NAME
        CMD_WRITE/ETD_WRITE -- write sectors of data to a disk.

   FUNCTION
        These commands transfer data from a supplied buffer to the track
        buffer. If the track that contains this 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 the new track is read in for
        modification. ETD_WRITE will write the sector label area if
        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_WRITE or ETD_WRITE
        io_Flags        0 or IOF_QUICK
        io_Data         pointer to the buffer where the data should be put
        io_Length       number of bytes to write, must be a multiple of
                        TD_SECTOR.
        io_Offset       byte offset from the start of the disk describing
                        where to write data to, must be a multiple of
                        TD_SECTOR.
        iotd_Count      (ETD_WRITE only) maximum allowable change counter
                        value.
        iotd_SecLabel   (ETD_WRITE 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_read, td_format