A computer built from NOR gates: inside the Apollo Guidance Computer

New post by Ken Shirriff on the AGC. This one takes a look at how some of its components (the registers, ALU, etc) are built using NOR gates.

We recently restored an Apollo Guidance Computer1, the computer that provided guidance, navigation, and control onboard the Apollo flights to the Moon. This historic computer was one of the first to use integrated circuits and its CPU was built entirely from NOR gates.2 In this blog post, I describe the architecture and circuitry of the CPU.

This has been a whirlwind tour of the Apollo Guidance Computer’s CPU. To keep it manageable, I’ve focused on the ADS addition instruction and a few of the control pulses (A2X, RG, and WY) that make it operate. Hopefully, this gives you an idea of how a computer can be built from components as primitive as NOR gates.

The most visible part of the architecture is the datapath: arithmetic unit, registers, and the data bus. The AGC’s registers are built from simple NOR-gate latches. Even though the AGC’s arithmetic unit can only do addition, the computer still manages to perform a full set of operations including multiplication and division and Boolean operations.9


Whoops. Forgot to include the link to the post. Here it is: [ https://www.righto.com/2019/09/a-computer-built-from-nor-gates-inside.html ]

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Thanks for sharing this. Ken’s investigations and write-ups are great - I will eventually see this in my RSS feeds, but I’m way behind.

It’s just come to me, a recollection that the (rather later!) Cray-1 is also built from NOR gates. Ah yes, Wikipedia says:

The Cray-1 used only four different IC types, an ECL dual 5-4 NOR gate (one 5-input, and one 4-input, each with differential output), another slower MECL 10K 5-4 NOR gate used for address fanout, a 16×4-bit high speed (6 ns) static RAM (SRAM) used for registers and a 1,024×1-bit 48 ns SRAM used for the main memory

and here’s the 1977 paper in PDF form:
RM Russell, “The CRAY-1 Computer System”, Comm. ACM, January 1978, pp. 63–72.


I do like Ken’s use of footnotes. In this case, even the footnotes have footnotes. A good read!

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