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  Description:  Device Wait      [AT] [PS/2]
 
  Invoked by the ROM BIOS fixed disk, floppy disk, printer, network, and
  keyboard drivers prior to performing a programmed wait for I/O completion.
 
  Input                                      Output
  ═══════════════════════════════            ═══════════════════════════════
  AH=90H                                     If no wait (driver must perform
  AL=device type                                         its own time-out)
     00H-7FH serially reusable               Carry flag=clear
             devices                         AH=00H
     80H-BFH reentrant devices               If wait was performed
     C0H-FFH wait-only calls,                Carry flag=set
             no corresponding
             Post function
  ES:BX=segment:offset of request
                block for device
                types   80H-FFH
                                    -♦-