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Subleq

2026 Open source · Public domain Online

A small SUBLEQ one-instruction-set computer (OISC) authored for emulators.org and running in the browser. Every instruction is a triple (A, B, C): it subtracts Mem[A] from Mem[B] and branches to C when the result is less than or equal to zero. A negative address selects character I/O. It boots printing "Hello, world!" and plugs into the shared debugger, so you can single-step the one instruction, set breakpoints on the instruction pointer, and watch memory change.

Runs on: Web browser

Subleq Online Emulator

Play Subleq using JavaScript directly in your browser.

Configurations

ConfigurationEmulatorMachineOSLegal
Hello, world!SubleqSubleq (OISC)openOpen ⛶

Machines emulated

Subleq (OISC)

Chips

Subleq (OISC)

Notes

Embedding

subleq is a self-contained one-instruction computer authored for this site. Vendor the two source files and drive the machine from your own loop; the core never touches the DOM, so the debugger can pause, step and breakpoint it:

var vm = new SubleqVM({ onOutput: putChar, onInput: getChar });
vm.load(SubleqVM.printProgram('Hello, world!\n'));   // a ready-to-run image
while (vm.step()) {}                                // step() = one instruction

The machine is a plain object. Everything the debugger needs is a live field. There is no wasm heap to reach into:

MemberKindWhat it does
vm.step()methodExecute exactly one SUBLEQ instruction (the single-step primitive). Returns false once halted or blocked on input.
vm.ipfieldThe instruction pointer (a word index). Halts when it goes negative.
vm.memfieldThe word-addressed memory: a plain array of signed integers.
vm.rd(a) / vm.wr(a,v)methodSide-effect-free read / write of one word, what the debugger's hex view and watchpoints call.
SubleqVM.printProgram(s)staticAssemble a self-contained "print this string" image (one OUT per character + a halt).

Because the CPU and memory are ordinary JavaScript, breakpoints and watchpoints are host-side checks around vm.step() and vm.wr() - no changes to the core.

Debugger integration

The boot shim publishes window.EMU_BOOT with a full transport: pause / resume / isPaused, stepInsn(n) (one instruction) and step(n) (a burst), reset, plus breakpoints (a Set of instruction-pointer values the loop checks before each step) and watchpoints (a Set of memory addresses, checked inside vm.wr). A new CPU decoder - /debugger/src/cpus/subleq.js - disassembles each triple as one instruction: subleq A, B, C, or OUT A / IN B when an operand is -1. It reads the true signed words off EMU_BOOT.vm.mem, because the shared views hand decoders a byte-masked reader that would truncate a word operand.

Architecture

SUBLEQ (SUBtract and branch if Less-than or EQual to zero) is the canonical one-instruction machine. There is a single instruction, so there is no opcode. Every three consecutive words are an instruction:

  • (A, B, C) - Mem[B] = Mem[B] - Mem[A]; if the result is <= 0 the instruction pointer jumps to C, otherwise it falls through to the next triple.
  • A == -1 - read one character from the input stream into Mem[B].
  • B == -1 - write Mem[A] out as a character.
  • Execution halts when the instruction pointer becomes negative (programs branch to -1 to stop).

Despite having one instruction, SUBLEQ is Turing-complete: subtraction plus a conditional branch is enough to synthesise moves, adds, copies and loops. The bundled "Hello, world!" image emits each character with its own OUT instruction, then zeroes a scratch cell and branches to -1 to halt.