[Coco] CoCo 1/2 VDG & SAM interaction.

[Zippster]

Thanks Darren, I understand how this works a little better now.  :)

- Ed


> On May 31, 2015, at 5:10 PM, Darren A <mechacoco at gmail.com> wrote:
> 
> On Sun, May 31, 2015 at 3:56 PM, Zippster wrote:
> 
>> Ah, interesting.  Does anyone know what the input CB1 on the PIA gets used
>> for?
>> 
>> 
> 
> There are two PIAs.  For each of them, CB1 is used as an interrupt source
> for the CPU.  One is connected to the CART pin of the cartridge connector
> and can be used to assert FIRQ* for the cartridge auto-start interrupt.
> The other is connected to the FS* output of the VDG and can be used to
> assert IRQ* on each frame of video. The 60Hz TIMER function in Extended
> Basic uses this for incrementing the value and Disk Basic uses it for the
> Motor shut-off timer.
> 
> - Darren
> 
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