Apollo Guidance Computer: Dipstiks and Reverse Engineering the Core Rope Simulator
canopic jug writes:
Ken Shirrif writes about restoring the computer which helped guide spacecraft from the Earth to the moon and back again. The Apollo Guidance Computer restoration team, consisting of Mike Stewart, Carl Claunch, Marc Verdiell, and Ken Shirrif ran into challenges simulating the permanent storage. In flight the persitent storage was made of core ropes, though these were simulated on the ground. The team is currently reverse engineering the rope simulators, which were used on the ground and originally built with 7400-series TTL integrated circuits, so as to avoid having to thread an overwhelming multitude of cores with each new version. Once completed the programs were hard-wired into the computers by passing wires through magnetic rings, a process which took many weeks and had to be done correctly the first time. The restoration team opted for a simpler method and built theirs from a BeagleBone.
The Apollo Guidance Computer held six core rope modules, each storing just 6 kilowords of program information (about 12 kilobytes).2 Core rope modules were a bit like a video game ROM cartridge, holding software in a permanent yet removable format. Programs were hard-wired into core rope by weaving wires through magnetic cores. A wire passed through a core for a 1 bit, while a wire going around a core was a 0 bit. By weaving 192 wires through or around each core, each core stored 192 bits, achieving much higher density than read/write core memory that held 1 bit per core.
Earlier on SN:
Her Software Put Men on the Moon. Fifty Years Later, Margaret Hamilton got a Glowing Moonlit Tribute (2019)
The Machine That Made the Moon Missions Possible (2019)
NASA Restores Mission Control (2019)
First Moon Landing Manual Could Fetch $9 Million at Auction (2019)
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