A cycle-accurate NES emulator in a single HTML file, with serverless WebRTC multiplayer.

Open nes.html in a browser. That's the whole install.

• Full 6502 CPU, cycle-accurate at the bus level (every hardware dummy read/write modeled)
• Dot-based PPU: loopy scrolling, sprites, sprite-0 hit, $2002/NMI races, odd-frame skip
• Full APU: 2 pulse, triangle, noise, DMC (with DMA + IRQ), frame counter — AudioWorklet output
• Mappers: NROM, MMC1, UNROM, CNROM, MMC3 (A12-filtered IRQ), AOROM — ~85% of the licensed library
• Savestates, battery-save persistence (localStorage), gamepad support, 2-player keyboard
• Netplay: deterministic lockstep over a WebRTC DataChannel with copy-paste signaling — no server anywhere. Host sends full state on connect; desyncs auto-heal.
• Built-in freely-licensed game (LJ65 by Damian Yerrick) for zero-setup testing

Passes: nestest (all 8,991 log lines), blargg instr_test-v5, all 10 ppu_vbl_nmi tests, all 11 sprite-hit tests, all 8 apu_tests, MMC3 1/2/3/5 (+13/14 of scanline_timing), and SingleStepTests 65x02 with exact per-cycle bus activity. See DOCS.md for the full matrix.

• DOCS.md — documentation roadmap, architecture notes, test status
• tools/run-nestest.mjs — CPU trace diff vs nestest.log
• tools/run-singlestep.mjs — SingleStepTests incl. bus-activity validation (SST_DIR env to point elsewhere)
• tools/run-blargg.mjs <rom> — blargg $6000-protocol test runner
• tools/run-determinism.mjs — replay + savestate-resume hash checks
• tools/run-netplay-sim.mjs — two-peer lockstep simulation with latency + desync recovery
• tests/ — test ROMs (nestest, blargg suites); tools/fetch-singlestep.sh re-fetches the big JSON suites

The emulator core is DOM-free between the //<NES-CORE> markers in nes.html; the Node harnesses extract and run it directly, so the single file stays the source of truth.

Emulator code: MIT. tests/lj65.nes is free software by Damian Yerrick (see tests/README.txt); test ROMs by blargg and others are redistributed from the community nes-test-roms collection.