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random.S revision 1.2 
      1 /*	$NetBSD: random.S,v 1.2 1998/08/15 03:02:49 mycroft Exp $	*/
      2 
      3 /*
      4  * Copyright (c) 1995 Charles M. Hannum.
      5  * Copyright (c) 1990,1993 The Regents of the University of California.
      6  * All rights reserved.
      7  *
      8  * Redistribution and use in source and binary forms, with or without
      9  * modification, are permitted provided that: (1) source code distributions
     10  * retain the above copyright notice and this paragraph in its entirety, (2)
     11  * distributions including binary code include the above copyright notice and
     12  * this paragraph in its entirety in the documentation or other materials
     13  * provided with the distribution, and (3) all advertising materials mentioning
     14  * features or use of this software display the following acknowledgement:
     15  * ``This product includes software developed by the University of California,
     16  * Lawrence Berkeley Laboratory and its contributors.'' Neither the name of
     17  * the University nor the names of its contributors may be used to endorse
     18  * or promote products derived from this software without specific prior
     19  * written permission.
     20  * THIS SOFTWARE IS PROVIDED ``AS IS'' AND WITHOUT ANY EXPRESS OR IMPLIED
     21  * WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF
     22  * MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.
     23  *
     24  * Here is a very good random number generator.  This implementation is
     25  * based on ``Two Fast Implementations of the "Minimal Standard" Random
     26  * Number Generator'', David G. Carta, Communications of the ACM, Jan 1990,
     27  * Vol 33 No 1.  Do NOT modify this code unless you have a very thorough
     28  * understanding of the algorithm.  It's trickier than you think.  If
     29  * you do change it, make sure that its 10,000'th invocation returns
     30  * 1043618065.
     31  *
     32  * Here is easier-to-decipher pseudocode:
     33  *
     34  * p = (16807*seed)<30:0>	# e.g., the low 31 bits of the product
     35  * q = (16807*seed)<62:31>	# e.g., the high 31 bits starting at bit 32
     36  * if (p + q < 2^31)
     37  * 	seed = p + q
     38  * else
     39  *	seed = ((p + q) & (2^31 - 1)) + 1
     40  * return (seed);
     41  *
     42  * The result is in (0,2^31), e.g., it's always positive.
     43  */
     44 #include <machine/asm.h>
     45 
     46 	.data
     47 randseed:
     48 	.long	1
     49 	.text
     50 ENTRY(random)
     51 	movl	$16807,%eax
     52 	imull	randseed
     53 	shld	$1,%eax,%edx
     54 	andl	$0x7fffffff,%eax
     55 	addl	%edx,%eax
     56 	js	1f
     57 	movl	%eax,randseed
     58 	ret
     59 1:
     60 	subl	$0x7fffffff,%eax
     61 	movl	%eax,randseed
     62 	ret
     63