Ken Shirriff @righto.com · 24d

The second board is memory (PROM), holding microcode for the I/O Processor, which is a (very strange) computer, independent of the CPU. Each gold-lidded chip holds 2K bits in tiny metal fuses. The chip is programmed by blowing the fuse for each 1 bit, literally burning the PROM.

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Ken Shirriff · 24d

IBM had a series of aerospace computers called System/4 Pi. These computers all used standard-sized boards (top). Except that the I/O Processor's boards (bottom) were one inch wider. I guess they needed the extra space for more circuitry.