[Coco] Video band width and accessing video memory

[Alex Evans]

The coco3 also uses 16-bit reads for reading ram for it's video so the
available bandwidth is twice what you're calculating.

On Sun, Jun 2, 2019, 00:58 Stephen Fischer <SFischer1 at mindspring.com> wrote:

> The bytes seen by the CPU for execution and the bytes seen by the
> graphics display are read alternately, the memory chips are busy on both
> tasks at the same time.
>
> This alternation of memory is explained in the service manual if my
> memory (wet) is correct.
>
> SHF
>
> On 6/1/2019 9:40 PM, Walter wrote:
> > I'm just intrigued at what mechanism the video output device is using to
> > access memory .
> >
> >
> >
> > The reason is this.
> >
> >
> >
> > A CoCo's max freq operates at 1788000hz (1.788mz).
> >
> >
> >
> > So in 1 second there can be 1788000 cycles.  So at maximum of 1 cycle per
> > memory
> > byte access, a total 1788000 bytes can be accessed.
> >
> >
> >
> > Dividing that into 60 frames per second that would equate to 1788000 /
> 60 =
> > 29,800 bytes per
> > frame can be accessed.
> >
> >
> >
> > But the video modes for the CoCo support 320x225x4 = 320x225x(1/2) =
> 36000
> > bytes per screen.
> >
> >
> >
> > So how does the video device access 36000 bytes per screen in 1/60 of a
> > second when the
> > memory bandwidth will only allow a maximum of 29,800 bytes per second!
> >
> >
> >
> > Does the video device read two bytes in one cycle?
> >
> >
> >
> > Or are my assumptions all wrong?
> >
> >
> >
> > Walter
> >
> >
>
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