Re: [NTLK] Virtual Memory was Re: Toshiba PC Card with ATA Support

From: Paul Guyot (pguyot_at_kallisys.net)
Date: Sun May 04 2003 - 11:04:30 PDT


[Oops, found this old one I never replied to]
Il me semble que le 9/2/03 à 14:27 +1100, Finn Thain nous racontait:
> > > ...ATA backed VM?
>>
>> This is feasible. I just don't think the system really needs it, except
>> on MP2000s.
>
>Is this because the CPU speed of the MP2000s is able to make better use of
>more heap? Or is ATA throughput too slow on older models? Or is NOS 2.1
>needed?

It might be interesting on 2.0 models indeed. But they do not really
need more heap, as far as I know. The only Newton which is not
perfectly sized for memory is the eMate 300 without the upgrade and
the MP2000.

>I was actually thinking of those NOS2.0 newtons that have quite limited
>memory (like my MP120 :) An ATA card large enough to be partitioned into
>extra heap as well as normal Newton stores would be a help even on those
>models.

Do you feel the need for more heap?

> > Actually, what is required is to go to supervisor mode. Currently this
> > isn't possible unless we design a system patch letting us go to
> > supervisor mode.
>
>Can the NOS extend it's heap dynamically in supervisor mode? I have only a
>cursory understanding of VM operation, but it seems to me that to extend
>the heap, you would need a page fault exception handler that could access
>the ATA backing store... but I guess you already have a good insight into
>that problem :) Could it be that ATA cards are already memory mapped?

ATA cards are not memory mapped. ATA Support was a realistic project
(if I dare say) because NewtonOS 2.x does not require cards to be
memory mapped. In fact, it does not use the fact that linear cards
are memory mapped (except for ROM cards and cards formatted in
NewtonOS 1.x).

Once in supervisor mode, we can talk to the MMU and do anything like
(a) replacing pages of the ROM by some pages of ours (in order to
change the pagefault handler), (b) define virtual pages that are
backed with the ATA card.

Still, this would be a lot of work.

Paul

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