NAME
        CMD_CLEAR/ETD_CLEAR -- mark the track buffer as containing invalid
                               data.

   FUNCTION
        These commands mark the track buffer as invalid, forcing a
        reread of the disk on the next operation. ETD_UPDATE or cmd_update
        would be used to force data out to the disk before turning the motor
        off. ETD_CLEAR or CMD_CLEAR are usually used after having locked out
        the trackdisk.device via the use of the disk resource, when you
        wish to prevent the track from being updated, or when you wish to
        force the track to be re-read. ETD_CLEAR or CMD_CLEAR will not do an
        update, nor will an update command do a clear.

   IO REQUEST INPUT
        io_Device       preset by the call to opendevice()
        io_Unit         preset by the call to opendevice()
        io_Command      CMD_CLEAR or ETD_CLEAR
        io_Flags        0 or IOF_QUICK
        iotd_Count      (ETD_CLEAR only) maximum allowable change counter
                        value.

   IO REQUEST RESULT
        io_Error - 0 for success, or an error code as defined in
                   <devices/trackdisk.h>

   SEE ALSO
        cmd_write, cmd_update