[Coco] Raster position

[Robert Gault]

Sandro Mestre wrote:
> Hello, I'm new to this list.
>
> I would like to code a game for the Coco1/2. I usually code for the
> commodore c64 so I am aquainted with the 6502 but I also can code for the
> motorolas 6800, 6809 and 680000. During the last days I have coded a sprite
> engine for a computer with a 6809E and it is working great and then I
> thought that I could code also the game for the Coco/Dragon computers.
>
> I need to know if there is a register that I can poll to check the current
> raster position

There is no register that tracks the current raster position. If you write a 
program in Basic, then Basic keeps track of the current screen position but 
that's not much use to a machine language program.

> or if there is another way to know when the VDP starts
> painting the horizontal borders.

I think you are asking for the size of the blanking pulse but am not sure. The 
size may differ from Coco to Coco but is I think about 70 lines. If that is your 
question and you have a Coco, run some tests. If you don't have a Coco, then get 
some alpha testers to help you.

> I am asking this because as many will know
> if I paint the sprites there(borders) there will be no flickering. Another
> option I have would be to use double buffering but then this would eat much
> more memory and that would be perhaps a no go.
>
> How safe is it to use the high speed poke on a game? Do coco games use it?
>

On most or all versions of the Coco1, using the full fast poke caused a loss of 
video. I'm not sure if that carried over to the Coco2. However, there is a 
half-fast poke which is address dependent for speed and code in RAM is fast 
while code in ROM is slow. Video works in this mode.
There is no problem with the fast clock rate on the Coco3 which will run 
programs written for the Coco1 or 2 graphics modes.

> Thanks
>
> Wood
>