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From: mjl@tropix.UUCP (Mike Lutz)
Newsgroups: comp.unix.wizards
Subject: Re: Why "u." and not "u->" ?  (I'm reading Bach book)
Message-ID: <276@tropix.UUCP>
Date: Thu, 21-May-87 10:23:32 EDT
Article-I.D.: tropix.276
Posted: Thu May 21 10:23:32 1987
Date-Received: Sat, 23-May-87 11:32:31 EDT
References: <626@vixie.UUCP> <3568@spool.WISC.EDU> <500@rust.DEC.COM> <8047@utzoo.UUCP>
Reply-To: mjl@tropix.UUCP (Mike Lutz)
Organization: GCA/Tropel Div. Rochester, NY 14450
Lines: 31

In article <8047@utzoo.UUCP> henry@utzoo.UUCP (Henry Spencer) writes:
>> I worked on a system were kernel mode ran unmapped. We were unable to
>> use u. and therefore had to solve some very difficult problems. We even
>> considered copying u areas to a fixed location at every context switch.
>
>Don't laugh, it's been done.  I'm told it works okay, if a bit slowly.

When we ported Unix V7 to the old Motorola ExorMacs system, the ublock
relocatability problem was the most difficult one we had to face.  The
ExorMacs MMU only worked in user mode, so the standard technique of
using the OS's map in supervisor mode wouldn't cut it.

The trickiest part, of course, is handling the system stack for the
current process (residing at the top of the UBLOCK).  Stacks have lots
of addresses in them, many of which cannot be located, making
on-the-fly software relocation impossible.  We considered the software
copy in/copy out to a fixed memory area, but abandoned this as it would
have created intolerable interrupt latency.  Finally, we resorted to
running most of the kernel code in user mode, with software "logical"
states mapped onto the hardware "real" states.  Overall performance was
between that of a PDP11/34 and an 11/45.  If you want to know more
about the types of shenanigans required, and the issues that arise,
look at the paper by Mike Shon and me in the 1983 Toronto USENIX
Conference Proceedings.

By the way, I believe Motorola's Unix port to the ExorMacs *did* use
software copying; I know that in a simple "cc hello.c" benchmark, our
version blew them out of the water.

Mike Lutz
tropix!mjl	(no longer an ExorMacs, thank God!)