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

[<gr...@gm...>]

I'd use watchpoints on the entire screen, like

. watch store 0400 07e7
. x

(assuming your screen memory is at 0400)

then when the first wrong character pops up break into the monitor again, and 
you'll see a list of writes to the screen. find out which one wrote the wrong 
character, and you know where it happens.

that said, i took a quick peek at the source - it seems you are mixing conio 
and stdio functions (eg scanf, and even a direct jsr to $ffd2) - that is a 
nono and can lead to all kind of undesirable effects.


Am Dienstag, 2. März 2021, 21:29:42 CET schrieb Bill Kendrick:
> 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.


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