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Apollo 11 Apollo Guidance Computer
By Martin English | July 27, 2009
I came across an article on how someone built a copy of the 1964 prototype of the Apollo Guidance Computer (or AGC). It’s hard to compare these, which had a clock speed of 1.024 MHz (that was internal – the external signaling was half that), to modern desktop / laptops with speeds of 2ghz and above (1Ghz is 1,000Mhz is 1,000Khz is 1,000hz).
Going back to something where comparison makes sense, not just in speed, but in memory size, you can look at the IBM PC “XT”, released in 1981. In the cheapest configuration it had 8 times more memory – 16k, vs the 2k used by the final version of the AGC (the AGC also had read-only storage of 32k). The IBM PC XT ran at a clock speed of 4.77MHz (0.00477 GHz, if you want to compare it with your machine). The Apollo’s Guidance Computer ran at less than a quarter of that.
What blows me away is the functionality that NASA got out of the Apollo Guidance Computer. The real-time operating system in the Apollo 11 spacecraft could multi-task up to 8 jobs at a time, something we take entirely for granted today, but a major feat in its time. However, multi-tasking had to be exercised from within the programs – modern operating systems are in control of the execution and can stop any program at any time and hand off some computing power to another. The Apollo Guidance Computer relied on non-preemptive multi-tasking, where programs had to be written so that they relinquish control back to the OS periodically (or nothing else got run)
The Apollo version of the AGC also had a virtual machine which offered more complex instructions, and could be used to perform more advanced mathematics. This was all written within the limitation of 2k of memory and 32k of storage – no external storage whatsoever. Keep in mind that the Apollo 11 was actually the advanced “Block II” version of the AGC, and that earlier missions had relied upon as little as 24k of core read-only storage, and 1k of main memory.
By comparison, the Space Shuttle uses the AP-101 avionics computer which shares its general architecture with the System/360 mainframes, of the 1960’s. It remains in service because it works and is flight-certified, whereas to certify a replacement would cost hundreds of thousands or millions of dollars.
Topics: Hardware, History, Technology | No Comments »
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