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

[Michael P. <mp...@bl...>]

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