[stella] Help Interpreting Code

Subject: [stella] Help Interpreting Code
From: "Roagie" <roagie@xxxxxxx>
Date: Sat, 13 Jul 2002 11:35:18 -0700
Hi all. I'm making good progress in my newbie-ism. I'm also compiling a
newbie FAQ as I go, so it's nice and authentic :)

Here's my latest question... a fellow did a rewrite of a bunch of demos and
slapped into one BIN. It's called alpha.asm. I was interested in Crackers
bat with moving sprite demo so I stripped it out of the alpha demo.

At this point I am examining code and tracing each step to see how it works,
but I can not figure out what is going on here:
(all comments are my own, please correct if wrong)
vertPos = current vertPos of sprite
GRP0SIZE = height of sprite

begin code snippet:

     DEY              ; Y = Y - 1

     CPY vertPos     ; compare vertPos and Y, returns 1 if they are the same
     BNE sprite1     ; if Y = vertPos, then branch

     LDA #27         ; set sprite height
     STA GRP0SIZE    ; sprite is 27 scanlines high

     LDX GRP0SIZE    ; load X with current line of sprite being drawn

     BMI grfx2 ; branch on minus (N = 1). See if finished drawing sprite,
does X = -1?

     LDA p0data,X ; if X <> -1 then store sprite data
     STA GRP0 ; store data to TIA P0 graphics reg.
     BEQ OverScan ; check to see if Y = 0, if so frame is done
     BNE LoadGrfx ; if not, load next scanline data


As I read it, if the current scanline equals the sprite's vertPos, then it
bypasses reseting the sprite height to 27. But let's say that we haven't hit
that point and that the next 10 scanlines do not contain the sprite. So it
keeps loading GRP0SIZE = 27. And it keeps storing the player data 'LDA
p0data,X' where X = 27 every scanline. So WHY isn't there a large vertical
bar of the sprite's 27th byte down the scanlines? The player graphic data
for that offset is $ff.

My Understanding of the Routine: (something should be wrong in here)
- The routine drops down into sprite1, so X is always loaded with the
GRP0SIZE, which is always 27 if the vertPos is not the same as the current
- The storing of the player data is bypassed if X = -1 (AKA bit 7 = 1), but
as far as I can tell, X = 27 = #%00011011. bit 7 is not a 1. So this command
should be executed each time, but for some reason isn't.
- I see how the sprite is disabled, because his P0data,0 is $00, which
disables him in his last scanline of being drawn. So I know that X can not
be set back to 27 unless Y = vertPos, but I don't understand how. :(

Another thing that funks me out is that Y = vertPos for only one line! So it
shouldn't be drawing the sprite after the first scanline!


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