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random.S revision 1.5 
      1 /*	$NetBSD: random.S,v 1.5 2009/01/04 17:10:46 pooka 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  *
     19  * THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS
     20  * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
     21  * TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
     22  * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS
     23  * BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
     24  * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
     25  * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
     26  * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
     27  * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
     28  * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
     29  * POSSIBILITY OF SUCH DAMAGE.
     30  */
     31 
     32 /*
     33  * Copyright (c) 1990,1993 The Regents of the University of California.
     34  * All rights reserved.
     35  *
     36  * Redistribution and use in source and binary forms, with or without
     37  * modification, are permitted provided that: (1) source code distributions
     38  * retain the above copyright notice and this paragraph in its entirety, (2)
     39  * distributions including binary code include the above copyright notice and
     40  * this paragraph in its entirety in the documentation or other materials
     41  * provided with the distribution, and (3) all advertising materials mentioning
     42  * features or use of this software display the following acknowledgement:
     43  * ``This product includes software developed by the University of California,
     44  * Lawrence Berkeley Laboratory and its contributors.'' Neither the name of
     45  * the University nor the names of its contributors may be used to endorse
     46  * or promote products derived from this software without specific prior
     47  * written permission.
     48  * THIS SOFTWARE IS PROVIDED ``AS IS'' AND WITHOUT ANY EXPRESS OR IMPLIED
     49  * WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF
     50  * MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.
     51  *
     52  * Here is a very good random number generator.  This implementation is
     53  * based on ``Two Fast Implementations of the "Minimal Standard" Random
     54  * Number Generator'', David G. Carta, Communications of the ACM, Jan 1990,
     55  * Vol 33 No 1.  Do NOT modify this code unless you have a very thorough
     56  * understanding of the algorithm.  It's trickier than you think.  If
     57  * you do change it, make sure that its 10,000'th invocation returns
     58  * 1043618065.
     59  *
     60  * Here is easier-to-decipher pseudocode:
     61  *
     62  * p = (16807*seed)<30:0>	# e.g., the low 31 bits of the product
     63  * q = (16807*seed)<62:31>	# e.g., the high 31 bits starting at bit 32
     64  * if (p + q < 2^31)
     65  * 	seed = p + q
     66  * else
     67  *	seed = ((p + q) & (2^31 - 1)) + 1
     68  * return (seed);
     69  *
     70  * The result is in (0,2^31), e.g., it's always positive.
     71  */
     72 #include <machine/asm.h>
     73 
     74 	.data
     75 randseed:
     76 	.long	1
     77 	.text
     78 ENTRY(random)
     79 	movl	$16807,%eax
     80 	PIC_PROLOGUE
     81 	imull	PIC_GOTOFF(randseed)
     82 	PIC_EPILOGUE
     83 	shld	$1,%eax,%edx
     84 	andl	$0x7fffffff,%eax
     85 	addl	%edx,%eax
     86 	js	1f
     87 	PIC_PROLOGUE
     88 	movl	%eax,PIC_GOTOFF(randseed)
     89 	PIC_EPILOGUE
     90 	ret
     91 1:
     92 	subl	$0x7fffffff,%eax
     93 	PIC_PROLOGUE
     94 	movl	%eax,PIC_GOTOFF(randseed)
     95 	PIC_EPILOGUE
     96 	ret
     97