Re: [hatari-devel] New blitter code for xcount=1 nfsr=1

[Laurent <laurent.sallafranque@xxxxxxx>]

Hi Nicolas,


That's a good news. Is the Falcon blitter also concerned ?

Should I test some falcon demos/games for regression ?

regards

Laurent


Le 10/09/2020 à 17:44, Nicolas Pomarède a écrit :
> Hi
>
> some times ago we had several examples of wrong blitter emulation when 
> xcount=1 and nfsr=1 at the same time.
>
> Various tests showed some different result depending on "source x 
> increment" values and other parameters maybe.
>
> Thanks to the work by Ijor who did a Verilog implementation of the 
> blitter, based on some reverse engineering as well as on the blitter's 
> schematics that Christian Zietz found some years ago, I was able to 
> fix this behaviour under Hatari and get all the cases to work under 
> Hatari using the small test program that Christian wrote at this time 
> (see attached BLIT.ZIP for reference)
>
> To get correct results, blitter's code had to be rewritten in a more 
> "low level" way, where data are fetched at the start of the blit in a 
> specific order (fxsr, src, dst) instead of doing it "on demand" 
> depending on the OP/HOP modes (which could sometimes lead to read dst 
> before src, which is an error if we want to emulate xcount=1/nfsr=1)
>
> The resulting code should be also smaller and maybe easier to read, 
> with two big functions Blitter_ProcessWord and Blitter_Step that 
> should better "mimic" the tests' logic made in the real blitter.
>
> Basically, if I didn't made error in my implementation, we need to add 
> 2 extra shift/read in the case where xcount=1 and nfsr=1 ; but the 
> main difference is that those 2 reads are dummy read (no cycle taken), 
> returning the latest content of the bus instead of really reading from 
> RAM ! This is what explain the difference we see depending on "source 
> x increment" being >0 or <0.
>
> With current hatari version, the attached program BLITEMU.TTP should 
> now output a file BLITEMU.TXT that matches the result file ST1.TXT (it 
> doesn't match the FALCON*.TXT files, but I wonder if the data cache 
> was really disabled when those tests were made ?)
>
> I tried some games / demos that make use of the blitter and didn't see 
> any regression so far (Lethal Xcess, We Were by Oxygene, rgbbeast by 
> agression, sea of colors by dhs, ...)
>
> Given the number of changes, it's possible some cases were forgotten. 
> Please test your favorite STE games/demos that use the blitter and 
> report any problem.
>
> Next Hatari release will be soon now, so better fix everything before ;-)
>
> Nicolas