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From: eachus@mbunix.mitre.org (Robert Eachus)
Newsgroups: comp.sys.amiga.tech
Subject: Re: MEMF_PHYSICAL?
Summary: The reason was address space!
Keywords: virtual memory, address space
Message-ID: <54912@linus.UUCP>
Date: 2 Jun 89 02:12:31 GMT
References: <8906010252.AA11810@jade.berkeley.edu> <367@xdos.UUCP>
Sender: news@linus.UUCP
Reply-To: eachus@mbunix.mitre.org (Robert I. Eachus)
Organization: The MITRE Corporation, Bedford, Mass.
Lines: 83

In article <367@xdos.UUCP> doug@xdos.UUCP (Doug Merritt) writes:
>In article <8906010252.AA11810@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
>
>I disagreed, and Valentin said in the early 60's ram was limited even on
>mainframes. True, but the point is that limited ram is *not* why separate
>address space support was designed into MMU's and OS's. It was because
>this was considered to be a feature. Each program thought it owned the
>whole machine. Again, this is desirable for *most* programs, even on
>the Amiga, due to the way most programmers write code. Some programs
>behave very well and take the whole environment into account, but far more
>do not.

     I was there Charlie! And the reason most 70's computers gave each
user his own address space was that they had run out of address bits,
and putting each user in his own box, postponed the architectural doom
date for a few more years.

     The clearest example is the PDP-11 which started life with 16-bit
hardware and instruction-set adresses. As this limit became more and
more intolerable, the hardware addresses were upped (to a final figure
I think of 22-bits), but addresses in programs were still required to
be 16-bits max.  Having separate data and instruction address spaces
was added to give programmers one more effective address bit, but boy
did it make things messy.

     Writing programs which do page table swapping is even messier,
but many of the early sixties architectures were even more brain-dead
than the PDP-11. The PDP-1 had 12-bit address (to 18-bit words), and
four pages.  The PDP-8 had 128-word pages (of 12-bit words), and the
in the IBM-360 most addresses were 10-bit offsets from 32-bit
registers (but the addresses were calculated modulo 2**24 -- 16 Meg).
Fortunately, all machines which require page table swapping in user
progams have been obsolete for years and have all probably been
scrapped by now...or in the case of IBM-PC's and clones, should have
been. :-)

     Of course, some people, forced by necessity or brain death
to use such obsolete architectures, claim that in memory paging is an
advantage.  But I would never, even with a gun to my head, buy a
machine which supported more physical memory than was directly
addressable in normal address modes.


     If all users can share a portion of their address space with the
operating system, many operations become vastly easier.  When the
Amiga makes full use of the virtual memory capabilities of the 2620
boards or the 68030, I expect that it will still maintain a model
where all addresses are unique, so that messages paseed between
processes can contain pointers, etc... At least until the Amiga 9000
is released, with support for 16 Gigabytes or more of RAM. (Or maybe
it will change in the Amiga 8000, with only 4 Gig, because of virtual
memory.  :-)

     Gordon Bell said it all: (this may not be exact) "No computer
architecture has ever become obsolete for any reason other than a lack
of address bits."  He should know, he designed the PDP-15 nee PDP-1
instruction set archecture (with other incarnations as the PDP-4,
PDP-7, and PDP-9), the PDP-8 nee PDP-5 ISA, the DECsystem-20 nee PDP-6
ISA (a.k.a PDP-10, and Decsystem-10) and the PDP-11 ISA all of which
bit the dust for just that reason.  (There are still some DEC-20's
around.  By then the address space had been increased to 256K of
36-bit words. But the others are all dead, dead, dead.)

     The Amiga architecture may outlive the VAX, because it has one or
two more effective address bits available. (The VAX uses the top two
bits of 32-bit addresses as mode bits, but I expect that when they hit
the limit that will change.)  Recently the need for address bits has
been increasing at a rate of almost one a year.  If we say that with 1.3
Amigas require 1-Meg, then most of the current Amigas will run out of
physical address space in about 3 or 4 years, but it will take another
10 years to hit the architectural limit of the 680x0 family.  Note
that reserving 4 Meg of address space for CHIP RAM was a foresighted
decision.  But a new address map will be needed in say 1992, but that
one would be needed to support more than 16 Meg on the FAST bus anyway.

					Robert I. Eachus

with STANDARD_DISCLAIMER;
use  STANDARD_DISCLAIMER;
function MESSAGE (TEXT: in CLEVER_IDEAS) return BETTER_IDEAS is...