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

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

On Wed, 3 Mar 2021, gr...@gm... wrote:
> 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);
>
> :)

And that sound you just heard was my head banging on my keyboard.

I guess the next question is... why doesn't the X-16 behave the same
way?

> 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 :)

Thank you very much for digging into this!

Also, thanks for additional info on how to debug.  I've used gdb on *nix
hosts to debug C, but most of what I know about poking around in ML
monitors I've learned from watching various 8-bit Youtubers.

I figured it was in that loop, since that was where I saw it happening,
but I didn't know enough about how to examine what was going on with the
monitor to pin it down.

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

As far as compiler/language bugs go, is what I'm seeing with 'cscanf()'
expected behavior?

#include <conio.h>

int main() {
         unsigned char name[16];

         clrscr ();

         cprintf ("what is your name? ");
         cscanf ("%s", &name);
         cprintf ("hello, %s\n", name);

         return 0;
}

Start typing something at the prompt, hit backspace and it treats it
like a return instead?

-- 
Michael Parson
Pflugerville, TX