Re: [stella] Submitted for your (dis)approval: griddemo

Subject: Re: [stella] Submitted for your (dis)approval: griddemo
From: Eckhard Stolberg <Eckhard_Stolberg@xxxxxxxxxxxxxxxxxxxxx>
Date: Sat, 18 Sep 1999 19:37:44 +0200
At 12:35 17.09.99 -0700, you wrote:

>One thing I'm having problems with is why the board is so far to the
>left side of the screen.  If I add one more NOP to delay it, I go over
>the scan line limit and break the display.  Can anyone give a
>suggestion on how to center the display? 

The current routine to display one scanline takes 73 cycles from
one WSYNC to the next. If you add another NOP, it would be 75
cycles and cycle 75 is too late for WSYNC to catch the end of
the current scanline.

What you could do is add a 3 cycle instruction, like STA $3f, to
the NOPs. Then have the DEX/BNE ... thing loop to the instruction
after the WSYNC at scanloop. This way you would be doing 76 cycles
per scanline which fills the whole scanline, so that the next time
through scanloop everything would be executed at the same cycle
in the scanline as before. This would shift the display 9 pixels to
the right.

Another thing that you could do is taking out the DEX/BNE ... at
the end of the loop and put in a DEX/BEQ EXITLOOP before the NOPs.
That would take 5 cycles from behind the display and put 4 cycles
in front of the display which would shift the display 12 pixels to
the right.

BTW: Your demo does only 251 scanlines per frame. You should change
that to 262 lines per frame, just to make sure you stay in Atari's
specification.


Ciao, Eckhard Stolberg



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