NAME
       ADCMD_WAITCYCLE -- wait for an audio channel to complete the current
                          cycle of a write

   FUNCTION
       ADCMD_WAITCYCLE is a command for a single audio channel (io_Unit).
       If the allocation key (ioa_AllocKey) is correct and there is a write
       (CMD_WRITE) in progress on selected channel, ADCMD_WAITCYCLE does not
       reply (mn_ReplyPort) until the end of the current cycle.  If there is
       no write is progress, ADCMD_WAITCYCLE replies immediately.  If the
       allocation key is incorrect, ADCMD_WAITCYCLE returns an error
       (ADIOERR_NOALLOCATION).  ADCMD_WAITCYCLE returns an error
       (IOERR_ABORTED) if it is canceled (AbortIO) or the channel is stolen
       (ADCMD_ALLOCATE).  ADCMD_WAITCYCLE is only asynchronous if it is
       waiting for a cycle to complete, in which case it clears the quick
       flag (IOF_QUICK); otherwise, it is synchronous and only replies if the
       quick flag (IOF_QUICK) is clear.  Do not use ADCMD_WAITCYCLE 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, or if a write is in
                     progress on the selected channel and a cycle has
                     completed
       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 wait for cycle (bits 0 thru 3
                     correspond to channels 0 thru 3), if more then one bit
                     is set lowest bit number channel is used
       io_Command  - command number for CMD_WAITCYCLE
       io_Flags    - flags, must be cleared if not used:
                     IOF_QUICK - (CLEAR) reply I/O request
                                 (SET) only reply I/O request if a write is
                                       in progress on the selected channel
                                       and a cycle has completed
       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 channel that successfully waited for cycle
                     (bits 0 thru 3 correspond to channels 0 thru 3)
       io_Flags    - IOF_QUICK flag cleared if a write is in progress on the
                     selected channel
       io_Error    - error number:
                     0                    - no error
                     IOERR_ABORTED        - canceled (AbortIO) or channel
                                            stolen
                     ADIOERR_NOALLOCATION - allocation key (ioa_AllocKey)
                                            does not match key for channel