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

See also: 1590

1591 - OS HOOK - DEVICE POST (AT,PS)