[hatari-devel] OS X frameskip issue

[Kåre Andersen]

On Thu, Jul 23, 2009 at 7:44 PM, Eero Tamminen<eerot at users.berlios.de> wrote:
> Hi,
>
> (I changed the subject as this isn't anymore about the convertors)
>
> On Thursday 23 July 2009, Kåre Andersen wrote:
>> >> > -int nFrameSkips;          /* speed up by skipping video frames */
>> >> > +int nFrameSkips=0;        /* speed up by skipping video frames */
>> >>
>> >> It's possible that because you explicitly set the variable, it goes
>> >> to a different section in the binary (.data instead of .bss)[1], but
>> >> that should not have any effect on it's value as globals are
>> >> automatically zeroed. (Apple documentation I referred in my earlier
>> >> mail already stated that .bss should be zeroed.)
>> >
>> > I agree with Eero here. I hardly can believe that the .bss is not
>> > initialized to zero on Mac OS X (it's a BSD/Unix based OS, and all such
>> > systems clear the BSS as far as I know).
>>
>> Ok, talking to my guru gives the same answer: The C standard defines
>> that bss should be cleared. And Eero found the info that i could not
>> find, so I back down from this... Sorry for jumping the gun - again -
>> bad habit. I guess the fix was just too simple to be correct....
>>
>> > So the real problem (a wrong memory access) might still persist, it's
>> > just not visible anymore since the variable has been moved to another
>> > location in memory.
>>
>> I will try to track this down as best as I can tomorrow daytime.
>> Better get as much done as possible before holidays end....
>
> Could you remove zeroing of the nFrameSkips and do following change:
> --------------------
> --- a/src/video.c       Tue Jul 21 23:37:35 2009 +0300
> +++ b/src/video.c       Thu Jul 23 20:40:53 2009 +0300
> @@ -2483,6 +2483,14 @@
>  static void Video_DrawScreen(void)
>  {
>        bool bScreenChanged;
> +
> +       static int OldFrameSkips = 0;
> +
> +       if (nFrameSkips != OldFrameSkips) {
> +               fprintf(stderr, "DEBUG: nFrameSkips, old: %d, new: %d\n",
> +                       OldFrameSkips, nFrameSkips);
> +               OldFrameSkips = nFrameSkips;
> +       }
>
>        /* Skip frame if need to */
>        if (nVBLs % (nFrameSkips+1))
> --------------------
>
> Then run Hatari in a way that earlier produced the issue and mail
> us what it outputs.
>
> I want to know what the problematic value is and whether it changes.
>
> If it's because of overwrite from some other variable, the bogus frameskip
> value(s) could tell what does the overwriting.
>
>
>        - Eero
>

Indeed, but I can do you one better... :)

Gotcha!

The problem is with the ShifterLines array in the SHIFTER_FRAME
struct. Either there is an off-by-one error someplace or the number of
scanlines per frame should be 502 for high res.

So, temporary solution is increase the value of
MAX_SCANLINES_PER_FRAME in video.h to 502 - and all is peachy. Does
this turn any light bulbs on for you, or should i dig deeper into the
scanline code?

I am your humble servant :)

/Kåre