[stella] Supercharger RPG bare beginnings - request for help

[bob.montgomery at thomson.com]

Hi,

>Actually Andrew's code is doing something very clever and maybe this
would >work for your code too: 
>When scrolling, he compares the new tile with the old one and then
skips 
>updating the buffer if the tiles are identical.

That's a good idea.  

>One question: How (or where) you do merge the data of the two tiles 
>which belong to the same PF register?

They are pre-merged, if that makes sense.  All the necessary
combinations are stored in ROM (well, SC RAM, but I don't modify them).


>I suppose you are only switching pointers inside the kernel, right?

I'm not sure what you mean.


>I saw this code:
>	lda (PtrTemp),Y	;<--have both tiles
>	sta Temp+1
>	and #$F0		;left tile * 16...
>	sta Temp
>	lda Temp+1		;...plus right tile
>	and #$0F
>...
>
>which can be optimized into:
>	lda (PtrTemp),Y	;<--have both tiles
>	and #$F0		;left tile * 16...

I don't follow this.  I have two lookup "addresses," each of which is 4
bits.  I need the left one, times sixteen, added to twice the right one.
My comments aren't completely clear there, here's a bigger snippet for
folks who don't want to wade through the whole source:

	;	Y is zero already
	lda (PtrTemp),Y	;<--have both tiles
	sta Temp+1
	and #$F0		;left tile * 16...
	sta Temp
	lda Temp+1		;...plus right tile
	and #$0F
	asl
	;...times two. (carry is clear after ASL)
	jmp HaveTileBitmapIndex

[elsewhere]

HaveTileBitmapIndex
	adc Temp		;(carry is clear after LSR)
	tay			;Is our index into the array of tile
bitmaps.
...
Thanks a bunch for looking at this. :)

-bob