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From: jfh@killer.UUCP (John Haugh)
Newsgroups: comp.unix.wizards
Subject: Re: Why "u." and not "u->" ?  (I'm reading Bach book)
Message-ID: <895@killer.UUCP>
Date: Sat, 16-May-87 15:06:48 EDT
Article-I.D.: killer.895
Posted: Sat May 16 15:06:48 1987
Date-Received: Sat, 23-May-87 10:30:52 EDT
References: <626@vixie.UUCP> <3568@spool.WISC.EDU> <500@rust.DEC.COM>
Organization: The Unix(tm) Connection, Dallas, Texas
Lines: 19

In article <500@rust.DEC.COM>, mhl@rust.DEC.COM (Mark Lucovsky) writes:
> 
> The u. vs u-> is really an issue that deals with the location of the
> kernel stack for the current process. Traditional ports assume that all
> kernel stacks are based at the same virtual address (inside the u area).
[ munch munch ]
> 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.

This is how it is done in TRS-Xenix so far as I can tell.  All 4K of the
U-page is copied on each context switch.  It is very expensive to say the
least, something like 4K * (8 cycles per word + 2 wait states) * 6MHz does
not give a whole lot of context switches per second.  They seem to get
smart and only copy from the base of the U-page to the end of struct user
and from the end of the U-page to the SSP.  This leaves out all that unneeded
stuff in the middle.

- john.