xref: /src/sys/lib/libkern/arch/x86_64/random.S

/*	$NetBSD: random.S,v 1.3 2017/10/30 17:13:39 maxv Exp $	*/

/*
 * Copyright (c) 1998 The NetBSD Foundation, Inc.
 * All rights reserved.
 *
 * This code is derived from software contributed to The NetBSD Foundation
 * by Charles M. Hannum.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS
 * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
 * TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
 * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS
 * BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
 * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 * POSSIBILITY OF SUCH DAMAGE.
 */

/*
 * Copyright (c) 1990,1993 The Regents of the University of California.
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that: (1) source code distributions
 * retain the above copyright notice and this paragraph in its entirety, (2)
 * distributions including binary code include the above copyright notice and
 * this paragraph in its entirety in the documentation or other materials
 * provided with the distribution, and (3) all advertising materials mentioning
 * features or use of this software display the following acknowledgement:
 * ``This product includes software developed by the University of California,
 * Lawrence Berkeley Laboratory and its contributors.'' Neither the name of
 * the University nor the names of its contributors may be used to endorse
 * or promote products derived from this software without specific prior
 * written permission.
 * THIS SOFTWARE IS PROVIDED ``AS IS'' AND WITHOUT ANY EXPRESS OR IMPLIED
 * WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF
 * MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.
 *
 * Here is a very good random number generator.  This implementation is
 * based on ``Two Fast Implementations of the "Minimal Standard" Random
 * Number Generator'', David G. Carta, Communications of the ACM, Jan 1990,
 * Vol 33 No 1.  Do NOT modify this code unless you have a very thorough
 * understanding of the algorithm.  It's trickier than you think.  If
 * you do change it, make sure that its 10,000'th invocation returns
 * 1043618065.
 *
 * Here is easier-to-decipher pseudocode:
 *
 * p = (16807*seed)<30:0>	# e.g., the low 31 bits of the product
 * q = (16807*seed)<62:31>	# e.g., the high 31 bits starting at bit 32
 * if (p + q < 2^31)
 * 	seed = p + q
 * else
 *	seed = ((p + q) & (2^31 - 1)) + 1
 * return (seed);
 *
 * The result is in (0,2^31), e.g., it's always positive.
 */
#include <machine/asm.h>

	.data
randseed:
	.long	1
	.text
ENTRY(random)
	movl	$16807,%eax
	imull	randseed(%rip)
	shld	$1,%eax,%edx
	andl	$0x7fffffff,%eax
	addl	%edx,%eax
	js	1f
	movl	%eax,randseed(%rip)
	ret
1:
	subl	$0x7fffffff,%eax
	movl	%eax,randseed(%rip)
	ret
END(random)