[torontocbm] C64 ASM Question
- From: "wbochar" <admin@xxxxxxxxxxx>
- To: <torontocbm@xxxxxxxxxxxxx>
- Date: Fri, 12 Jun 2009 17:03:09 -0400
I was wondering if someone out there can give me some guidance on an
assembler problem I am having.
I have three screens of characters stored in memory (40x25x3 character bytes
at $3000 or "BG1"
Anyways, I can draw the bg1 to the screen (the first 40*25), it works fine
that way. But I want to smooth scroll it, so I got that working with out
issue.
But at the end of the smooth scroll vertically, I want to increment the
display to go up one line.
I have the following static subroutine:
DrawBG1
LDA BG1 + 0, X
STA 1024, X
LDA BG1 + 200, X
STA 1024 + 200, X
LDA BG1 + 400, X
STA 1024 + 400, X
LDA BG1 + 600, X
STA 1024 + 600, X
LDA BG1 + 800, X
STA 1024 + 800, X
INX
CPX #200
BCC DrawBG1
LDX #0
RTS
If I manually add "+ 40"
LDA BG1 + 0 + 40, X
STA 1024, X
LDA BG1 + 200 + 40, X
STA 1024 + 200, X
It will change address reference up one character line
If I create a variable
CharRow := 40
And do this
LDA BG1 + 0 + CharRow, X
STA 1024, X
LDA BG1 + 200 + CharRow, X
STA 1024 + 200, X
Same thing works. But I can't reassign the value at runtime
I put a value of 2 at $C000
LDA BG1 + 0 + ($C000 * CharRow), X
STA 1024, X
LDA BG1 + 200 + ($C000 * CharRow), X
STA 1024 + 200, X
This doesn't work either, because it's looking at $C000 as a value not the
value at $C000 address.
How do I dynamically adjust this address based on a value stored somewhere
else?
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