c64-cia-audit

name

c64-cia-audit

description

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.

C64 CIA Audit

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.