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ELI5 Yahaaa

And that is the boring half

Your Phone Beats the Computer That Flew to the Moon

The Apollo Guidance Computer had about 72 kilobytes of memory and its programs were woven by hand with wire. The interesting question is not why it was small. It is why it was enough.

Apollo Guidance Computer ~72 KB of program, ~4KB of working memoryThe phone in your pocket Millions of timesmore of both
One of them has landed on the Moon.

  1. Step 1 of 5

    The numbers really are as small as you have heard.

    Working memoryAbout 4 KBProgram memoryAbout 72 KBOne phone photoAround 3,000 KBA phone's memoryMillions of KB
    The bars for Apollo are drawn as thick as they can be and still be visible.

    The Apollo Guidance Computer ran at about 0.043 MHz — roughly a hundred thousand times slower than a modern phone. It held about 72 kilobytes of program and 4 kilobytes of working memory. A single photograph on your phone is bigger than everything that flew.

  2. Step 2 of 5

    The program was not loaded. It was woven.

    A wire through thering reads as 1A wire around itreads as 0Woven by hand, overweeksImpossible tooverwrite by accident
    NASA called it LOL memory, for Little Old Lady memory. The name has not aged well; the reliability has.

    Program memory was core rope: wires threaded through or past tiny magnetic rings, where through meant one and past meant zero. Once woven, it was physically unchangeable — and physically unable to be corrupted. Skilled workers, mostly women, wove it by hand at a rate measured in weeks per program.

    Core rope was dense for its era — several times more compact than any writable memory available — which is exactly why it was worth the enormous labour of weaving it.

  3. Step 3 of 5

    During the landing itself, the computer told them it was overloaded.

    1 A radarpiles onspare workMore jobsthan time torun them2 Alarm1202: noroom leftThe crew has30 secondsto decide3 Softwareshedsspare jobsGuidance andcontrol keeprunning4 MissionControlsays goEagle landswith secondsof fuel

    At 30,000 feet the alarms started: 1202, then 1201. A misconfigured radar was flooding the computer with work it did not need. The software had been written to expect exactly this: it dropped the low-priority jobs, restarted, and kept the landing tasks running. Mission Control recognised the code, said go, and Eagle landed.

  4. Step 4 of 5

    This is the actual lesson, and it is not about speed.

    Why 72 KB wassufficientOne known job, fullyspecified in advanceEvery byte written forthat job by handIt could not do anything else at all

    The computer was not a small version of a general-purpose machine. It did one job — fly this spacecraft — and every byte was spent on that job by people who knew precisely what it had to do. No operating system to host, no unknown future software to accommodate, no graphics, no spare capacity for anything it would never be asked to do.

  5. Step 5 of 5

    Modern computers are not mostly doing arithmetic.

    Parts spent onpictures, sound,readinessThe part that isarithmetic likeApollo'sTimes the Apollocomputer failedin flight: zero

    Your phone's power goes on drawing millions of pixels sixty times a second, decoding video, running a general-purpose operating system, and being ready for any program anyone might ever write. The navigation maths that flew to the Moon is a rounding error inside that. Apollo did not need a bigger computer. It needed a computer that never failed.

The short version

The Apollo Guidance Computer was slower than a modern kitchen appliance and its program was woven by hand — and it was enough, because it did exactly one fully understood job and nothing else.

Try it yourself

Check the size of one photo on your phone. Then remember the whole Apollo landing program was about 72 kilobytes — usually a fortieth of that single image.

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