NAME
        CMD_READ/ETD_READ -- read sectors of data from a disk.

   FUNCTION
        These commands transfer data from the track buffer to a supplied
        buffer. If the desired sector is already in the track buffer, no disk
        activity is initiated. If the desired sector is not in the buffer, the
        track containing that sector is automatically read in. If the data in
        the current track buffer has been modified, it is written out to the
        disk before a new track is read. ETD_READ will read the sector label
        area if the iotd_SecLabel is non-NULL.

   IO REQUEST INPUT
        io_Device       preset by the call to opendevice()
        io_Unit         preset by the call to opendevice()
        io_Command      CMD_READ or ETD_READ
        io_Flags        0 or IOF_QUICK
        io_Data         pointer to the buffer where the data should be put
        io_Length       number of bytes to read, must be a multiple of
                        TD_SECTOR.
        io_Offset       byte offset from the start of the disk describing
                        where to read data from, must be a multiple of
                        TD_SECTOR.
        iotd_Count      (ETD_READ only) maximum allowable change counter
                        value.
        iotd_SecLabel   (ETD_READ only) NULL or sector label buffer pointer.
                        If provided, the buffer must be a multiple of
                        TD_LABELSIZE.

   IO REQUEST RESULT
        io_Error - 0 for success, or an error code as defined in
                   <devices/trackdisk.h>
   NOTES
        Under versions of Kickstart earlier than V36, the io_Data had to
        point to a buffer in chip memory. This restriction is no longer
        present as of Kickstart V36 and beyond.

   SEE ALSO
        cmd_write