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Comment by hinkley | Hacker News Reader

Analog computers have to be rebuilt if it turns out the program is wrong though, don’t they?

In the context of this thread, I believe even a digital computer would have to be rebuilt if the program is wrong... :P

Unless you typically salvage digital computers from the wreckage of a failed rocket test and stick it in the next prototype. If the FCC is wrong, kaboom.

I'm pretty sure you can perform tests and find defects without actually flying the rocket

Presumably they meant a program being discovered to be wrong before the computer was actually launched. And meant literally building a whole new computer, not just recompiling a program.

Yeah, though to be fair, some of the programs Apollo ran were on hand woven ROMs, so I may be making too fine a distinction. The program itself was built, not compiled. It if we are comparing with today, it would just be installed, not constructed.

For the Apollo Guidance Computer, changing the program meant manually re-weaving wires through or around tiny magnet rings. A good part of the cost of the computer was the time spent painstakingly weaving the wires to store the program.

There's a very nice video about the assembly lines MIT made just for building the Apollo computer [0].

0. https://www.youtube.com/watch?v=ndvmFlg1WmE

Pardon me, but why would you have to re-weave wires around magnetic rings? The magnetic rings are for storing data; the whole point is that you can change the data without rewiring the memory. If you have to re-wire permanent storage (e.g. program storage), that's equivalent to creating a mask ROM, which is basically just two funny-shaped sheets of conductor. There's no need for magnetic rings.

No, I'm not talking about magnetic core memory. Core rope memory also used little magnetic rings.

https://en.wikipedia.org/wiki/Core_rope_memory input wires were energized, and they were coupled (or not) to the output wires depending on if they shared a magnetic ring (or not).

Thanks! I'd never heard of rope memory.

https://www.youtube.com/watch?v=hckwxq8rnr0

Because it’s not RAM, it’s ROM.

Only if the bug was caught after the computer had been assembled for the mission. For development, they used a simulator. Basically, a cable connected to a mainframe, with the bigger computer simulating the signals a bundle of core rope would produce.

I had assumed it meant more simple things like balanced balancing pneumatic or mechanical components that always put you at the the correct ratio sort of like a carburetor vs fuel injection.

Typically they were reprogrammed by changing the jumpers. The analogous digital change would be replacing the band of cards in a jaquard loom.

Much less than “rebuilding”.

There have been some hybrids too.

Yes though they tend to be mechanically tuned. So like a pneumatic computer or will get tuned to operate into some range of inputs and you probably bench prototype it before you mass produce it

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