Int 15 Fn 91  - Os Hook - Device Post (at,ps)                              [B]

   AH = 91h
   AL = device type (see #0431)
   ES:BX -> request block for type codes 80h through BFh
   CF clear

Return: AH = 00h

Note:  this function should be hooked by a multitasker to allow other tasks
     to execute while the BIOS is waiting for I/O completion; the default
     handler merely returns with AH=00h and CF clear

See Also: AH=90h