If you think programming now is difficult, VisiCalc was written in assembly language for an Apple II. Here are a few words from this web page that describe this code:
Dan Bricklin, inventor/creator of VisiCalc, the first spreadsheet program for personal computers, has created this page of historical notes and images about his work. His work came long before my interest in computers and programming, so I enjoy reading about it from a historical perspective. He shows a TI calculator and very large state diagram on this page. I remember seeing calculators like that in stores, and the work he put into the state diagram looks like a modern mind map.
If you’re into history, it’s all very cool.