[Coco] Fw: CoCo Hardware Questions

[Alex Evans]

On Jan 24, 2005, at 12:56 PM, Dino Steeman wrote:

> Now I have a couple of questions:
>
> * There is only an E-line going to the 6809 and not to the VDG. How 
> does the
> VDG
> "know" when to access memory? Does the 'clock'-pin do the trick?

Actually the SAM accesses memory for the VDG and it knows the state of 
the E and Q clocks, and generates the clock signal for the VDG as well.

> * What does the Q-signal do?

In stead of stepping through one state every cycle like many CPUs, the 
6809 breaks a single cycle into four states using the combination of 
the E and Q clocks to determine which state the CPU is currently in.  
This allows the CPU to get more done in a single cycle (it in a way 
looks at it as four ticks for one cycle), and makes sharing memory with 
another device like the SAM potentially very straightforward.

> * As the VDG is reading one byte at a time, and as it must manage to go
> through
> the entire videoram, is there a screen image stored anywhere for the
> video-signal? (I know, complex question, but still...)

I'm not really sure what you're asking.  If it is a text screen, the 
data read from the computer's memory is translated using the current 
row number and the value of the byte read to give a value that is 
rotated through a barrel shifter to drive the elements that physically 
convert into your actual video signal,  for a graphics screen the 
translation is a little more direct.  At the same time it keeps track 
of a horizontal and vertical counter so that it knows when to go into 
horizontal and vertical blanking and when to issue horizontal and 
vertical sync signals.

-- 
Theodore (Alex) Evans | 2B v ~2B = ?