INT 21 - DOS 1+ - DIRECT CONSOLE INPUT
AH = 06h
DL = FFh
Return: ZF set if no character available
ZF clear if character available
AL = character read
Notes:  ^C/^Break are NOT checked
if the returned character is 00h, the user pressed a key with an
extended keycode, which will be returned by the next call of this
function
reads from standard input, which is always the keyboard under DOS 1.x,
but may be redirected under DOS 2+

See also: 210B

2106 - DOS 1+ - DIRECT CONSOLE INPUT