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

Subject: [stella] RE: Illegal opcodes...another question
From: "Dennis Debro" <ddebro@xxxxxxxxxxxxx>
Date: Mon, 13 Jan 2003 23:13:45 -0500
Sorry for the multiple questions but I want to understand how these
illegal opcodes work as I think they may help me...

Thomas, I'm looking at the kernel you did for Glenn for Death Race and I
have a question. In this code snippet...

; draw car #0:
    lda     #CARHEIGHT-1    ; 2
    dcp     P0_Y            ; 5
    bcs     .doDraw0        ; 2³
    lda     #0              ; 2
    .byte   $2c             ;-1
    lda     (P0_Ptr),y      ; 5
    sta     GRP0            ; 3 = 18    @20

I see you are using dcp (opcode $c7, decrement and compare). What does
this have to do with the y-register? You're not updating y for each
scanline, right? Does dcp effect the y-register is some way? I had this
question back when you posted skipDraw
(http://www.biglist.com/lists/stella/archives/200102/msg00282.html) but
I never asked :) Apparently, I'm the only one here that doesn't get it

Take care,

Archives (includes files) at http://www.biglist.com/lists/stella/archives/
Unsub & more at http://www.biglist.com/lists/stella/

Current Thread