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

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

Am Mittwoch, 3. März 2021, 17:59:35 CET schrieb Michael Parson:
> OK, put together a stripped down version.  No score keeping, no stdio.h
> calls, no filesystem access, just the basic gameplay.  All screen
> manipulation should be going through conio. Since this is a C-64
> test, I even removed the asm bits and used a POKE to kick back into
> uppercase/PETSCII mode.  I'd still prefer a cross-platform 'safe' way to
> do that.
> 
> https://github.com/mparson/gridgame/blob/main/c64/minimal.c
> 
> Still getting the same behavior.
> 
> For grins, I compiled it for C-128, also got the errant chars.

it happens in the inner loop

gotoxy (mx,my);
c = cpeekc ();
textcolor (COLOR_RED);
cputcxy (c,mx,my);		// BUG, should be mx,my,c
textcolor (COLOR_WHITE);

:)

here is what i did:

first compile and generate a VICE label file:

cl65 -Ln minimal.lbl -o minimal.prg minimal.c

start in VICE:

x64sc -autostartprgmode 1 -moncommands minimal.lbl minimal.prg

break into the monitor (alt-h)

use "sc" to find out where the screen under the playfield starts

watch 06d0 07e7

x to go on

now press fire, it will break when the bug happens

chis to show the cpu history

now you can see its in cputc, and scrolling up a bit reveals that it comes 
after a "textcolor(2)", which is that textcolor (COLOR_RED) in the main loop

now i replaced the cputcxy call by gotoxy/cputc ... and that worked. obviously 
the cputcxy must be going wrong. Looked again, AHA! argument order :)

so there :) (i almost hoped for another compiler bug being triggered, but no - 
its just an ordinary bug in this code :))

-- 

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