Int 15 Fn 90  - OS Hook - Device Busy (at,ps)                              [B]

   AH = 90h
   AL = device type (see below)
   ES:BX -> request block for type codes 80h through BFh
   CF clear

Return: CF set if wait time satisfied
   CF clear if driver must perform wait
       AH = 00h

Notes: type codes are allocated as follows:
     00-7F non-reentrant devices; OS must arbitrate access
     80-BF reentrant devices; ES:BX points to a unique control block
     C0-FF wait-only calls, no complementary INT 15/AH=91h call
   floppy and hard disk BIOS code uses this call to implement a timeout;
     for device types 00h and 01h, a return of CF set means that the
     timeout expired before the disk responded.
   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=91h,INT 13/AH=00h,INT 17/AH=00h,INT 1A/AH=83h

Values for device type:
 00h   disk
 01h   diskette
 02h   keyboard
 03h   PS/2 pointing device
 21h   waiting for keyboard input (Phoenix BIOS)
 80h   network
 FBh   digital sound (Tandy)
 FCh   disk reset (PS)
 FDh   diskette motor start
 FEh   printer