Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!mailrus!ames!apple!motcsd!xdos!doug
From: doug@xdos.UUCP (Doug Merritt)
Newsgroups: comp.sys.amiga.tech
Subject: Re: MEMF_PHYSICAL?
Message-ID: <360@xdos.UUCP>
Date: 30 May 89 23:53:59 GMT
References: <8905270516.AA18668@jade.berkeley.edu>
Reply-To: doug@xdos.UUCP (Doug Merritt)
Organization: Hunter Systems, Mountain View CA (Silicon Valley)
Lines: 76

In article <8905270516.AA18668@jade.berkeley.edu> 451061@UOTTAWA.BITNET (Valentin Pepelea) writes:
>allowed to access each other's memory. The only reason current Minis and
>Mainframes, give tasks their own address space is because two decades ago,
>processors could access only a limited amount of ram. (say 64K) Thus for

It's sad to see people get confused about such recent history. Your
statement about RAM availability was true for microprocessors and some
mini's but certainly not mainframes (or even all minis).

And even there, address space limitations has certainly never been
the only reason for giving tasks their own address space.

One of the biggest reasons has always been for protection. It has
generally been considered to be desirable, on say a timesharing system,
to prevent any given process from clobbering some other poor person's
program.

>multi-tasking, the computer would swap the entire programs in and out of that
>same physical space at ever context switch. Even Unix uses this *poor* memory
>model, indeed it was first develloped for the PDP-11, which had a 64K
>instruction space and 64K data space.

This is almost true. Unix *once* used that model. Berkeley Unix has
had virtual memory since 1979. Prior to that, on PDP-11's, it
absolutely *had* to swap (no paging support in hardware), so it wasn't
a "poor memory model" at the time, on that hardware.

It's true that AT&T fought tooth and nail to keep VM out of their
versions of Unix, but even they gave up years ago, so both standard
flavors of Unix have had VM for a long time. Longer than the Amiga has
been out.

BTW, the PDP 11 had 8 segment registers (8K each) defining each 64K
address space, so shared memory on 8K boundaries could have been done.
Wasn't in standard V6 or V7 Unix, though.

In any case, having an MMU good enough to support swapping sure beats
the crumby setup on Amiga's where you can't even do that! I really like
Amiga's, but that's no reason to bad mouth swapping. Swapping is a pain
*only* if you can outdo it, i.e. if you've got demand paged VM.

>If they had processors able to access 4 Gigabytes of physical ram, then Unix
>too would enjoy the superior functionality available in single-addressing space
>operating systems.

This is a really bizarre (as well as false) idea.
	A) Unix had 4Gig addressing on VAXen in 1979

	B) Exclusively single address space OS's have traditionally been
	   considered a major *defect* in OS design circles.

	C) It's true that for some things it's handy to allow shared address
	   spaces, but only once you first have multiple address spaces.
	   The fact that the Amiga guru's all the time demonstrates this
	   quite easily -- crashes are *very* undesirable. Shared address
	   space is great in real time systems, but ideally as an *option*,
	   not as the only way to go!

You young whippersnappers think you've got all the answers, but haven't
even done your homework. For shame!

>I stand by the model of the ideal real-time message-based operating system.

Ok, sure, you're on solid ground there!

>"An  operating  system  without         Name:   Valentin Pepelea
> virtual memory is an operating         Phonet: (613) 231-7476 (New!)
> system without virtue."                Bitnet: 451061@Uottawa.bitnet

I like your proverb.
	Doug
-- 
Doug Merritt		{pyramid,apple}!xdos!doug
Member, Crusaders for a Better Tomorrow		Professional Wildeyed Visionary

"Welcome to Mars; now go home!" (Seen on a bumper sticker off Phobos)