[Coco] Video band width and accessing video memory

[tim franklinlabs.com]

   One thing I'd like to point out is that your math is slightly off. 320
   * 200 * 4 is correct but there's four pixels per bite so the final
   calculations is 18000 bytes not 36000.

   On June 1, 2019 at 11:40 PM Walter < [1]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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References

   1. mailto:zambotti at iinet.net.au
   2. mailto:Coco at maltedmedia.com
   3. https://pairlist5.pair.net/mailman/listinfo/coco