[Coco] Video band width and accessing video memory

[Dave Philipsen]

But it *is* feasible to do a 160x200 mode in 256 colors. 

Dave

> On Jun 2, 2019, at 11:18 PM, Walter Zambotti <zambotti at iinet.net.au> wrote:
> 
> Very interesting!
> 
> That’s what I expected.
> 
> The video hardware must access 2 bytes per cycle (or on one edge of the cycle).
> 
> And the video frequency must always be 1788000 regardless of CPU speed.
> 
> And if that is the case then the maximum accessible bytes would be double 29800 or 59600.
> 
> Now if you divide that figure by:
> 
> 320 horizontal pixels of 4 bits (160 bytes) then the max theoretical vertical resolution is 372.
> 
> 320 horizontal pixels of 8 bits (320 bytes) then the max theoretical vertical resolution is 186.
> 
> And 186 does not match any known CoCo vertical resolution.
> 
> So a hidden CoCo 3 graphics modes that allows 256 colors must simply not exist as there is insufficient bandwidth to display 320x200x8bit.  As I calculated the max height could only be 186(.25).
> 
>