Margaret Hamilton (seen above with a printout of the Apollo 11 guidance software) was the first programmer hired for the Apollo project at MIT. In 1965, she was put in charge of the MIT Instrumentation Laboratory team who wrote the software for the Apollo Guidance Computer.

The computer was primitive by today's standards. It had the equivalent of 5000 or so transistors, 74KB of OS and navigation programming, 4K of RAM work space, operated at about 2 MHz, and had about 20 operation codes in its instruction set. It weighed in at about 87 pounds with the display/keyboard module. The command module had a similar computer. The only backup was a sextant and a slide rule.

The software was written on paper, transcribed to punched cards, compiled on a computer, and embedded onto magnetic core memory "ropes" by hand.

The AGC worked flawlessly, even when it failed. During the lunar landing, Neil Armstrong reported numerous "1202" errors that almost caused the landing to be aborted. NASA mission control contacted the MIT team who told NASA they could ignore it. It appears the rendezvous radar was in an incorrect setting and it was overloading the computer which was busy with the landing radar. The software was designed to cache the data, reboot itself, and start back up where it left off.

President Obama awarded Hamilton the Medal of Freedom, saying “her example speaks of the American spirit of discovery that exists in every little girl and little boy who know that somehow to look beyond the heavens is to look deep within ourselves." You can read more about her at the above link.

Trivia: The AGC source code was posted on Github by some guy who transcribed it from scanned images of the printouts. Check out some of the issue reports.