INT 15 - OS HOOK - DEVICE POST (AT,PS)
        AH = 91h
        AL = device type (see AH=90h)
        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