Apollo Stored Its Flight Program in Wires Threaded Through Magnetic Rings
Core rope put fixed program bits into the route a wire took, while erasable memory handled the changing work.
A software release could be a bundle of wires you could hold. Updating it involved a rather more demanding operation than pressing Deploy.
Restorer Ken Shirriff describes the Apollo Guidance Computer’s fixed memory as storing “192 bits per core.” Ordinary erasable core memory used a ring for each bit; rope memory squeezed many fixed bits around the same magnetic element by giving each sense wire a physical route.
Shirriff’s account distinguishes the computer’s 2,048 words of erasable memory from 36,864 words of rope ROM. Similar-looking rings were doing very different jobs.
The restoration also exposes a useful escape hatch. Ground-test simulator boxes supplied program words externally, so developers could try code without manufacturing another rope. Even this famously physical software had a way to rehearse a change.
The simulator makes the history especially satisfying: the flight program was hardwired, yet the people developing it still needed room to change their minds.