NAME
    TD_CHANGENUM -- return the current value of the disk-change counter.

FUNCTION
    This command returns the current value of the disk-change counter (as
    used by the enhanced commands). The disk change counter is incremented
    each time a disk is inserted or removed from the trackdisk unit.

IO REQUEST INPUT
    io_Device       preset by the call to OpenDevice()
    io_Unit         preset by the call to OpenDevice()
    io_Command      TD_CHANGENUM
    io_Flags        0 or IOF_QUICK

IO REQUEST RESULT
    io_Error  - 0 for success, or an error code as defined in
                <devices/trackdisk.h>
    io_Actual - if io_Error is 0, this contains the current value of the
                disk-change counter.