Posted By
Rybags
on 2010-11-09
20:17:02
|  Re: Assembly and interrupts: my first attempt. Help needed!
I maybe rushed in that idea of "strip sprites".
It's actually a method that was discovered in the Atari game "Zone Ranger".
The method isn't fully useful on C64/Plus4 bitmap modes because of the way the graphics data is arranged. In a linear scheme like Atari uses, you can address an entire "Y" position with "STA xxxx,X" because the last byte in the line is only 39 bytes ahead of the first.
But on the C= machines mentioned, the last byte of a scanline is actually 311 bytes away from the first, so you can't reach it with an Indexed addressing instruction
But not all is lost - you could still use the method, but would need 2 big blocks of code instead of 1 so that you could reach the far side of the screen.
Re time per frame etc.
The normal display area takes 200 scanlines. A normal PAL frame should be 312 scanlines, or 262 if an NTSC system.
So, that gives 112 or 62 non-display scanlines depending on what system you're on.
But, the bonus of the non-display scanlines is that the CPU will be running full speed, no slowdown due to TED needing to do graphics data fetches.
So, e.g. you might have a bit of code that takes 20 scanlines to execute if run during the display area, but it might only take 15 or so scanlines if running offscreen.
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