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To: KATO Takenori <kato@eclogite.eps.nagoya-u.ac.jp>
cc: current@FreeBSD.org
Subject: Re: TSS broken 
In-reply-to: Your message of "Mon, 01 Jan 1996 11:50:18 +0900."
             <199601010250.LAA00929@marble.eps.nagoya-u.ac.jp> 
From: David Greenman <davidg@Root.COM>
Reply-To: davidg@Root.COM
Date: Thu, 01 Feb 1996 02:30:47 -0800
Sender: owner-current@FreeBSD.org
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>In recent change of sys/i386/i386/swtch.s, the code
>	movl	%esp,PCB_ESP(%ecx)
>is changed into
>	leal	4(%esp),%eax
>	movl	%eax,PCB_ESP(%ecx)
>(in savectx).
>
>After this operation, the member tss_esp points not user stack
>pointer, but the address of TSS.
>
>This may causes panic when swapper is activated.

   I just read the above again a few times and I think I see now the
misunderstanding. The "leal" instruction moves the _address_ of its
argument. In this case, %esp+4. It is similar to:

	movl	%esp,%eax
	addl	$4,%eax
	movl	%eax,PCB_ESP(%ecx)

   The only difference being that leal is faster and also doesn't affect the
condition codes like the addl does (although in this case it doesn't matter
since we're not using them).

-DG

David Greenman
Core Team/Principal Architect, The FreeBSD Project