[Coco] Video band width and accessing video memory

[Stephen Fischer]

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
> 
>