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random.S revision 1.3 
      1 /*	$NetBSD: random.S,v 1.3 1998/08/15 04:49:50 mycroft Exp $	*/
      2 
      3 /*-
      4  * Copyright (c) 1998 The NetBSD Foundation, Inc.
      5  * All rights reserved.
      6  *
      7  * This code is derived from software contributed to The NetBSD Foundation
      8  * by Charles M. Hannum.
      9  *
     10  * Redistribution and use in source and binary forms, with or without
     11  * modification, are permitted provided that the following conditions
     12  * are met:
     13  * 1. Redistributions of source code must retain the above copyright
     14  *    notice, this list of conditions and the following disclaimer.
     15  * 2. Redistributions in binary form must reproduce the above copyright
     16  *    notice, this list of conditions and the following disclaimer in the
     17  *    documentation and/or other materials provided with the distribution.
     18  * 3. All advertising materials mentioning features or use of this software
     19  *    must display the following acknowledgement:
     20  *        This product includes software developed by the NetBSD
     21  *        Foundation, Inc. and its contributors.
     22  * 4. Neither the name of The NetBSD Foundation nor the names of its
     23  *    contributors may be used to endorse or promote products derived
     24  *    from this software without specific prior written permission.
     25  *
     26  * THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS
     27  * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
     28  * TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
     29  * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS
     30  * BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
     31  * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
     32  * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
     33  * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
     34  * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
     35  * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
     36  * POSSIBILITY OF SUCH DAMAGE.
     37  */
     38 
     39 /*
     40  * Copyright (c) 1990,1993 The Regents of the University of California.
     41  * All rights reserved.
     42  *
     43  * Redistribution and use in source and binary forms, with or without
     44  * modification, are permitted provided that: (1) source code distributions
     45  * retain the above copyright notice and this paragraph in its entirety, (2)
     46  * distributions including binary code include the above copyright notice and
     47  * this paragraph in its entirety in the documentation or other materials
     48  * provided with the distribution, and (3) all advertising materials mentioning
     49  * features or use of this software display the following acknowledgement:
     50  * ``This product includes software developed by the University of California,
     51  * Lawrence Berkeley Laboratory and its contributors.'' Neither the name of
     52  * the University nor the names of its contributors may be used to endorse
     53  * or promote products derived from this software without specific prior
     54  * written permission.
     55  * THIS SOFTWARE IS PROVIDED ``AS IS'' AND WITHOUT ANY EXPRESS OR IMPLIED
     56  * WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF
     57  * MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.
     58  *
     59  * Here is a very good random number generator.  This implementation is
     60  * based on ``Two Fast Implementations of the "Minimal Standard" Random
     61  * Number Generator'', David G. Carta, Communications of the ACM, Jan 1990,
     62  * Vol 33 No 1.  Do NOT modify this code unless you have a very thorough
     63  * understanding of the algorithm.  It's trickier than you think.  If
     64  * you do change it, make sure that its 10,000'th invocation returns
     65  * 1043618065.
     66  *
     67  * Here is easier-to-decipher pseudocode:
     68  *
     69  * p = (16807*seed)<30:0>	# e.g., the low 31 bits of the product
     70  * q = (16807*seed)<62:31>	# e.g., the high 31 bits starting at bit 32
     71  * if (p + q < 2^31)
     72  * 	seed = p + q
     73  * else
     74  *	seed = ((p + q) & (2^31 - 1)) + 1
     75  * return (seed);
     76  *
     77  * The result is in (0,2^31), e.g., it's always positive.
     78  */
     79 #include <machine/asm.h>
     80 
     81 	.data
     82 randseed:
     83 	.long	1
     84 	.text
     85 ENTRY(random)
     86 	movl	$16807,%eax
     87 	imull	randseed
     88 	shld	$1,%eax,%edx
     89 	andl	$0x7fffffff,%eax
     90 	addl	%edx,%eax
     91 	js	1f
     92 	movl	%eax,randseed
     93 	ret
     94 1:
     95 	subl	$0x7fffffff,%eax
     96 	movl	%eax,randseed
     97 	ret
     98