Ideally the sound device would have exclusive use of all the sound registers. However the cassette device uses some of the sound registers for output and timing purposes. The sound driver is designed to set up all sixteen sound registers on each interrupt so it will recover very quickly if its registers are corrupted.
The keyboard device has to make an audible click when a key is pressed.
This event is triggered from the keyboard device's interrupt routine and
so cannot be done with an EXOS call. Instead the keyboard device calls a
globally defined routine KEYCLICK in the sound driver which
uses tone channel zero to produce a click without interfering with any
other sound which may be on this channel. The key click sound takes about
1/100 second and the routine does not return untill it has finished.