From mboxrd@z Thu Jan  1 00:00:00 1970
Date: Mon, 20 Nov 2006 22:03:08 -0500
From: William Josephson <jkw@eecs.harvard.edu>
To: Fans of the OS Plan 9 from Bell Labs <9fans@cse.psu.edu>
Subject: Re: [9fans] Creating a custom jmp_buf;
	libthread implementation question
Message-ID: <20061121030308.GA40174@mero.morphisms.net>
References: <64c8196ab25721b971e64c96b6f8e065@plan9.jp>
	<7871fcf50611131933h1b7ee706vc56be7b37cd6b58b@mail.gmail.com>
	<775b8d190611140139i4e28842dndd5bede4255b4b11@mail.gmail.com>
	<ee9e417a0611140434m13221df3l2b3583bf7b52196b@mail.gmail.com>
	<7871fcf50611201903m3a8aaefdk10b544067ef8999c@mail.gmail.com>
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On Mon, Nov 20, 2006 at 10:03:09PM -0500, Joel Salomon wrote:
> On 11/14/06, Russ Cox <rsc@swtch.com> wrote:
> >You really care about _threadinitstack (386.c) and setjmp(2).
> >"tos" means top of stack.
>
> What does the line
>        tos = (ulong*)&t->stk[t->stksize&~7];
> specifically stksize&~7, mean?

Um.  It means just what it says :-) I.e. round
down to the nearest multiple of 8 by clearing
the low three bits.  Recall which way the stack
grows on IA-32...