Re: Undocumented APIs
Undocumented Internals was superb.
And don't forget Ralf Brown's Interrupt List. https://en.wikipedia.org/wiki/Ralf_Brown%27s_Interrupt_List
A treasure trove of information, much of it about what other programs were doing - helpful in ensuring anything you were writing wouldn't conflict (too badly) with anything else.
Or so that you could do things like send commands to disk cache TSRs, and so forth...
Biting the hand that feeds IT
