Z3JS

An in-browser emulator of the Zuse Z3 — the electromechanical computer Konrad Zuse completed in Berlin in May 1941, generally credited as the first working, programmable, fully automatic digital computer.

Live demo →

What's actually emulated

• The real 22-bit semi-logarithmic word format — 1 sign bit, 7-bit exponent, 14-bit mantissa — encoded and decoded bit-for-bit.
• The real 64-word memory.
• The real arithmetic unit: add, subtract, multiply, divide, and hardware square root — including genuine ±infinity / "undefined" exception values on divide-by-zero or √(negative), instead of just crashing.
• The real absence of conditional branching. Programs run straight through, top to bottom, exactly like a punch tape from 1941 — see docs/ARCHITECTURE.md for why that's more interesting than it sounds.
• Both instruction sets: write in the modern mnemonics below, or the Z3's original nine — Pr, Ps, Ls1/La, Ls2/Ls, Lm, Li, Lw, Lu, Ld — including the real register-occupancy behavior that let the original machine skip naming which register an instruction targets. The two can be freely mixed.
• A small assembler turning readable mnemonics into the binary tape format the machine actually reads.
• A lamp-panel UI so you can watch registers and memory flip bits live.
• Save, load, and delete your own programs as .z3asm files, on top of the three built-in examples.

Getting started

git clone https://github.com/SSMGAlt/Z3JS.git
cd Z3JS
npm install
npm test
npx serve .

Open the printed local URL. (Loading index.html directly via file:// can fail in some browsers due to ES module CORS restrictions — use a local server, or the live demo above.)

Writing a program

LOADI1 7
LOADI2 5
ADD
PRINT
HALT

Full opcode reference in docs/ARCHITECTURE.md. Example programs live in examples/ and are also loadable from the dropdown in the UI.

Project structure

src/core/     the machine itself — memory, ALU, CPU, assembler (no DOM dependency)
src/ui/       browser UI wiring
styles/       CSS
tests/        node --test unit tests
examples/     sample .z3asm programs
docs/         architecture & opcode reference

Scope note

The numeric format, memory size, and lack of branching are historically accurate. The exact instruction encoding is this project's own design, since the original 1941 punch-tape bit format isn't preserved in accessible public archives. Details in docs/ARCHITECTURE.md.

License

MIT — see LICENSE.