EDSAC

Boots with the classic squares demo already running: it computes 1, 4, 9, 16 ... 81 by summing successive odd numbers and stores each into the store, so the growing binary numbers light up the monitor tube, while the teleprinter counts 1-9. The amber marker is the sequence control (program counter) walking the orders. Use the debugger to single-step one order, set a breakpoint on an order line, or watch a store line for writes. The on-screen console gives Start / Stop / Single EP / Reset - EDSAC had no keyboard, it read paper tape and was started from switches.