Re: [hatari-devel] Blitter bug in STe mode

[Eero Tamminen <oak@xxxxxxxxxxxxxx>]

Hi,

On 6/9/20 12:31 AM, Christian Zietz wrote:
> Eero Tamminen schrieb:
> > Ok, so issue always starts at middle of screen, and continues (either up
> > or down) until EmuTOS
> > finishes drawing the line with blitter.
> >
> > Any idea why the garbage lines left behind would
> > be solid when the window outline is dotted?
>
> It's too late for me for a really detailed write-up. So, here's what
> goes wrong: Hatari sometimes restarts the Blitter in the middle of a TAS
> instruction; something that does not happen on the real CPU, because no
> other bus master is allowed until TAS completes.
>
> I added some code to cpuemu_13.c to print when TAS is entered and the
> values it reads and writes. I also added code to Blitter_Start() to make
> note of the CTRL register at the beginning and and the end of a Blitter
> round.
>
> Normally I get output like this:
> Blitter_Start: ctrl=80
> Leaving Blitter_Start: ctrl=81
> TAS: before read cycle
> TAS: after read cycle = 81
> TAS: after write cycle = 81
> Blitter_Start: ctrl=81
>
> So the Blitter modifies it's control register, TAS reads the control
> register and restarts the Blitter. Everything's fine.
>
> In the error case I see something like this:
> TAS: before read cycle
> Blitter_Start: ctrl=80
> Leaving Blitter_Start: ctrl=81
> TAS: after read cycle = 80
> TAS: after write cycle = 80
> Blitter_Start: ctrl=80
>
> So the Blitter modifies it's control register, *during* a TAS
> instruction, but, importantly, *after* TAS has read it. Therefore TAS
> writes back an *old* value to the control register.

Interesting that it always happens exactly at
middle of screen...  Nicolas, any idea why
that would happen?


> This sets the index
> into halftone RAM to the wrong value. Since EmuTOS uses the halftone RAM
> to draw a dotted line, setting the index off by one causes pixels to be
> drawn where they should not be and vice-versa.

I didn't know that EmuTOS did the dotted line
with halftone registers, but your explanation
make complete sense.


> I hope this is comprehensible. Otherwise I can expand on it tomorrow.

Great investigation, thanks!

And I think your write-up was pretty detailed. :-)


	- Eero