[Coco] DAC sound output and keyboard input

[Nick Marentes]

On 19/06/2014 10:51 PM, Mark McDougall wrote:
> Hi Gurus,
>
> I have a strange problem and I'm not sure whether it is an issue with 
> my code, the Coco, or MESS.
>
> If I configure the Coco3 for DAC sound output, occasionally I get 
> incorrect results when reading the keyboard to detect if *ANY* key is 
> pressed (ie, the column select is zero (0) which ordinarily would 
> return the results for all rows). In some cases - and there's 
> seemingly no pattern to it - bit 7 is reset, indicating a key has been 
> depressed in that column. And once on, it is stuck on. It also *seems* 
> to happen more often when I use <F10> (toggle throttle) in MESS, which 
> suggests that it's perhaps an emulation problem).
>
When reading $FF00, are you masking out bit 7 (the joystick comparator 
input)?

This will add 127 to the value read depending on where the joystick is 
positioned.

There really is no need to use the DAC for sound if you are only 
emulating the Apple II 1-bit sound.

Nick