[Coco] Video band width and accessing video memory

[Walter]

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