Can a machine store its own instructions?

That time, that place

ENIAC, built during the war, computed fast and had a strange weakness.

To run a different calculation you had to unplug cables, replug them, and reset switches one by one — days of work. Seconds to compute, days to prepare.

Why this question

The machine already has places to hold numbers. What if the sequence of operations were held there too?

Encode the sequence as numbers and changing the computation stops being a matter of rewiring and becomes a matter of writing different numbers.

What was found

This is the stored-program design. Turing's universal machine — the one that reads its own rule table off the tape — had become an actual circuit.

The consequences were large. Changing a program dropped to seconds. And since programs sat in the same store as data, programs could read, write, and modify other programs. Compilers and operating systems come from here.

The report describing it circulated widely in draft. Only von Neumann's name appeared, which obscured Eckert's and Mauchly's contributions — but because it was public, no patent was possible, and so anyone could build on it.