Re: [hatari-devel] Allowing/supporting additional ST-RAM amounts?

[Eero Tamminen <oak@xxxxxxxxxxxxxx>]

Hi,

On 4/4/20 12:12 AM, Nicolas Pomarède wrote:
> Le 03/04/2020 à 23:05, Eero Tamminen a écrit :
> > What else than the attached patch is needed to properly support 
> > additional ST-RAM amount?
> >
> > I'm asking because the real max ST-RAM limit for TT and MegaSTE 
> > machines is 10MB (not 8MB nor 14MB that Hatari currently allows).
> >
> > There e.g. seems to be this magic for Falcon:
> > https://git.tuxfamily.org/hatari/hatari.git/tree/src/stMemory.c#n388
> >
> > So I'm wondering whether the memory validation should actually be done 
> > per machine, instead of using same validation for all of them?
>
> well it depends, some machine are HW limited (for example a real Falcon 
> can't have TT RAM except if changing the cpu for a 32 bit address 
> version), so same for ST RAM, we allow some amount that might not be 
> possible on real HW as a convenience.
>
> As we allow TT RAM for "basic" Falcon where it should not exist in fact, 
> should be really limit ST RAM amount to real case values ?

If it has impact on emulated code, then we should
at least give user a warning about potential
issues.

So, first question is what are all the effects
that should be emulated for different ST-RAM
amounts in current Hatari code, and where they
differ between different machines?

For example:
* Are the some other emulated machine registers
  that need different values for different (>4MB)
  amounts of ST-RAM, than the Falcon one I found?


> Or do you have a case where this gives a problem
> / bug ?

Not yet.  But I spend a while debugging why
specifying 10MB resulted in Hatari using 1MB
(fixed in commit 6922af3c8).

Because real 68000 machines couldn't have more than 4MB, any amount 
above that probably works
on them as well.

=> If there are issues with different >4MB ST-RAM
   amounts, it should matter only to TT & Falcon,
   and their SW.

I.e. OSes (TOS, EmuTOS, Magic, MiNT, Linux),
memory managers (e.g. outside) and demos that
by-pass OS memory management.  Linux has other
issues still, and I haven't looked into others
beside TOS/EmuTOS.


	- Eero