[karmakaze]

The thing that I really liked was that you felt like you were programming for the hardware and not really for an OS. Most systems ran a single program at a time so you pretty much had full control except for interrupts. On DOS TSR (terminate, stay resident) interrupt handlers could also do amazing things.

Knowing a lot about my personal computer (Atari 400 and later Atari ST) made it so easy to do work on embedded systems as a student intern. Without internet, learning about your hardware/ROM was paramount from magazines and word-of-mouth.

Another thing that I learned from that time is how to make designs fast and small. This is a skill that's not really taught, shared, or practiced much and makes a huge positive impact on a codebase keeping it light on abstraction depth and high on flexible capability. Huge improvement to maintainability and extension into unforeseen directions too.