Re: Virtual Memory

[Hartmut Keller <Keller@informatik.uni-stuttgart.de>]

On Tue, 14 Feb 1995, Stephan Haslbeck wrote:

> [...]
>
> I think it will be very diffcult (if not even impossible) only to implement 
> some simple way of swapping on 68000s. It's just the missing MMU. All 

Just a silly idea: how about placing the program in memory regions
that create bus errors and converting the addresses in the bus error
exception routine? Well, forget it, it'd be terrible slow.

> programs would have to know that their pointers might change and some 
> memory they just accessed is not valid any more. Maybe there is a way of 
> putting this into the MiNTLibs. But it will be hard work, no question. 
> And porting programs will be even harder, because most programs rely on 
> non changing pointers.
> 
> I have a good old 520 ST, also. I won't have any use of VM on it, too. So 
> should we leave it all as it is? A program that won't run in 4 MB on an 
> 68000, won't run in 4 MB on an 68030 without VM either.

Correct. But *with* VM they *can* run programs >4MB. Why shouldn't
people having a better CPU profit from it? I think if it is possible
to provide VM for 68030 then it should be done. 68000 users don't
profit from the memory protection, but it does not hurt them either.
But it provides useful security for 68030 users. So it was included.
Period. 

I think a MiNT with VM, swapping and shared libraries (ok, I'm
dreaming away, but who cares? :-) ) is always an alternative to Linux.
And as all those 68030'ers stay at MiNT instead of converting to Linux
the 68000'ers have their advantage too: further development of MiNT
and MiNT aware software. Even if it runs slower or less comfortable on
their machines. In a little time MiNT without VM will be no competitor
to Linux anymore.

> I don't think someone will try to port Linux to 68000s, so for 68000 
> users MiNT is the only way to get some Unix compatible OS. VM then is 
> only a gift for those who have newer CPUs.

So I absolutely recommend implementing VM into MiNT as (in my opinion)
it is the only future of MiNT.


So long,

  Hartmut     (520 STM user)

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