All of that probably has to be done in order for the AP-101 to be flight certified. As far as I can recall, they wrote their own operating system and didn't rely on any external libraries.

Even when it was new, the AP-101 was part of a family of flight computers that NASA had already been using, and the same is true for the HAL/S compiler used for most of the code.

If you want to explore how it all worked, this SE thread contains some great information and also serves as a good launching point for further research:

https://space.stackexchange.com/questions/19006/what-operati...

People named a computer HAL, and sent it to space??

One of the tools that helped people and other computers into spaces was named, in part, HAL