Re: [hatari-devel] Blitter emulation corner case

["markus@xxxxxxx" <markus@xxxxxxx>]

Hmmm.

I'm not convinced that your code is completely correct. You appear to 
use the blitter in NO-HOG mode and simplify the blitter start to

while (LINE_NUM & 0x80)
    continue;

As far as I understand, this will assign only one single portion of 64 
"NOHOG"- bus cycles to the blitter. As you do not restart the blitter 
until finished (which required a RMW cycle to LINE_NUM[BUSY] until its 
eventually signalling a zero bit, to my understanding), it might get 
into some kind of undefined state (assuming it needed more than these 
initial 64 cycles to finish which is something I don't know) affecting 
further tests. We also do not know if there is any difference between 
Falcon and ST blitters regarding the number of cycles it might need for 
certain operations (at least I don't).

Of course this is a shot into the dark and could only explain any 
difference from the second round onwards but might be worth a try.



Am 17.05.2017 um 18:51 schrieb Christian Zietz:
> Nicolas Pomarède schrieb:
>
> > Although not 'random', it seems the xcount=1 case had no 'official'
> > defined behaviour, which makes it quite dangerous to use, especially
> > when using same code for STE and Falcon.
> >
> > I wonder if TOS tries not to use xcount=1 when using OS functions ?
> Atari TOS uses xcount=1, however of course by far not every possible
> permutation of x_inc, y_inc, NFSR, FXSR etc. Thorsten Otto sent me a
> disassembly of the Blitter routines from TOS 2.06/3.06, however, I
> didn't have the time to understand it yet.
>
> > I don't know if a logic pattern can be seen from the STE blitter
> > schematics, but if not it could be handled with a hardcoded table
> > combining those 8 possible cases (and a 2nd table for falcon)
> I'd be more comfortable if we found the logic pattern behind it, given
> that there are even more permutations than those currently tested by
> BLITEMU. For example, what if x_inc is positive (or zero) and y_inc is
> negative or vice-versa? It would result in a huge table, I'm afraid.
> I cannot promise, though, that I'll be able to reconstruct the ST/STE's
> Blitter inner workings from the schematic.
>
> Regards
> Christian