Re: [hatari-devel] Lethal Xcess: Dragon Blitter bug

[Nicolas Pomarède <npomarede@xxxxxxxxxxxx>]

Le 09/04/2018 à 14:07, Nicolas Pomarède a écrit :
> Le 09/04/2018 à 12:30, Vincent Rivière a écrit :
> > Hello.
> >
> > I have noticed an ugly thing in Lethal Xcess, with dragons. See 
> > screenshots.
> > You can notice that other dragons are fine.
> >
> > It only happens:
> > - with Hatari (I used a recent HG build for Cygwin)
> > - STe mode
> > - Blitter turned on in the game
> >
> > I never saw such problem on real hardware, or Steem, or Hatari STe 
> > with Blitter turned off.
> >
> > That might suggest a bug in Hatari Blitter emulation.
>
> hi
>
> nice catch, I confirm I can see this too. This seems like a regression 
> in hatari 2.1, because under 2.0 there's no problem.
>
> On the other hand, some low level blitter code was rewritten in 2.1 for 
> accurate bus sharing, so it's possible it broke a case with some logical 
> blitter operation.
>
> I will have a look at this.

Hi

This was a missing case in the new cycle exact bus sharing of the 
blitter emulation added in Hatari 2.1

During the timer B interrupts used to remove top border, the game 
temporarily stops the blitter (because the interrupt handler needs more 
than 64 bus accesses). But when blitter was resumed at the end of the 
interrupt, its context was not correctly restored, which could lead to 
an extra read of the same memory address, thus altering the transfer.

This was mainly visible on big sprites, but if you look closely to the 
main ship, you can see that it's sometimes altered too (or the 
background on its right is altered on a few pixels).

Works fine for me now, let me know if you still see some problems.

Nicolas