Path: utzoo!utgpu!utstat!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!bloom-beacon!apple!oliveb!amiga!cbmvax!daveh
From: daveh@cbmvax.UUCP (Dave Haynie)
Newsgroups: comp.sys.amiga.tech
Subject: Re: Virtual Memory
Message-ID: <6207@cbmvax.UUCP>
Date: 10 Mar 89 21:33:45 GMT
References: <420003@hpdml93.HP.COM>
Organization: Commodore Technology, West Chester, PA
Lines: 57

in article <420003@hpdml93.HP.COM>, daly@hpdml93.HP.COM (John Daly) says:

> / hpdml93:comp.sys.amiga.tech / bradch@microsoft.UUCP (Bradford Christian ms1) /  5:37 pm  Feb 15, 1989 /

> Is anyone out there in net-land with a 2500 (or some expansion card with
> an MMU) working on virtual memory?  The rom to ram thing sounds fun, but
> the real power of an mmu is preventing those nasty "Not enough memory"
> messages (or gurus :-().  I know at least one company is selling software
> for Macs with mmus to do this, and Apple's Gassee is quoted on the cover
> of the Jan 31 MacWeek saying "Apple will have virtual memory in 1989."

While you can't strictly use a Motorola MMU without some bit of memory
protection, the issues of virtual memory and protected memory are not
the same.  You can have virtual memory implemented globally, as this Apple
add-on apparently does, without any sort of task to task protection.  And
you can obviously have an MMU context attached to each task (not implying
that it's easy, or that it's a drop in with the current OS, just that it
can be done) without adding virtual memory.  I'd like to see both on the
Amiga, though I suspect that virtual memory would be easier to implement,
by some degree, than real protection.

Some people have looked into this; I personally haven't had the time (the
ROM to RAM hack was a 2-niter; anything beyond that would take some real
effort).

> I think we should beat them with a public domain virtual memory handler
> for properly equipped Amigas.  

That would be in the grand tradition; several commercial Mac Hacs have 
surfaced in equivalent form as PD/Shareware for the Amiga (VScreen, for
example).

> Are supposed to access the mmu directly, or does C/A have some sort of
> software interface we should use?  Am I babbling?  Where did I learn
> how to spell?

Currently, you have to access the MMU directly, and you have to be the only
one doing so.  You also have to assume that it will unconditionally break as
early as OS V1.4 time.  The MMU must be arbitrated by the operating system,
or an equivalent agent, to be used properly.  However, a user program has
no business even knowing about the MMU.  Given that we don't have OS support,
the only real thing that can be done is direct access.  I was early on trying
to get some support going for some kind of MMU.Library, that would be easy
to translate on a function-call basis to whatever surfaces in terms of 
support for a future OS.  Having since hacked a little MMU code, I'm on the
way to believing that would be a mistake -- the OS should be the only agent
that knows about the MMU.

> 	BradCh

> BTW: The Amiga 2500 is AWSOME!

		Gee, thanx!
-- 
Dave Haynie  "The 32 Bit Guy"     Commodore-Amiga  "The Crew That Never Rests"
   {uunet|pyramid|rutgers}!cbmvax!daveh      PLINK: D-DAVE H     BIX: hazy
              Amiga -- It's not just a job, it's an obsession