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From: dmr@alice.UUCP
Newsgroups: comp.unix.wizards
Subject: Why u.
Message-ID: <6909@alice.UUCP>
Date: Wed, 20-May-87 04:31:45 EDT
Article-I.D.: alice.6909
Posted: Wed May 20 04:31:45 1987
Date-Received: Fri, 22-May-87 01:19:47 EDT
Organization: AT&T Bell Laboratories, Murray Hill NJ
Lines: 17

The real reason the u area doesn't move is simply that it contains the
kernel stack, and moving it would require relocating addresses in the
stack, and possibly other u variables.  Given that you don't want to move its
virtual location during a swap, on many machines (including the 11)
it is better to access it as `u.' rather than as `up->'.

On the standard Cray version of Unix the u area is referred to as u->
and moves, but the stack part is exchanged to a fixed location when
context switches occur.  (The copy uses vector moves, copies only the
active stack, and is pretty fast).  The people of Chippewa told me that
they had a version that did not copy the stack, but instead traced it
back and fixed up the frame pointers.  They said they found only a few
places where stack-relative automatic pointers occurred and the code
was readily fixed to avoid the problem.  I guess I believe them, but
I'm surprised it was so easy.

	Dennis Ritchie