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