rnes/STATE.md

Status

Done

• Repo set up, backed up the original game ROMs to roms/backup (gitignored, stays local)
• Got a full set of CPU test ROMs in roms/test/cpu: kevtris nestest (with reference log) plus the blargg suite (instr tests, timing, interrupts, dummy reads, reset checks)
• Cartridge module done and working (src/cartridge.rs):
  • iNES header parser (magic, PRG/CHR sizes, mapper, mirroring, flags)
  • Mapper 0 (NROM) support only: 16KB PRG mirroring, 32KB direct, CHR-ROM and CHR-RAM (zeroed 8KB buffer)
  • read_prg/read_chr/write_chr with debug_assert guards
  • Split from_file (thin I/O wrapper) / parse_data (pure parser) for testability
  • 8 unit tests, all passing (header, PRG mapping, error cases, CHR-RAM roundtrip)
• Bus module done (src/bus.rs):
  • Bus trait (read/write) so the CPU is decoupled from the concrete memory layout
  • NesBus: 2KB mirrored CPU RAM, cartridge PRG routing, PPU/APU/SRAM/expansion regions stubbed (return 0)
• CPU skeleton done (src/cpu.rs):
  • Flag bit constants, registers (A/X/Y/SP/PC/P), set/clear/flag helpers, set_zp_flags
  • Memory helpers: fetch, fetch16, read, write, read16, push, pop
  • All 10 addressing mode helpers, including the JMP ($xxxx) page-wrap bug and page-cross cycle penalties
  • First 3 opcodes implemented (JMP abs, LDX imm, STX zp)
  • step dispatch shell + trace in nestest format
  • set_test_start for the nestest harness
• nestest harness working (main.rs): loads nestest.nes, runs from $C000 with CYC:7, dumps trace to ours.log
• Batch 1 done (subroutines + stack): JSR, RTS, PHA, PLA, PHP, PLP, RTI, BRK, plus NOP and SEC as quick wins. Verified SP:FD->FB across JSR (push high-then-low works)
• Batch 2 done (branches): all 8 branches (BEQ/BNE/BCC/BCS/BPL/BMI/BVC/BVS) via one shared branch() routine with condition param. Taken = +3 cycles, page-cross = +1 more. Verified both taken and not-taken paths in the log
• Flag set/clear ops done (part of Batch 10): CLC, CLI, CLV, CLD, SEI, SED (SEC done in Batch 1). SEI/SED set, the rest clear. Fixed a real bug where SEI/SED were clearing instead of setting
• Batch 8 done (inc/dec): INC (4 modes), DEC (4 modes), INX, INY, DEX, DEY. RMW cores (inc/dec) read-modify-write and correctly return base WITHOUT the page-cross extra (RMW cycles are fixed). Register ops are wrapping_add/sub with Z/N only. 124 of 151 official opcodes done. INY/INX carried the log from 506 to 678 lines
• Batch 9 done (shifts): ASL, LSR, ROL, ROR (accumulator + 4 memory modes each) via four shift cores (shift_asl/lsr/rol/ror) + wrappers. ROL/ROR read carry-in before overwriting it. Memory shifts follow the RMW fixed-cycle rule. 144 of 151
• Batch 10 done (transfers + JMP ind): TAX, TAY, TXA, TYA, TSX, TXS (all 2 cycles; TXS sets NO flags, the other five set Z/N), plus JMP (0x6C) which finally uses address_ind. Resolved the long-standing address_ind dead-code warning
• OFFICIAL CPU COMPLETE: all 151 official opcodes implemented. Trace matches the nestest reference log through line 5003 (entire official section), 0 mismatches. Stops at line 5004 (0x04, first illegal opcode) by design
• Fixed a latent Batch 4 bug the log caught at line 3319: the sta core added the page-cross extra, but stores have FIXED cycle counts. Fixed sta core to return base without extra, and corrected bases (sta_abx/sta_aby 5, sta_izy 6). Before the fix the diff had a +1 CYC drift from line 3319 onward
• Batch 3 done (loads): LDA (8 modes), LDX (5 modes), LDY (5 modes) via three shared cores (lda/ldx/ldy) + thin wrappers. Caught two real bugs in review: all 4 LDY wrappers called self.ldx (would write to X instead of Y), and 6 of 8 LDX/LDY wrappers had wrong base cycles (3 instead of 4). Both fixed
• Batch 4 done (stores): STA (7 modes), STX (3 modes), STY (3 modes) via three shared cores (sta/stx/sty) + thin wrappers. No flag changes. Refactored stx_zp to use the shared stx core for consistency. Fixed a comment typo (sta_abx was labeled 0x95 instead of 0x9D)
• Batch 5 done (compares): CMP (8 modes), CPX (3 modes), CPY (3 modes) via three shared cores (cmp/cpx/cpy) + wrappers + immediate variants. Sets C (reg >= mem), Z, N; no store. Uses wrapping_sub for the borrow math. 70 of 151 official opcodes done. No diff progress expected until the log reaches these opcodes
• Batch 6 done (arithmetic): ADC (8 modes), SBC (8 modes), BIT (2 modes) via shared add_with_carry core + adc/sbc/bit cores + wrappers. SBC adds the one's complement (A + ~mem + C), which makes the V-flag math correct. Decimal flag inert (binary only). 88 of 151 official opcodes done. Big diff progress: BIT/PHP/PLA/PHA carried the log from 38 to 73 lines
• Batch 7 done (logic): AND (8 modes), ORA (8 modes), EOR (8 modes) via three cores (and/ora/eor) + wrappers + immediate variants. Only Z/N flags affected, C/V untouched. Caught a real bug in review: all three _izx wrappers used base 5 instead of 6. 112 of 151 official opcodes done. Huge diff progress: carried the log from 73 to 506 lines
• Match compaction done: step and opcode_len both reformatted to max 4 opcodes per line. step match grouped by family with header comments; opcode_len keeps per-entry /* */ mnemonics. Diff verified unchanged after compaction
• trace bytes are length-aware via opcode_len (fixed-width byte field, columns align for awk diff)

Next

Official CPU milestone is COMPLETE (all 151 opcodes, nestest-verified through line 5003). The full 8991-line nestest log tests illegal opcodes too (225 distinct opcodes total; first illegal is 0x04 at line 5004). Official opcodes never seen in the log: BRK 0x00, CLI 0x58.

Follow-up options, in any order:

• Illegal-opcode phase: implement the undocumented opcodes (0x04 and friends) for the full 8991-line nestest pass. ~76 illegal opcodes present in the log. Reference: nesdev wiki undocumented opcode table
• MMC1 (mapper 1) in the cartridge so blargg official_only.nes / all_instrs.nes can run (authoritative official check)
• Run blargg mapper-0 suite (coredump, cpu_timing_test, ram_after_reset, registers)
• Then PPU/bus/timing, GUI (egui + winit) come after the CPU milestone

Decisions

• No external dependencies yet, keeping it pure std until the GUI milestone
• GUI choice: egui + winit (decided, not yet used)
• Scope: cartridge + CPU first, verified headless before graphics
• Bus is a trait (impl Bus) so the CPU works against any memory layout (real bus, test bus, debug bus)
• Flags stored as a raw u8 with named bit-mask constants (FLAG_CARRY etc.)
• opcode length is a growing match in opcode_len() (returns u8); migrate to a full 256-entry table when it gets big
• trace diff compares PC + A/X/Y/P/SP/CYC, ignoring disassembly/PPU columns
• Mapper 0 first, other mappers (MMC1, CNROM) implemented after CPU works
• Test ROMs are homebrew/open, no ethics issue; game backups stay in gitignored roms/backup