[stella] *Why* does the AtariVox work?

[Stephen Anthony]

Yes, I know that seems like a strange question.  However, I don't 
understand how writing SWACNT actually writes data to the controller 
port.  I thought writing SWACNT only acted as a 'mask' for further 
reads/writes done by SWCHA.  But it seems as if actually setting SWACNT 
causes writes to be sent to the controller port.  Is this correct?

I'm asking because the AtariVox support in Stella does just that; any 
writes to SWACNT then also do writes to the controller port, and these 
latter writes are what drives the AVox.  However, I think this may be a 
hack, and not how the real hardware works.  And if so, does anyone know 
how it should work?

The overall point of the this question is to get AVox/Savekey EEPROM 
emulation working, which is causing problems right now.

Does anyone have any advice??

Thanks,
Steve