Re: [stella] RE: Illegal opcodes...another question

Subject: Re: [stella] RE: Illegal opcodes...another question
From: Thomas Jentzsch <tjentzsch@xxxxxx>
Date: Tue, 14 Jan 2003 09:34:29 +0100
Dennis wrote:
> Sorry for the multiple questions but I want to understand how these
> illegal opcodes work as I think they may help me...

I am always glad if I can help. :-)

> I see you are using dcp (opcode $c7, decrement and compare). What does
> this have to do with the y-register? Does dcp effect the y-register is some 
> way? 

No. DCP doesn't effect any registers, except the flag register. P0_Y is simply initialized with the distance from the top of the screen/kernel and decremented every second scanline. When it goes below CARHEIGHT (=8) the car is drawn. After that P0_Y is decremented below zero and the compare fails again.

> You're not updating y for each scanline, right? 

Right, only every 2nd scanline (-> 2LK). All objects have a two line vertical resolution. The positioning is done with single line precision by using VDELPx (for the cars) and different kernels (for the zombies).

>Apparently, I'm the only one here that doesn't get it

Probably you are just the only one who tried and doesn't get it. ;-)


Ihre Freunde sind in Italien? Schicken Sie ihnen trotzdem eine SMS
mit WEB.DE FreeMail

Archives (includes files) at
Unsub & more at

Current Thread