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INT 13h,  15h (21)       Read DASD Type                                  many

    Returns the type of interface associated with a drive and also the
    number of sectors the drive contains. Can be used to determine if a
    diskette drive supports the sensing of a diskette change.

       On entry:      AH         15h
                      DL         Drive number

       Returns:       CF         Set if error, else cleared
                      AH         Disk type (see below)
                                 Status of operation if error (see Service 01)
                      CX:DX      If no error and AH <> 0, then 4-byte integer
                                 with number of sectors.

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       Notes:         This service is not available for PCs, PCjrs, and
                      XTs dated before 1/10/86.

                      This service allows you to determine whether a drive
                      supports the Change Line Status service (Service
                      16h). The Change Line Status informs the system
                      whether or not a diskette has changed since the last
                      access to it.

                      Use Service 17h (Set DASD Type) to set the type.

                              Disk Type
                              AH        Meaning
                              00h       Drive not present
                              01h       Diskette, no change line available
                              02h       Diskette, change line available
                              03h       Fixed Disk

                      For the AT, XT-286, and PC Convertible, the BIOS
                      executes INT 15h, Service 90h (Device Busy), for the
                      diskette (Type = 01h) and the fixed disk (Type =
                      00h) prior to waiting for the interrupt. INT 15h,
                      Service 91h (Interrupt Complete), is executed upon
                      completion. Also diskette operations that require
                      the diskette motor to be on will call INT 15h,
                      Service 90 (Device Busy), with the type equal to
                      "Diskette Drive Motor Start" (Type = FDh). This
                      allows the system to perform another task while the
                      drive motor is waiting to get up to speed.