Odp: [stella] AOTM AFOS problems

Subject: Odp: [stella] AOTM AFOS problems
From: "Shrek" <matzieq@xxxxxxxxxxxxxx>
Date: Thu, 1 Jul 2004 12:03:33 +0200
First of all: thanks, Erik. For a single second I didn't think, that this
code COULD actually be longer than it was supposed to be... shame on me!
Thanks again.


I'm extremely grateful for your answer, BUT....

...I think the main problem is, that I'm at my very very very beginning of
the 2600 programming adventure, and as I may have a small idea what this
sprite positioning code does (at last, there are places when one can insert
Sprite_Y, and Sprite_Height), I completely don't understand these routines
that you have written. And it is the first time I hear of something called
"illegal opcodes". What are these and why illegal?
Anyway, are these positioning routines covered somewhere in the stella
archive, or anywhere?

I wish to thank you again for your help... there is a chance, that I will
manage to finish this game :)

> Also, when looking at the code, I think the main problem is, that its
> using sprite code to do missiles. Drawing a missile is a way simpler
> thing, there's various shortcuts to draw it, depending on it's height.
> In Seawolf I draw them using the illegal opcaode DCP:
>      LDA #3
>      DCP torpedoyPos
>      ADC #2
>      STA ENABL
> And in Star Fire I use the stack trick:
>      LDX #$1F ; Set stack outside loop
>      TXS
> ...
>      LDY vline ; Draw both missiles and ball
>      CPY yposBackup+2    ; inside loop
>      PHP
>      CPY yposBackup+1
>      PHP
>      CPY yposBackup
>      PHP
> ...
>      LDX #$FF ; Restore stack outside loop
>      TXS
> There's more methods hidden in the archive, specialized to diverse
> situations or heights of missiles.
> Greetings,
> Manuel
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