NAME
       ADCMD_PERVOL -- change the period and volume for writes in progress to
                       audio channels

   FUNCTION
       ADCMD_PERVOL is a command for multiple audio channels.  For each
       selected channel (io_Unit), if the allocation key (ioa_AllocKey) is
       correct and there is a write (CMD_WRITE) in progress, ADCMD_PERVOL
       loads a new volume and period immediately or at the end of the current
       cycle depending on the sync flag (ADIOF_SYNCCYCLE).  If the allocation
       key in incorrect, ADCMD_PERVOL returns an error
       (ADIOERR_NOALLOCATION).  ADCMD_PERVOL is synchronous and only replies
       (mn_ReplyPort) if the quick flag (IOF_QUICK) is clear.  Do not use
       ADCMD_PERVOL in interrupt code at interrupt level 5 or higher.

   INPUTS
       mn_ReplyPort- pointer to message port that receives I/O request
                     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 load period and volume (bits 0
                     thru 3 correspond to channels 0 thru 3)
       io_Command  - command number for ADCMD_PERVOL
       io_Flags    - flags, must be cleared if not used:
                     IOF_QUICK      - (CLEAR) reply I/O request
                     ADIOF_SYNCCYCLE- (CLEAR) load period and volume
                                              immediately
                                      (SET) load period and volume at the end
                                            of the current cycle
       ioa_AllocKey- allocation key, must be set by (or copied from I/O block
                     set by) opendevice function or adcmd_allocate command
       ioa_Period  - new sample period in 279.365 ns increments (124 thru
                     65536, anti-aliasing filter works below 300 to 500
                     depending on waveform)
       ioa_Volume  - new volume (0 thru 64, linear)

   OUTPUTS
       io_Unit     - bit map of channels that successfully loaded period and
                     volume (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