Int 21 Fn 0B  - DOS 1+ - Get Stdin Status                                  [D]

   AH = 0Bh

Return: AL = 00h if no character available
      = FFh if character is available

Notes:  Break are checked, and INT 23 is called if either pressed
   standard input is always the keyboard under DOS 1.x, but may be
     redirected under DOS 2+
   if the interim console flag is set (see AX=6301h), this function
     returns AL=FFh if a partially-formed double-byte character is
     available

See Also: AH=06h"INPUT",AX=4406h