mos-6502/CLAUDE.md

Contributor notes

Commands

make              # build bin/mos6502
make test         # core checks
make debug        # strict warning build and checks
make sanitize     # ASan/UBSan build and checks
make snake        # assemble and run the SDL game (requires SDL2 + cc65)

Architecture

• CPU6502 contains CPU-visible and interrupt-line state.
• Bus6502 owns RAM and Easy6502 MMIO; CPU code only uses bus accessors.
• cpu_step executes one instruction or interrupt and returns status/opcode/cycles.
• src/opcode.c has a 256-entry descriptor table for all 151 official encodings.
• src/frontend.c is an optional SDL2 frontend with a dependency-free stub.
• tests/run_tests.c is the in-tree test harness; tests/run_suite.c runs external ROMs.

The target is the NMOS 6502. Preserve decimal-mode flag behavior, zero-page wrapping, the indirect-JMP page-wrap bug, stack push order, and page-cross cycle rules. Do not silently accept undocumented opcodes.

The virtual machine exposes game state at $00FB, high/game score at $00FC/$00FD, random input at $00FE, keyboard input at $00FF, and a 32x32 framebuffer at $0200-$05FF.