How NASA built the Apollo Guidance Computer

The Apollo computer had two zeros: +0 and -0

The Apollo Guidance Computer relied on one's complement math rather than modern two's complement. This design resulted in distinct binary representations for both positive zero and negative zero.

Why the Apollo Computer Had Two Different Zeros

The trick Apollo used to jump duplicate zero

Because one's complement arithmetic contains both positive and negative zero, crossing zero requires adding the carry bit back into the result. This mathematical workaround is known as the end-around-carry.

The End-Around-Carry Math Trick

How Apollo jumped memory banks without crashing

Changing ROM banks while executing code would instantly switch out the currently running instruction and crash the computer. Apollo engineers solved this issue by repurposing a double-word exchange to swap both bank and program counter atomically.

How Apollo Solved Bank Switching Without Crashing

A breakdown of the unusual quirks in the Apollo Guidance Computer, from overflow disabling interrupts to having no stack and shifting bits through RAM.

The Wildest Architecture Quirks of the Moon Computer

Apollo software was woven completely by hand

Core rope memory stored the guidance programs by physically threading copper wires through ferrite cores, requiring factory workers to hand-weave hundreds of thousands of bits.

The Moon Missions Ran on Hand-Woven Memory

How Apollo multitasking worked with only one CPU

The Apollo Guidance Computer used priority-based cooperative multitasking, requiring running jobs to yield every twenty milliseconds so critical tasks could cut the line.

How Apollo Ran Multitasking with One CPU

How Apollo astronauts talked to their computer

The Apollo Guidance Computer used a command line styled keypad known as DSKY, where every system request was entered as numerical verb and noun pairs.

How Apollo Astronauts Used the Verb-Noun Interface

The 3 software routines that landed humans on the Moon

The lunar module relied on three specialized AGC programs to execute powered descent. P63 handled blind braking, P64 pitched the spacecraft so astronauts could view the surface, and P68 managed touchdown.

How the Apollo Computer Landed on the Moon

The 1202 alarm that almost stopped the Moon landing

During Apollo 11 powered descent, an unintended radar hardware design sent thousands of extra instructions to the guidance computer. The operating system dropped lower-priority tasks and recovered within seconds, allowing the landing to continue.

Why Apollo 11 Almost Aborted Its Moon Landing

[09:33] Why the Apollo Computer Had Two Different Zeros
[10:53] The End-Around-Carry Math Trick
[20:15] How Apollo Solved Bank Switching Without Crashing
[27:46] The Wildest Architecture Quirks of the Moon Computer
[32:21] The Moon Missions Ran on Hand-Woven Memory
[35:45] How Apollo Ran Multitasking with One CPU
[44:38] How Apollo Astronauts Used the Verb-Noun Interface
[52:28] How the Apollo Computer Landed on the Moon
[57:38] Why Apollo 11 Almost Aborted Its Moon Landing

How NASA built the Apollo Guidance Computer

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