/COMMODORE/CIA/CIARegisters.cpp
    • include/COMMODORE/CIA/CIATimer.hpp, src/COMMODORE/CIA/CIATimer.cpp
    • include/COMMODORE/CIA/CIAClock.hpp, src/COMMODORE/CIA/CIAClock.cpp
    • include/COMMODORE/CIA/CIASerialPort.hpp, src/COMMODORE/CIA/CIASerialPort.cpp
    • include/C64/CIA1*, src/C64/CIA1*, include/C64/CIA2*, src/C64/CIA2*
    • include/C64/SerialPort.hpp, src/C64/SerialPort.cpp
  3. When changing C++ code, also use $emulators-framework-cpp-style.
  4. Separate generic 6526 behavior from C64-specific wiring. CIA1 and CIA2 are the same chip but expose different system effects.
  5. For each finding, identify the exact register, bit, read/write side effect, interrupt line, and expected cycle/timer consequence.

Audit Priorities

Prioritize correctness in this order:

  • Port read/write and DDR behavior, including active-low keyboard/joystick/IEC lines.
  • CIA1 wiring: keyboard matrix, joystick ports, paddles/control port selection, datasette FLAG, and IRQ output.
  • CIA2 wiring: VIC bank selection on PA0/PA1, IEC/user-port/RS-232 lines, and NMI output.
  • Timer A/B latch/counter load behavior, underflow reload, one-shot/continuous mode, force-load, CNT counting, and Timer B count sources.
  • ICR read-clear behavior and bit 7 set/clear mask semantics.
  • TOD BCD clock, alarm selection, 50/60 Hz selection, halt/latch behavior, and AM/PM bit.
  • Serial shift register and FLAG edge semantics.

Validation Hints

Useful local assets:

  • docs/C64Data/CIA1Logic.xlsx
  • emulators/C64Programs/Code/Macros/MCIAS.asm
  • emulators/C64Programs/Code/Tests/TESTCIA1.asm
  • emulators/C64Programs/Code/Tests/TESTCIA1IRQ.asm
  • emulators/C64Programs/Code/Tests/TESTCIA2NMI.asm
  • emulators/C64Programs/Code/Tests/TESTKEYBOARD.asm
  • emulators/C64Programs/Code/Tests/TESTKEYBOARDIRQ.asm

Timer regressions are easiest to catch with minimal assembly programs that count IRQ/NMI firings or observe PB6/PB7 transitions.

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