Here's a talk about how the Voyager team fixed the flight data computer on Voyager 1 when a memory chip went bad on it a few years ago. It goes over how the flight computer works and he walks through a few assembly routines. https://www.youtube.com/watch?v=YcUycQoz0zg

Some of the challenges they had to deal with while developing the fix:

- The only source code they had for the flight data software was an OCR'd Microsoft Word document (with typos) that was likely scanned from a hard copy assembler listing printout.

- The processor runs a custom instruction set developed by JPL for the Voyager mission. The documentation they had on the processor was incomplete.

- Everybody who had designed the flight software was dead.

- They had no assembler, no debugger, and no processor simulator. They had no testbed, the only two FDS processors were in space.