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From: Peter Dufault <dufault@hda.com>
Message-Id: <199703180946.EAA21402@hda.hda.com>
Subject: Re: test and set instruction: how?
In-Reply-To: <199703172327.AAA01251@plm.xs4all.nl> from Peter Mutsaers at "Mar 18, 97 00:27:27 am"
To: plm@xs4all.nl (Peter Mutsaers)
Date: Tue, 18 Mar 1997 04:46:47 -0500 (EST)
Cc: freebsd-questions@FreeBSD.ORG
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> Is there a (portable ?) way for a test-and-set instruction under
> FreeBSD?

(...)

Compare and exchange is present on 486s and Pentiums.  Early 486s
have a different op code that collides with other instructions;
Pentiums support 64 bit flavors.  Here is what I've been using.
It will compile with CAS_TEST.

/* cas: Compare-and-swap using the 486 and Pentium "cmpxchg"
 *      instruction.
 *
 * Atomically change what "ptr" points to to "n" if it is "o"
 * and return success.
 */

#if defined(CAS_TEST)
	typedef unsigned long castype;
	extern int cas(volatile castype *, const castype, const castype);
#else
	#include "cas.h"
#endif

#if defined(CAS_DEBUG)
	#include <stdio.h>
#endif

int
cas(volatile castype *ptr, const castype o, const castype n)
{
	volatile int result = 0;

#if defined(CAS_DEBUG)
	fprintf(stderr, "cas %p %08lx %08lx %08lx, ", ptr, *ptr, o, n);
#endif
	__asm   __volatile(
	            "cmpxchg%L3 %3,%1; jne 0f; inc%L0 %0; 0:"
	    :       "=m"(result)
	    :       "m"(*ptr), "a"(o), "r"(n)
	);

#if defined(CAS_DEBUG)
	fprintf(stderr, "%s %08lx\n", result ? "OK  " : "Fail", *ptr);
#endif

	return result;
}


#if defined(CAS_TEST)

#include <stdio.h>

int main(int argc, char *av[])
{

	while (1)
	{
		volatile castype c;
		castype o, n;
		int result;

		printf("c old new > ");
		if (scanf("%ld %ld %ld", &c, &o, &n) != 3)
			break;

		result = cas(&c, o, n);
		printf("%d; %ld %ld %ld\n", result, c, o, n);
	}

	return 0;
}
#endif /* CAS_TEST */

-- 
Peter Dufault (dufault@hda.com)   Realtime Machine Control and Simulation
HD Associates, Inc.               Voice: 508 433 6936