Harwell Dekatron Computer (WITCH)

A working model of the Harwell Dekatron Computer (WITCH) — a 1951 DECIMAL computer whose memory is a wall of glowing DEKATRON tubes (each tube stores one decimal digit 0-9 as a neon glow sitting on one of ten cathodes), with relay control and a PAPER-TAPE program. It is famously slow — about one operation per second — so you can watch every carry ripple through the tubes. Press RUN to let the tape reader step the program (SPACE toggles run/stop); STEP executes one operation at a time so you can watch a single add; REWIND returns the tape to the start; the slower/faster buttons change the reader speed. Each row on the wall is a register: the accumulator ACC on top, then the store R0..R7, showing the dekatron glows, a sign lamp and the live decimal value. Open the debugger, or add ?debug, to single-step operations, read every dekatron register side-effect free, set a breakpoint on a paper-tape frame, or a write watchpoint on a register.

A working model of the Harwell Dekatron Computer (WITCH) — a 1951 DECIMAL computer whose memory is a wall of glowing DEKATRON tubes (each tube stores one decimal digit 0-9 as a neon glow sitting on one of ten cathodes), with relay control and a PAPER-TAPE program. It is famously slow — about one operation per second — so you can watch every carry ripple through the tubes. Press RUN to let the tape reader step the program (SPACE toggles run/stop); STEP executes one operation at a time so you can watch a single add; REWIND returns the tape to the start; the slower/faster buttons change the reader speed. Each row on the wall is a register: the accumulator ACC on top, then the store R0..R7, showing the dekatron glows, a sign lamp and the live decimal value. Open the debugger, or add ?debug, to single-step operations, read every dekatron register side-effect free, set a breakpoint on a paper-tape frame, or a write watchpoint on a register.

A working model of the Harwell Dekatron Computer (WITCH) — a 1951 DECIMAL computer whose memory is a wall of glowing DEKATRON tubes (each tube stores one decimal digit 0-9 as a neon glow sitting on one of ten cathodes), with relay control and a PAPER-TAPE program. It is famously slow — about one operation per second — so you can watch every carry ripple through the tubes. Press RUN to let the tape reader step the program (SPACE toggles run/stop); STEP executes one operation at a time so you can watch a single add; REWIND returns the tape to the start; the slower/faster buttons change the reader speed. Each row on the wall is a register: the accumulator ACC on top, then the store R0..R7, showing the dekatron glows, a sign lamp and the live decimal value. Open the debugger, or add ?debug, to single-step operations, read every dekatron register side-effect free, set a breakpoint on a paper-tape frame, or a write watchpoint on a register.