Harwell Dekatron Computer (WITCH)

Embedding

The machine is modelled from scratch in plain JavaScript. There is no binary CPU and no ROM: every number is held as an explicit column of dekatrons — a dekatron is a cold-cathode neon decade counter with ten main cathode positions round a ring, and a single glowing discharge marks the current decimal digit. Pulsing the guide cathodes walks the glow one step, so it counts 0→1→…→9→0 and carries on the roll-over. A bank of dekatrons is a decimal register; the "program" is a paper tape of decimal instruction frames stepped by a reader.

The tape reader is the loop. The host owns the run loop: it calls machine.stepOp() about once a second (authentically slow) so you can follow each carry; the renderer eases every glow toward its target so the transfer discharge is visible.

var w = new Witch('runsum');  // load a paper tape
w.stepOp();                    // execute one tape operation (one add, one transfer…)
var acc = w.accValue();       // the accumulator's decimal value
var r0  = w.regValue(0);      // store register R0

Debugger integration

witch-debug.js reads window.EMU_BOOT and hands the shared debugger a machine whose "CPU" is the dekatron store and whose "code" is the paper tape. Every debugger feature maps onto the real hardware.

• Registers = the dekatron banks. ACC and R0…R7 show the live decimal value of each bank of tubes and are editable — type a number and the dekatrons step to it. TAPE is the reader position and HALT the stop latch.
• Single-step = one operation. The Step control calls transport.stepInsn, which runs exactly one paper-tape instruction (one add, transfer, multiply…) so you can watch a single carry ripple through the tubes.
• Breakpoints on the tape. Execution breakpoints match the tape frame under the reader; the run loop stops when it reaches a marked frame.
• Write watchpoints on registers. A watchpoint fires when its dekatron register is written — a real "break when this number changes".
• Disassembly = the paper tape. A new witch decoder (debugger/src/cpus/witch.js) renders one row per tape frame — the raw decimal OO AA punch and its decoded mnemonic — so the code window literally lists the tape, advancing as the reader steps.
• Memory. Two chips: the paper tape (one frame per address, witch-disasm) and the dekatron digit store (one decimal digit 0-9 per tube), both read side-effect free.
• Controls. The Controls window carries a keypad — Run, Stop, Step, Rewind, Reset and slower/faster — the same handlers as the page's keys.

Architecture

The Harwell Dekatron Computer was built between 1949 and 1951 at the Atomic Energy Research Establishment, Harwell, by a team including Ted Cooke-Yarborough, Dick Barnes, Ken Hird and Gurney Thomas. It was designed to be cheap and reliable rather than fast — to take over routine calculation from human "computers" — and it succeeded: it ran for years, often overnight and unattended. In 1957 it was given to Wolverhampton and Staffordshire Technical College for teaching and renamed the WITCH (Wolverhampton Instrument for Teaching Computation from Harwell). Restored at The National Museum of Computing in 2012, it holds the record as the oldest surviving working digital computer.

How dekatrons store decimal. A dekatron is a cold-cathode gas-discharge tube: a central anode surrounded by a ring of ten main cathodes (with guide cathodes between them). A neon glow discharge sits on exactly one main cathode, and applying pulses to the guide cathodes walks the glow one position round the ring — a decade counter that counts 0-9 and emits a carry pulse when it rolls over from 9 back to 0. So one dekatron holds one decimal digit, visibly, and a column of them is a decimal register. The machine had roughly ninety dekatrons for its fast store, holding numbers of eight decimal digits and a sign. Because the store is the display, you could literally watch a number being built up.

Decimal arithmetic, by counting. Control was relay logic and arithmetic was serial and decimal. Addition pulsed the units dekatron and let carries ripple up the column, tube by tube. There was no multiplier: multiplication was done by repeated addition and division by repeated subtraction, which is why a multiply took several seconds while an add took about one. That deliberate slowness — around one operation per second — is the whole character of the machine, and this model keeps it: the reader steps once a second and the dekatron glows sweep so you can follow every carry.

What this model does and does not claim. The dekatron store, the visible stepping and carries, the decimal add/subtract/multiply-by-addition/divide-by-subtraction scheme, the paper-tape reader and the teleprinter listing are modelled. The exact relay control matrix, the original tape character encoding and the precise store size and instruction format are represented by a clean, decimal, accumulator-plus-store scheme in the spirit of the real machine rather than reproduced gate for gate — the WITCH's control was relays, not a documented micro-order ROM. The demonstration tapes (running sum, factorial, Fibonacci) are original CC0 programs, chosen so the dekatrons visibly compute. Silent: the real machine clicked with relays; that audio is not modelled, so there is no Sound button.