What is done in software is translation from raw scan codes to ASCII which is done by BIOS code (or overridden by DOS driver for non-US keyboard layouts) that h

What is done in software is translation from raw scan codes to ASCII which is done by BIOS code (or overridden by DOS driver for non-US keyboard layouts) that handles interrupts from keyboard controller and presents API on INT 16h. 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.