Thread crossing alert... Since the ZEBRA used a drum for storage, most instructions had to wait for the drum to rotate into position, so one instruction takes a variable amount of time. And so the computer had a moving-needle efficiency meter on the front panel, which presumably integrated a series of pulses (similar to the circuit Tony described earlier). Or I suppose it could have divided the rate of instruction execution by the rate of drum rotation (since there was a timing track on the drum anyway). 100% efficiency was attainable but only by simple programs such as a parity check of the drum. It also had a telephone dial, sense switches, the usual memory-tweaking switches, and a register display. In the second-generation (transistor) machine, the register display was a CRT. In the first-generation (tube) machine the register display was a bunch of Magic Eye units IIRC. -- Derek