Here's a wonderful post talking about the process of writing code that decoded floppy disk data read in realtime, on a 6502 processor. Readers who are geekly and of a certain age will identify with this. Younger folks will probably just shake their heads at the lameness of their elders :)
What struck me about this post was the way the author brings back the challenges of programming on those old, slow microprocessors – and the way we engineers glommed onto tricks like the “illegal” op codes (almost every microprocessor had them). I never wrote any code that decoded floppy data, but I did many other similarly challenging things. A couple that spring to mind: variable-slew stepper motor control (to speed up and quieten floppy disk positioners) and realtime FSK radioteletype decoding, with multipath and single-side dropout handling. Both of these would be trivial today; your average smartphone could do it in a high-level language. But back then, the microprocessors could only do things like that if you pulled out every trick in the book, and perhaps invented a new trick or two along the way...
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