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Comment by moring

13 hours ago

Actual application code was hardwired, entered manually with switches and lights, or with punch cards. Later, when ICs were sufficiently advanced, mask-programmed ROMs/PLAs.

Or diode matrix ROMs were pretty popular as well.

Electrically, essentially what happens in most mask ROMs, but as a circuit board that allowed you to solder in a diode or not in each bit location in order to specify a 1 or a 0.