
Unofficial Instructions in Production – Why Some NES Games Use Undocumented NOPs - JALsnipe
http://retrocomputing.stackexchange.com/q/1839/621
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logronoide
It's more than 30 years since I don't develop anything for a 8 bit computer,
and specifically for a 6502 processor, but recompiling as we understand it
nowadays it was not possible. You had to code right on the memory addresses,
and moving blocks of code was a feature that only advanced tools had.

Sometimes I left 'gaps' between pieces of code filled with NOPs just in case
you need to code something in the middle. The problem was when you did not
have enough 'gaps' or because of performance restrictions several NOPs could
impact in the overall performance of your game. Then I started to play with
non documented instructions until I was able to adjust performance and 'gaps'
for example.

Maybe those guys had the same problem I had when I developed for Oric
computers...

