INT 15 - OS HOOK - DEVICE BUSY (AT,PS)
        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
SeeAlso: 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

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