The Apollo Guidance Computer was the onboard brain of the crewed lunar missions.
It had two kinds of memory, and the difference mattered.
Working memory was about 4 kilobytes: magnetic rings that could be rewritten while the computer ran. Less silicon than the chip in a singing greeting card.
The program lived in core rope memory, a web of copper wires threaded through magnetic cores, holding about 72 kilobytes. Its contents were fixed during flight.
How did the code get in there?
Women at Raytheon threaded copper wire through tiny magnetic cores, one bit at a time. A wire through a core meant one. A wire around it meant zero. The software was literally woven.
On July 20, 1969, the lunar module Eagle was descending toward the Moon. Alarms 1201 and 1202 lit up.
What did they mean?
The computer was overloaded. It discarded low-priority tasks and kept essential functions running.
In Houston, 26-year-old guidance officer Steve Bales had about 15 seconds to decide.
He called “Go.”
The computer was not broken.
It did exactly what it was built to do.
It drew about 55 watts, less than a modern laptop charger.
So little working memory.
And people on another world.
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