What a weird coincidence in one day.
The real debouncer hardware is also in the keyboard itself for all standard external keyboards ever used with PC.
On the other hand there were simple keyboard interfaces which involved reading out the whole keyboard as if it was one big shift register and debouncing was then done on the host side in software, two examples from top of the head are Symbolics machines and Wyse terminals. Interface for (S)NES controllers (really for Nintendo controllers up to GameCube) is similar, but I'm not sure whether debouncing is done by controller hardware or not.
http://underscorejs.org/#debounce
I never would have guessed that debouncing was first used to tame physical keys bouncing on contacts.
> The default keyboard handling on a DOS PC are meant for typing. So any time you hit a key there is a debounce before the key is allowed to repeat continuously, usually on the order of a quarter second to a half second.
What he is describing is the keyboard driver autorepeat feature, if you hold down a key for a while the driver will generate a sequence of virtual keypresses. This is just a software convention, it's also possible to read the state of the keys directly (pressed down or not) instead of treating them in terms of key-press events. Using the word debouncing for this is incorrect.
Debouncing happens on a millisecond time scale, it's due to the key actually physically bouncing.
More precisely: it is due to the contacts bouncing, they don't close just once when they touch each other, they rebound and close again, this repeats a number of times before they finally settle.
This code works by tracking state of modifier keys (and setting the keyboard LEDs appropriately) and converts key down events for other keys into keycode/character pairs.
DOS in turn provides function INT 21, AH=07 which is thin wrapper on top of INT 16h which waits for key press and returns it as either ASCII character or zero followed by extended scan code (returned on second invocation) for non-ASCII keys (this DOS function is what is called by readey() in Borland Pascal/C++).
Edit: it is not as thin wrapper as it looks like, because it does not blindly call into BIOS and block there, but loops in DOS code and calls INT 28h until key is ready. INT 28h is intended as a hook for TSR to either do unimportant background processing (kind of poor mans multitasking) or call into DOS when it is known to be safe to do so. Particularly interesting thing to do in INT 28h handler is executing HLT in order to conserve power as any condition that would cause DOS to exit from this loop generates interrupt.