Re: [cc65-devel] c64 - getting seemingly random chars on screen

[Bill K. <nb...@so...>]

Y'know, I had a similar problem with my cc65 port of my
game "Gem Drop" (originally written in Action! back in 1997, so it
was relatively easy to port to C).  I never figured out what was up
with it. :-/

  http://www.newbreedsoftware.com/gemdrop_deluxe/

And similarly, since the screen data basically _defines_ the game
board, any corruption there breaks the game. :-(

-bill!

On Tue, Mar 02, 2021 at 02:06:32PM -0600, Michael Parson wrote:
> I'm pretty new to C programming in general, but cc65 and being able to
> target the C-64 and Commander X-16 piqued my interest and gave me the
> motivation to actually jump into it.
> 
> First few small projects went fine, but then I decided to write a
> C-64/X-16 version of an old Flash game I wasted a lot of time playing,
> "Grid Game".
> 
> https://github.com/mparson/gridgame
> 
> The X-16 version has been fine, the problems come in when I try to build
> it for the C-64.  A few #ifdefs for system specific things, but most of
> the code is the same.
> 
> On the C-64, when you hit the joystick button to start a move, I get
> a random (red) character printed to the screen.  My first thought was
> that maybe I was bumping into screen ram somehow, so I #ifdef'ed out
> a bunch of stuff to make the C-64 version have fewer features, wrote
> a joystick interface instead of the mouse like the X-16 version uses
> (haven't actually verified that this generates smaller code yet, maybe I
> should), etc, but I'm still getting the random letters.
> 
> Using Vice, I set up a few break points, tracked it down to just a
> few lines after the button is clicked:
> 
> #1 (Stop on  exec 094c)  296 034
> .C:094c  20 2A 15    JSR ._processQ - A:86 X:2D Y:00 SP:f4 N.-..... 219124114
> ^^^ no garbage chars yet
> 
> (C:$094c) n
> .C:094f  4C 5F 09    JMP $095F      - A:00 X:11 Y:02 SP:f4 N.-..... 219185007
> ^^^ garbage appears, in this case, a red 'P' at col 26, row 16
> (C:$094f) .C:095f  AD 09 2C    LDA ._global_editmode - A:00 X:11
> Y:02 SP:f4 N.-.....  219185010
> 
> Digging around in memory with the Vice monitor, it looks like I should
> have plenty and don't see any reason for some random byte of screen RAM
> to be changed.
> 
> The game seems to otherwise run fine, as long as the random char doesn't
> wind up on the playboard, messing up the matching.  A board reset fixes
> it, until the next time one appears...
> 
> I'm at a loss here, would appreciate any pointers.
> 
> Thanks.
> 
> -- 
> Michael Parson
> Pflugerville, TX
> 
> 
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-- 
-bill!
Sent from my computer