Verify sectors

        Entry:
          AH = 04h
          AL = number of sectors to verify
          CH = track (for hard disk, bits 8,9 in high bits of CL)
          CL = sector
          DH = head
          DL = drive
        Return:
          CF = set if error occurred
          AH = status (see AH=1 above)
          AL = number of sectors verified

        Notes:  AWARD AT BIOS extended to handle more than 1024 cylinders by
                placing bits 10 and 11 of the cylinder number into bits
                6 and 7 of DH apparently, the AMI BIOS also follows this
                convention