Re: Unix without an MMU

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5 total messages Started by dan Tue, 07 Jan 1986 17:05

#1918

Author: dan
Date: Tue, 07 Jan 1986 17:05

22 lines
2105 bytes

Mark Crispin's message did indeed list all the practical or semi-practical ways
to implement Unix w/o an MMU.  The people who responded to his message don't
seem to understand what the Unix fork() system call does; the Amiga TOS method
will not work in Unix.  The fork() system call, remember, generates a new
process which is an EXACT COPY of the old process.  Any pointers in the old
process's data are copied to the new process.  But the semantics of fork()
require that those copied pointers now magically point to the COPY of the data
in the NEW PROCESS.  Mark's message listed all the possible ways this could be
accomplished w/o MMU, save one, which is to somehow figure out which random
words of data in the data space are pointers and readjust them at fork time.
This is quite impractical, as anyone who knows something about C will realize.

As an aside, the Sage IV 68000 system had no MMU, and did run Unix.  Or rather,
it ran IDRIS, the Unix look-alike (which does have a Unix-style fork()).  I
don't know which of the methods Mark listed they used, but I suspect the first
one: running only one process at a time in memory, and swapping it out before
starting another one.  At any rate, let the Sage IV be the existence proof that
you CAN run Unix w/o an MMU, and let Mark's message be the explanation of how,
and let's get on with topics more closely related to the 520 ST, which I don't
believe will ever run Unix (though something else from Atari might).

	Dan Franklin (dan@bbn-prophet.arpa)

Re: Unix without an MMU

#1933

Author: nnamrreH cirE
Date: Wed, 08 Jan 1986 11:05

8 lines
1141 bytes

There are several implementations of UNIX-like operating systems,
with multitasking, on the MMU-less IBM PC and compatibles.  However
it is done, it seems to be at least nominally compatible with UNIX.
What methods are used here?  Could they be applied to a 68000-based
system?  Am I missing the point entirely?  (Perhaps PC UNIX's
fork() does NOT act like real UNIX.)

      Ranjit Bhatnagar

Re: Unix without an MMU

#1981

Author: Dave Brownell
Date: Fri, 10 Jan 1986 02:45

16 lines
1348 bytes

It was recently asserted here that UNIX runs on the IBM PC without an
MMU.  Well ... sort of.

The segment registers can be used as an MMU, if you keep programmers
from using them.  Since that can't really be done on an 8086/88, PC/IX
(and VENIX, Xenix, ...) on IBM PCs doesn't really count as secure.
(Ergo it's not UNIX.)  Processes can mangle each other at will.

In short, it's not really UNIX there, and it pretty much DOES have an
MMU.  Better than nothing, but I wouldn't buy it.  UNIX needs at LEAST
memory mapping.  This can be real cheap!  but the 520ST doesn't have it.

-- 
David Brownell
EnMasse Computer Corp
...!{harvard,talcott,genrad}!enmasse!dave

Re: Unix without an MMU

#2104

Author: John Moore
Date: Sun, 12 Jan 1986 14:49

33 lines
2343 bytes

In article <11372@ucbvax.BERKELEY.EDU> ranjit@bugs.UUCP (Ranjit Bhatnagar) writes:
>There are several implementations of UNIX-like operating systems,
>with multitasking, on the MMU-less IBM PC and compatibles.  However
>it is done, it seems to be at least nominally compatible with UNIX.
>What methods are used here?  Could they be applied to a 68000-based
>system?  Am I missing the point entirely?  (Perhaps PC UNIX's
>fork() does NOT act like real UNIX.)
>
>      Ranjit Bhatnagar

We have several Unix systems (not Unix-like) running on the PC. They are
VENIX (Unix V-7), VENIX-V (Unix System V), and PCIX (Unix system 3). These
are all AT&T licensed Unix's and work the same as one other systems (we
also have a CT Megaframe running UNIX System-V, and we port back and forth
all the time).
	However....
	These Unix's DO have the use of limitted MMU capability. The 8086
family has a (disgusting) memory management scheme which uses base registers.
All programs which use less than 64K dataspace are relocatable by simply
changing the contents of the base registers. This is better than absolute
addressing in that even pointers are relocated. It also adds the very
severe limitation of 64K data spaces.
	I agree tha`i MMU's would be nice on the ST. However, it is
possible to make a UNIX (or something very close to it) without
MMU's.

-- 
John Moore (NJ7E/XE1HDO)
{decvax|ihnp4|hao}!noao!terak!anasazi!john
{hao!noao|decvax|ihnp4|seismo}!terak!anasazi!john
terak!anasazi!john@SEISMO.CSS.GOV
(602) 952-8205 (day or evening)
5302 E. Lafayette Blvd, Phoenix, Az, 85018 (home address)

Re: Unix without an MMU

#2123

Author: John Moore
Date: Tue, 14 Jan 1986 14:24

28 lines
1848 bytes

>(and VENIX, Xenix, ...) on IBM PCs doesn't really count as secure.
>(Ergo it's not UNIX.)  Processes can mangle each other at will.
            ^^^ ^^^^ Nonsense! It works find as UNIX. It is true
that it is not hacker protected, but most single computer UNIX's
have only one user anyway. To me UNIX is a portable environment
offering a wide selection of nifty tools. That's just what I have
here on my VENIX.
>
>In short, it's not really UNIX there, and it pretty much DOES have an
>MMU.  Better than nothing, but I wouldn't buy it.  UNIX needs at LEAST
>memory mapping.  This can be real cheap!  but the 520ST doesn't have it.
^^^^^^^^^^^^^^^ You just haven't shown this. The extreme purist view that
PC UNIX's aren't really UNIX doesn't wash. On the other hand, there is
no denying the value of memory mapping
>
>-- 
>David Brownell
>EnMasse Computer Corp
>...!{harvard,talcott,genrad}!enmasse!dave


-- 
John Moore (NJ7E/XE1HDO)
{decvax|ihnp4|hao}!noao!terak!anasazi!john
{hao!noao|decvax|ihnp4|seismo}!terak!anasazi!john
terak!anasazi!john@SEISMO.CSS.GOV
(602) 952-8205 (day or evening)
5302 E. Lafayette Blvd, Phoenix, Az, 85018 (home address)

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