Doomsday algorithm, implemented in assembly language by hand from scratch

Conway's Doomsday algorithm — the trick for working out the weekday of any date in your head — written as 723 lines of 8086 assembly. No libraries, no runtime, no framework. Just org 100h, int 21h, and arithmetic done by hand.

Along with it, a website that live-demonstrates the program running. Real DOSBox compiled to WebAssembly, with real NASM installed inside it. Your source is written into the emulated filesystem, assembled with nasm -f bin, and the resulting .COM is executed — in the browser, with nothing installed and no server anywhere.

Try it

▶ Watch it run	The finished program. Source read-only on the left, a live DOS screen on the right, a debug panel underneath. It assembles the moment the page loads — click the screen and type a date.
Write your own	The same toolchain as a scratchpad. Type 8086 assembly, press Run, and NASM assembles it inside DOS exactly as it would on a real machine — errors, line numbers and all.

Because it is genuinely DOS and genuinely NASM, anything that works here works in a real DOSBox. It is a faster way to run the same toolchain, not a simulation of it.

The algorithm

Every year has a weekday — its doomsday — that a set of easy-to-remember dates all fall on: 4/4, 6/6, 8/8, 10/10, 12/12, and the last day of February. Find that one weekday for the year and any date is a short hop from the nearest anchor.

It is designed to be done in your head. Doing it in 8086 assembly means doing the division yourself — div bl, remainder in AH, and no help from anyone.

The same algorithm was implemented in C++ first; that version and a longer explanation of the arithmetic live in the sibling Doomsday project. This repository is the third implementation.

What is actually running

your source ──▶ prog.asm written into the DOS filesystem
                     │
                     ▼
              nasm -f bin -o prog.com -s      ← real NASM, inside real DOSBox
                     │
        diagnostics ─┴─▶ parsed → gutter markers, and the program is not run
                     │ ok
                     ▼
                 prog.com executes
                     │  int 21h AH=09h/02h ──▶ the screen
                 keystrokes ──▶ int 21h AH=01h/0Ah

Everything is static. No backend, no build step, no package manager, no trackers, and nothing leaves your browser.

Repository

docs/                  the GitHub Pages site — everything served lives here
├── index.html         landing page
├── doomsday.html      the showcase: read-only source + big DOSBox + debug panel
├── ide.html           the editable scratchpad
├── runner.html        one throwaway iframe per run (js-dos can't re-init twice)
├── asm/
│   ├── doomsday.asm   ← the project: the algorithm, in 8086 assembly
│   └── smoke.asm      toolchain fixture: prints, prompts, reads, echoes, exits
├── js/
│   ├── dosengine.js   the only module that knows js-dos exists
│   ├── nasm-errors.js pure diagnostic parser — no DOM, testable with node
│   ├── runlog.js      the assemble → run → exit phase machine
│   └── asm-view.js    read-only source viewer with gutter markers
├── css/vscode.css     the VS Code Dark+ chrome
└── vendor/            js-dos 8.4.1 (GPL-2.0) + NASM 2.16.03 DOS (BSD-2)

DESIGN.md              decisions, spike results, traps, and the NFRs
tools/                 node test for the diagnostic parser

DESIGN.md is worth reading if you care how this was built. It records the decisions, the seven traps that cost real hours (js-dos cannot be initialised twice in one document; never shell-redirect a DJGPP program; NASM's errors need -s or a failed build reports success), the measured timings, and the non-functional requirements — including two that are deliberately not implemented, with reasons.

Running it locally

No build step. Serve docs/ and open it:

python -m http.server 8000 --directory docs

Then visit http://localhost:8000. The parser has a node test:

node tools/test-nasm-errors.js

If the DOS screen freezes, check the tab is in front. js-dos drives the emulator from requestAnimationFrame, so a backgrounded tab stops the machine dead. It looks exactly like a hang, and it isn't one.

Status

Working end to end. doomsday.asm assembles with zero NASM errors, runs, accepts typed input through all three prompts on desktop and mobile, and reports the correct weekday.

Verified by hand: 2026 / 08 / 30 → Sunday (doomsday 2026 is Saturday; 8/8 is an anchor, +22 days) and 2000 / 12 / 12 → Tuesday (12/12 is an anchor).

Credits

Runs on js-dos (DOSBox, GPL-2.0) and NASM (BSD-2-Clause). Attribution in docs/vendor/NOTICE.md.

Because this ships and runs js-dos, the repository is licensed GPL-2.0 — see LICENSE.