Command          Operation
-------          ---------
 adcmd_allocate    allocate one or more of the four audio channels.

 adcmd_finish      abort the current write request on one or more of the
                   channels.  Can be done immediately or at the end of the
                   current cycle.

 adcmd_free        free one or more audio channels.

 adcmd_lock        lock one or more audio channels.

 adcmd_pervol      change the period and volume for writes in progress. can
                   be done immediately or at the end of the cycle.

 adcmd_setprec     set the allocation precedence of one or more channels.

 adcmd_waitcycle   wait for the current write cycle to complete on a single
                   channel.  Returns at the end of the cycle or immediately
                   if no cycle is active on the channel.

 cmd_flush         purge all write cycles and waitcycles (in-progress and
                   queued) for one or more channels.

 cmd_read          return a pointer to the i/o block currently writing on a
                   single channel.

 cmd_reset         reset one or more channels their initialized state.  all
                   active and queued requests will be aborted.

 cmd_start         resume writes to one or more channels that were stopped.

 cmd_stop          stop any write cycle in progress on one or more
                                   channels.

 cmd_write         start a write cycle on a single channel.


Exec Functions as Used in This Chapter
--------------------------------------
abortio()        abort a command to the audio device. if in progress, it
                 is stopped immediately, otherwise it is removed from the
                 queue.

beginio()        initiate a command and return immediately (asynchronous
                 request).

checkio()        determine the current state of an i/o request.

closedevice()    relinquish use of the audio device.

opendevice()     obtain use of the audio device.

wait()           wait for a signal from the audio device.

waitport()       wait for the audio message port to receive a message.


Exec Support Functions as Used in This Chapter
----------------------------------------------
allocmem()       allocate a block of memory.

createport()     create a signal message port for reply messages from the
                 audio device.  Exec will signal a task when a message
                 arrives at the reply port.

deleteport()     delete the message port created by createport().

freemem()        free a block of previously allocated memory.