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Date: Sat, 27 Oct 2007 21:48:39 -0700
From: "H. Peter Anvin" <hpa AT zytor DOT com>
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CC: djgpp AT delorie DOT com
Subject: Re: Using inline asm
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DJ Delorie wrote:
>>>>    asm ("cpuid" : "=a" (iRet));
>> Huh?  I'm sure I missed something...  That takes care of eax for cpuid, but
>> that ignores ebx, ecx, and edx.  cpuid trashes those four registers whether
>> he returns them or not, correct?   How does GCC correct the other three?
> 
>    asm ("cpuid" : "=a" (iRet) : : "ebx", "ecx", "edx");
> 
> Or something like that.

CPUID takes %eax as an input (and sometimes %ecx):

static inline unsigned int cpuid_eax(unsigned int level)
{
	unsigned int eax;

	asm("cpuid" : "=a" (eax) : "a" (level)
		: "ebx", "ecx", "edx");
	return eax;
}

Factoring inline assembly out into small inline functions is good 
programming practice.

	-hpa