On-board flight software for the manned missions was developed for both the Command and Lunar Module computers. In addition to the operating system, the AGC had both an assembly language and a sophisticated software interpreter developed at the MIT lab that could handle more complex ‘pseudo’ instructions than the AGC. These instructions could simplify navigation programs and handle complex navigation equations in the background so as not to overwhelm the AGC power and memory capabilities.
The famous 1201 and 1202 priority alarm displays that interrupted and replaced the astronauts' normal displays with the Priority Displays during the Apollo 11 lunar descent signaled executive overflow was caused because the rendezvous radar was left on during the landing sequence and was stealing precious “cycles” from the AGC. This is in fact exactly what the computer and the software were meant to do– The MIT IL team (led by Margaret Hamilton) had intentionally designed the software with a priority scheduling capability that could identify the most important commands and that allows those to run without interruption pushing less important commands to the side.
The story about the Apollo 11 landing and the Priority Displays was one of error detection and recovery in real time. It was about the astronauts, mission control, the software and the hardware; and how they all worked together during an emergency as an integrated system of systems. It was about creating new, man-machine and software engineering concepts to do things never done before. Unlike a system where the software (or hardware) might "know" of a serious problem without the pilot's knowing it, the Priority Displays were able to determine right away if a particular alarm had occurred that fell within the category of an "emergency alarm" and they let the astronauts know about it too.
No known software errors ever occurred during any of the Apollo missions. The AGC software influenced the design of systems and software for future spacecraft including Skylab, the Space Shuttle and digital fly-by-wire aircraft systems.
