NAME
       CMD_START -- start device processing (like ^Q)

   FUNCTION
       CMD_START is a standard command for multiple audio channels.  For each
       selected channel (io_Unit), if the allocation key (ioa_AllocKey) is
       correct and the channel was previously stopped (CMD_STOP), CMP_START
       immediately starts all writes (CMD_WRITE) to the channel.  If the
       allocation key is incorrect, CMD_START returns an error
       (ADIOERR_NOALLOCATION).  CMD_START starts multiple channels
       simultaneously to minimize distortion if the channels are playing the
       same waveform and their outputs are mixed. CMD_START is synchronous an
d
       only replies (mn_ReplyPort) if the quick flag (IOF_QUICK) is clear.  D
o
       not use CMD_START in interrupt code at interrupt level 5 or higher.

   INPUTS
       mn_ReplyPort- pointer to message port that receives I/O request after
                     if the quick flag (IOF_QUICK) is clear
       io_Device   - pointer to device node, must be set by (or copied from
                     I/O block set by) opendevice function
       io_Unit     - bit map of channels to start (bits 0 thru 3 correspond
                     to channels 0 thru 3)
       io_Command  - command number for CMD_START
       io_Flags    - flags, must be cleared if not used:
                     IOF_QUICK - (CLEAR) reply I/O request
       ioa_AllocKey- allocation key, must be set by (or copied from I/O block
                     set by) opendevice function or adcmd_allocate command

   OUTPUTS
       io_Unit     - bit map of channels successfully started (bits 0 thru 3
                     correspond to channels 0 thru 3)
       io_Error    - error number:
                     0                    - no error
                     ADIOERR_NOALLOCATION - allocation key (ioa_AllocKey)
                                            does not match key for channel