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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 "Sat, 03 Feb 1996 02:15:50 +0900."
             <199602021715.CAA00247@marble.eps.nagoya-u.ac.jp> 
From: David Greenman <davidg@Root.COM>
Reply-To: davidg@Root.COM
Date: Fri, 02 Feb 1996 10:25:17 -0800
Sender: owner-current@FreeBSD.org
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>> you've misunderstood the operation of the 'leal' instruction. The
>> code as I've written it appears to be correct.
>
>Your code depends on compiler options. 
>
>I know the kernel should be compiled only with '-O', but I usually do
>with -fomit-frame-pointer option.  After I applied the change of
>swtch, the kernel compiled with -fomit-frame-pointer causes panic.
>
>When cpu_switch restores context, saved eip is restored as
>	movl	PCB_EIP(%edx),%eax
>	movl	%eax,(%esp)
>After this operation caller address of savectx overwrites argument to
>savectx.  So esp of child process just after 'call _savectx' points
>the argument to bcopy.

   You are right. I didn't realize that cpu_switch() clobbers the top of
stack with the EIP and then does a return, making the assumption that the
return address wasn't previously popped off. The change I made is wrong - I
didn't realize the interaction between cpu_fork and cpu_switch. I'll back
out that change.

-DG

David Greenman
Core Team/Principal Architect, The FreeBSD Project