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       CMD_WRITE -- normal I/O entry point

       CMD_WRITE is a standard command for a single audio channel (io_Unit).
       If the allocation key (ioa_AllocKey) is correct, CMD_WRITE plays a
       sound using the selected channel; otherwise, it returns an error
       (ADIOERR_NOALLOCATION).  CMD_WRITE queues up requests if there is
       another write in progress or if the channel is stopped (CMD_STOP).
       When the write actually starts; if the ADIOF_PERVOL flag is set,
       CMD_WRITE loads volume (ioa_Volume) and period (ioa_Period), and if
       the ADIOF_WRITEMESSAGE flag is set, CMD_WRITE replies the write
       message (ioa_WriteMsg).  CMD_WRITE returns an error (IOERR_ABORTED) if
       it is canceled (AbortIO) or the channel is stolen (ADCMD_ALLOCATE).
       CMD_WRITE is only asynchronous if there is no error, in which case it
       clears the quick flag (IOF_QUICK) and replies the I/O request
       (mn_ReplyPort) after it finishes writting; otherwise, it is synchronous
       and only replies if the quick flag (IOF_QUICK) is clear.  Do not use
       CMD_WRITE in interrupt code at interrupt level 5 or higher.

       mn_ReplyPort- pointer to message port that receives I/O request after
                     the write completes
       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 channel to write (bit 0 thru 3 corresponds to
                     channel 0 thru 3), if more then one bit is set lowest
                     bit number channel is written
       io_Command  - command number for CMD_WRITE
       io_Flags    - flags, must be cleared if not used:
                     ADIOF_PERVOL       - (SET) load volume and period
                     ADIOF_WRITEMESSAGE - (SET) reply message at write start
       ioa_AllocKey- allocation key, must be set by (or copied from I/O block
                     set by) OpenDevice function or ADCMD_ALLOCATE command
       ioa_Data    - pointer to waveform array (signed bytes (-128 thru 127)
                     in custom chip addressable ram and word aligned)
       ioa_Length  - length of the wave array in bytes (2 thru 131072, must
                     be even number)
       ioa_Period  - sample period in 279.365 ns increments (124 thru 65536,
                     anti-aliasing filter works below 300 to 500 depending on
                     waveform), if enabled by ADIOF_PERVOL
       ioa_Volume  - volume (0 thru 64, linear), if enabled by ADIOF_PERVOL
       ioa_Cycles  - number of times to repeat array (0 thru 65535, 0 for
       ioa_WriteMsg- message replied at start of write,  if enabled by

       io_Unit     - bit map of channel successfully written (bit 0 thru 3
                     corresponds to channel 0 thru 3)
       io_Flags    - IOF_QUICK flag cleared if there is no error
       io_Error    - error number:
                     0                    - no error
                     IOERR_ABORTED        - canceled (AbortIO) or channel
                     ADIOERR_NOALLOCATION - allocation key (ioa_AllocKey)
                                            does not match key for channel

       If CMD_WRITE starts the write immediately after stopping a previous
       write, you must set the ADIOF_PERVOL flag or else the new data pointer
       (ioa_Data) and length (ioa_Length) may not be loaded.

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