Re: [hatari-devel] add support for TT ram -> problem with HD emulation and DTA

[Nicolas Pomarède <npomarede@xxxxxxxxxxxx>]

Le 17/12/2014 16:50, Roger Burrows a écrit :
> On 17 Dec 2014 at 14:19, Nicolas Pomarède wrote:
>
> > Le 17/12/2014 14:05, Uwe Seimet a écrit :
> > > Hi,
> > >
> > > By the way, remember that one cannot transfer data from/to TT RAM using
> > > ACSI/DMA. So for full compatibility it should be ensured that such a
> > > transfer does not work.
> >
> > Hi,
> >
> > yes, but it's the TOS or the specific program's responsability to
> > allocate the correct kind of memory for DMA operations, there's nothing
> > we can do at the HW emulation level to fix this (at best, we can print a
> > log and ignore the read or only send '0' bytes if it's a write)
> Exactly!  If you're using Hatari to develop programs for real machines, you
> don't want Hatari to allow something that real machines don't.

yes, but problem is that those bad behaviours are not necessarily 
documented.

For example, does the ASCI write 0 bytes when dma points to bad memory, 
or does it reads bytes after masking the lower 24 bits ? Sometimes you 
will point to a region where RAM is mirrored, but sometimes you will 
point to some bus error regions (in which case the DMA won't do a bus 
error, but what value will be sent to the HDC ?)

This is not obvious to handle in all cases, as it requires lots of tests 
on all types of machines we don't always have access to.

Those memory issues are really difficult to handle, it requires lot of 
time to test/document them. We can add some cases as we encounter them 
with real examples, but it would take too much time to try to handle 
everything.

Nicolas