[XT] [AT]
Expects: AH    00H
Returns: AH    scan code
AL    ASCII character code or extended ASCII keystroke
Info: If a keystroke is available in the keyboard buffer, this removes
it from the queue and returns it in AH and AL.

If no key is available, this waits for a keystroke.  If you want
to avoid the wait, use INT 16H 01H to see if a key is ready.

Notes:   On ATs, when the keyboard buffer is empty, this invokes
INT 15H 90H, allowing an operating system to switch tasks or
whatever, when awaiting a keystroke.

  Also on ATs, this updates the keyboard LEDs to match the
current shift status.

  Extended key filtering: For compatibility with older 83-key
'boards, this converts duplicated keys into their older
equivalent keys.  Use INT 16H 10H to get unfiltered key codes.

See Also: INT 16H (BIOS keyboard services)
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INT 16H 00H: Read (Wait for) Next Keystroke