[Coco] Video band width and accessing video memory

[RETRO Innovations]

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?

I can't speak to all of your assumptions, but the GIME does indeed read 
2 bytes per half cycle

The base CoCo is 128kB, but the memory is really represented by a 256kW 
array (256kilobits by 16 bit words)

On the each half of the E clock, the GIME reads the low byte of the RAM 
array while E is low, and holds the upper byte in U13, transferring to 
the GIME following the low byte.

Not sure if that helps explain things, though, as I thought GIME only 
handles video memory for half of each cycle.

Jim