Inside the Apollo Guidance Computer's core memory
The Apollo Guidance Computer (AGC) provided guidance, navigation and control onboard the Apollo flights to the Moon. This historic computer was one of the first to use integrated circuits, containing just two types of ICs: a 3-input NOR gate for the logic circuitry and a sense amplifier IC for the memory. It also used numerous analog circuits built from discrete components using unusual cordwood construction.
Erasable core memory and core rope both used magnetic cores, small magnetizable rings. But while erasable core memory used one core for each bit, core rope stored an incredible 192 bits per core, achieving much higher density.2 The trick was to put many wires through each core (as shown above), hardwiring the data: a 1 bit was stored by threading a wire through a core, while the wire bypassed the core for a 0 bit. Thus, once a core rope was carefully manufactured, using a half-mile of wire, data was permanently stored in the core rope.
This historic computer was one of the first to use integrated circuits and its CPU was built entirely from NOR gates. In this blog post, I describe the architecture and circuitry of the CPU.