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Date: Thu, 28 Jul 2005 09:59:28 +0200
From: Uwe Doering <gemini@geminix.org>
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To: Rink Springer <rink@stack.nl>
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Cc: hackers@freebsd.org, Ludvig Strigeus <strigeus@gmail.com>, ed@fxq.nl
Subject: Re: Problem with pic16l_setled
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Rink Springer wrote:
> * Ludvig Strigeus (strigeus@gmail.com) wrote:
> 
>>I see this code:
>>
>>+ENTRY(pic16l_setled)
>>+	push	%ebp
>>+	mov	%esp,%ebp
>>+
>>+	movl	0x8(%ebp),%ebx
>>+	orl	$0x800,%ebx
>>+	call	pic16l_ledhlp
>>+	movl	$0x701,%ebx
>>+	call	pic16l_ledhlp
>>+
>>+	leave
>>+	ret
>>
>>With the standard x86 calling convention, you're not supposed to
>>modify ebx, esi or edi without saving them first. Try adding a
>>push/pop ebx around.
> 
> I doubt this is the problem (I've looked at other .s files like
> i386/i386/support.s, function ssdtosd, and they don't seem to do this).

Well, at least in 'support.s' of RELENG_4 register ebx _does_ get pushed 
onto the stack in function 'ssdtosd'.  I suppose this is true also for 
the other FreeBSD branches.

If you call a function from the C level only registers eax, edx and ecx 
may be altered without preserving their original contents on the stack 
(or wherever else).  That is, in C the calling level expects that the 
other registers survive a function call unharmed.  As pointed out that's 
the standard x86 calling convention which applies to all x86 compilers, 
not just GCC.

    Uwe
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