Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!mnetor!uunet!husc6!mit-eddie!ll-xn!ames!ucbcad!ucbvax!jade!eris!mwm
From: mwm@eris.BERKELEY.EDU (Mike (My watch has windows) Meyer)
Newsgroups: comp.sys.amiga
Subject: Re: MY Wishlist for A3000
Message-ID: <5887@jade.BERKELEY.EDU>
Date: Fri, 13-Nov-87 00:03:37 EST
Article-I.D.: jade.5887
Posted: Fri Nov 13 00:03:37 1987
Date-Received: Sun, 15-Nov-87 03:44:19 EST
References: <74@mithras> <33314@sun.uucp> <2224@sputnik.COM>
Sender: usenet@jade.BERKELEY.EDU
Reply-To: mwm@eris.BERKELEY.EDU (Mike (My watch has windows) Meyer)
Organization: Missionaria Phonibalonica
Lines: 44
Keywords: Commodore A3000 Do it RIGHT!

In article <2224@sputnik.COM> kurt@tc.fluke.COM (Kurt Guntheroth) writes:
<(1) Protected virtual memory.  I will not buy ANY computer without this
<    feature from now on.  This is the critical feature that keeps a
<    multitasking machine from crashing when a program goes crazy.  With VM,
<    I can reliably keep the system up for days.

Carefull. That's two features, one of them ambiguous. Virtual memory
means either "demand paging" (i.e. - the memory looks like it's there,
but may not be) or "mapped memory" (i.e. - the memory isn't where you
think it is, but it is in memory), depending on where you studied.

Protected memory means that you can't read and/or write and/or execute
the memory in question. It can be (and has been) done without virtual
memory of either flavor. However, you still have to have enough of an
MMU that mapping is nearly free. Since it buys you lots of other nice
things, it's usually worth doing (even Seymour Cray thinks so).

On the other hand, demand paging costs more than mapping, and buys you
soft failure when processes get larger than real memory.
Debate/questions should go to comp.arch.

<I don't need a 68030, or even a 68020.  I need a 68010 so I can have
<the VM I require.

It sounded like you really wanted protected memory. As explained, this
doesn't require VM. The 68000 will support memory mapping just fine.
It has problems with demand paging - it doesn't even *have* a page
fault, much less do it right :-). It does have a line to use when it
tries to address memory that's not there. The stack frame doesn't have
sufficient information to let the OS restart/continue (choose one,
depending on religion) the faulting instruction after paging things
in. These didn't stop the sufficiently dedicated/devious from doing
demand paging on the 68000 (or two of them, in some cases).

If you're going to do demand paging, real memory quits affecting
what/how much you can run, and just makes things go faster.

	<mike

--
Teddies good friend has his two o'clock feast		Mike Meyer
And he's making Teddies ex girl friend come		mwm@berkeley.edu
They mistook Teddies good trust				ucbvax!mwm
Just for proof that Teddy was dumb.			mwm@ucbjade.BITNET