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

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

To wrap up my interruption into this thread...

I _believe_ I cleared up my option by making sure to use
inline assembly (rather than C, where cc65 was doing a lot of
jsr's to subroutines for doing math and comparisons)
for a simple Vertical Blank Interrupt I wrote.  I haven't seen
any glitches (though it was nearly 2am, so I didn't do extensive
gameplay testing.)

It's only like a dozen lines of 6502 assembly, but it was quite a
thrill to actually write some.  Hard to get to sleep.
I'm finally catching up with skills I should've had 30 years ago ;)

I was spoiled by Action!, I guess, which compiled the same kind
of code to much simpler machine language.

-bill!

On Tue, Mar 02, 2021 at 12:29:42PM -0800, Bill Kendrick wrote:
> 
> 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
> > 
> > 
> > _______________________________________________________
> > cc6...@li...
> > https://lists.sourceforge.net/lists/listinfo/cc65-devel
> > 
> 
> -- 
> -bill!
> Sent from my computer

-- 
-bill!
Sent from my computer