[Coco] Video band width and accessing video memory

[Dave Philipsen]

Just one other thing to keep in mind is that the access to the video memory is not necessarily limited by the CPU bus speed. It’s just a requirement that the video subsystem reads memory during the 1/2 of the cycle when the CPU is not accessing memory. If the RAM was fast enough (which it’s not) the video system could feasibly do multiple reads of memory during that 1/2 cycle and buffer them up. 

Dave

> On Jun 1, 2019, at 11:40 PM, Walter <zambotti at iinet.net.au> 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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