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Booting the IBM 1401: how a 1959 punch-card computer loads a program

How do you boot a computer from punch cards when the computer has no operating system and no ROM? To make things worse, this computer requires special metadata called “word marks” that can’t be represented on a card. In this blog post, I describe the interesting hardware and software techniques used in the vintage IBM 1401 computer to load software from a deck of punch cards. (Among other things, half of each card contains loader code that runs as each card is read.) I go through some IBM 1401 machine code in detail, which illustrates the strangeness of the 1401’s architecture and instruction set compared to a modern machine.

I simply cannot imagine what wizardry these newfangled computers must’ve felt like to the people of the ’50s, when computers first started to truly cement themselves in the public consciousness. Even though they’ve been around for twice as long, I find a world without cars far, far easier to imagine and grasp than a world without computers.

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