Try it in your browser at https://8086.jyotiprakash.org

µSystem 8086

It boots FreeDOS. The PC/XT preset — an 8088, 74LS glue, 8237A DMA, µPD765 floppy, 8259A/8253/8255, Hercules video and an XT keyboard, wired pin by pin on the canvas — runs GLaBIOS from its EPROM, POSTs, loads the boot sector over DMA channel 2, and brings FreeDOS 1.3 up on the phosphor CRT.

CPU core: 100.00% exact on the full SingleStepTests/8088 suite — 3,007,000 test cases, every documented and undocumented instruction, every trap path, every "undefined" flag bit matching real silicon (including microcode-faithful CORD/CORX division and multiplication, DAA/DAS quirks, SETMO, and the REP-negates-MUL quirk).

A fully static, single-file, cycle-accurate 8086/8088 system simulator for students — build a computer from period-correct chips on a canvas, assemble real 8086 code, step it cycle by cycle, probe any wire, and read the waveforms. No server, no install: one index.html on GitHub Pages.

• Design — place 8086/8088 CPUs, 8284A clock gens, SRAM/EPROM, 74LS glue, ports, LEDs, 7-segment displays; wire pins or whole buses; the DRC flags strict violations (VCC–GND shorts, bus contention) and weak ones (floating inputs, partial decode).
• Run — assemble your program into an EPROM, press play. Once running, the board is frozen; you can only touch what a real operator could: keys, switches, floppies.
• Probe — hover any wire for its live value; add any signal or bus to the waveform analyzer; step by cycle or by instruction; rewind; export PNGs for your teacher.
• Boot (roadmap) — HGC video on a phosphor CRT, XT keyboard, floppy + DMA, GLaBIOS → FreeDOS A:\>.

Develop

No dependencies. node build.mjs produces index.html and refreshes dev.html (which loads src/ files individually for debugging). Open either in a browser.

node build.mjs            # build single-file index.html
node tests/cpu-basic.mjs  # CPU core sanity
node tests/asm-basic.mjs  # assembler round-trips
node tests/sim-board.mjs  # pin-level board integration (boot, blink, IRQ lab)
node tests/memmap.mjs     # netlist memory-map analyzer
node tests/browser.mjs    # full UI pass in headless Chromium (needs playwright-core)
node tests/singlestep.mjs --download   # fetch & run SingleStepTests/8088 (CPU validation)

See ARCHITECTURE.md for the design, and guide/ for the illustrated guide.

License

µSystem 8086 is free software under the GNU GPL, version 3 or later. See LICENSE for the full text and COPYRIGHT for the notice.

The built index.html additionally embeds GLaBIOS (GPL-3), the FreeDOS 1.3 boot disk (GPL-2+) and public-domain VGA fonts, each verbatim and under its own license. See assets/LICENSES.md.