X-Authentication-Warning:	delorie.com: mail set sender to djgpp-bounces using -f
NNTP-Posting-Date:	Sat, 27 Oct 2007 23:48:39 -0500
Message-ID:	<472414A7.7070602@zytor.com>
Date:	Sat, 27 Oct 2007 21:48:39 -0700
From:	"H. Peter Anvin" <hpa AT zytor DOT com>
User-Agent:	Thunderbird 2.0.0.5 (X11/20070727)
MIME-Version:	1.0
Newsgroups:	comp.os.msdos.djgpp
CC:	djgpp AT delorie DOT com
Subject:	Re: Using inline asm
References:	<471FEAC8 DOT 5179 DOT 2FAB88DB AT gerritvn DOT gpvno DOT co DOT za> <200710250120 DOT l9P1KEb2032374 AT envy DOT delorie DOT com> <ffp07f$66t$1 AT aioe DOT org> <200710250306 DOT l9P36dqi002391 AT envy DOT delorie DOT com>
In-Reply-To:	<200710250306.l9P36dqi002391@envy.delorie.com>
Lines:	25
X-Usenet-Provider:	http://www.giganews.com
NNTP-Posting-Host:	67.169.144.158
X-Trace:	sv3-ZTEPNVqEV5NLgEt0NWpypR2+tgm7bfmrb2WwTdZnBFTawWpn17Fh+Py5nn6t0n4sDnfyhpJZF3wxqAJ!cAcpvNYdUZ1Iqn4TEJgHOdkaCmt7kRXha8dfrvEzt8YVAotPjB65N6Ax2apmZRUMnHRLDDh1t3Lq!WF2RD3Y=
X-Complaints-To:	abuse AT comcast DOT net
X-DMCA-Complaints-To:	dmca AT comcast DOT net
X-Abuse-and-DMCA-Info:	Please be sure to forward a copy of ALL headers
X-Abuse-and-DMCA-Info:	Otherwise we will be unable to process your complaint properly
X-Postfilter:	1.3.35
To:	djgpp AT delorie DOT com
DJ-Gateway:	from newsgroup comp.os.msdos.djgpp
Reply-To:	djgpp AT delorie DOT com

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

- Raw text -