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Annotation of sys/arch/i386/i386/random.s, Revision 1.1.1.1

1.1       nbrk        1: /*     $OpenBSD: random.s,v 1.4 2003/04/17 03:42:14 drahn Exp $        */
                      2: /*     $NetBSD: random.s,v 1.5 1995/01/15 23:20:33 mycroft Exp $       */
                      3:
                      4: /*
                      5:  * Copyright (c) 1995 Charles Hannum.
                      6:  * Copyright (c) 1990,1993 The Regents of the University of California.
                      7:  * All rights reserved.
                      8:  *
                      9:  * Redistribution and use in source and binary forms, with or without
                     10:  * modification, are permitted provided that: (1) source code distributions
                     11:  * retain the above copyright notice and this paragraph in its entirety, (2)
                     12:  * distributions including binary code include the above copyright notice and
                     13:  * this paragraph in its entirety in the documentation or other materials
                     14:  * provided with the distribution, and (3) all advertising materials mentioning
                     15:  * features or use of this software display the following acknowledgement:
                     16:  * ``This product includes software developed by the University of California,
                     17:  * Lawrence Berkeley Laboratory and its contributors.'' Neither the name of
                     18:  * the University nor the names of its contributors may be used to endorse
                     19:  * or promote products derived from this software without specific prior
                     20:  * written permission.
                     21:  * THIS SOFTWARE IS PROVIDED ``AS IS'' AND WITHOUT ANY EXPRESS OR IMPLIED
                     22:  * WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF
                     23:  * MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.
                     24:  *
                     25:  * Here is a very good random number generator.  This implementation is
                     26:  * based on ``Two Fast Implementations of the "Minimal Standard" Random
                     27:  * Number Generator'', David G. Carta, Communications of the ACM, Jan 1990,
                     28:  * Vol 33 No 1.  Do NOT modify this code unless you have a very thorough
                     29:  * understanding of the algorithm.  It's trickier than you think.  If
                     30:  * you do change it, make sure that its 10,000'th invocation returns
                     31:  * 1043618065.
                     32:  *
                     33:  * Here is easier-to-decipher pseudocode:
                     34:  *
                     35:  * p = (16807*seed)<30:0>      # e.g., the low 31 bits of the product
                     36:  * q = (16807*seed)<62:31>     # e.g., the high 31 bits starting at bit 32
                     37:  * if (p + q < 2^31)
                     38:  *     seed = p + q
                     39:  * else
                     40:  *     seed = ((p + q) & (2^31 - 1)) + 1
                     41:  * return (seed);
                     42:  *
                     43:  * The result is in (0,2^31), e.g., it's always positive.
                     44:  */
                     45: #include <machine/asm.h>
                     46:
                     47:        .data
                     48:        .globl  _C_LABEL(_randseed)
                     49: _C_LABEL(_randseed):
                     50:        .long   1
                     51:        .text
                     52: ENTRY(random)
                     53:        movl    $16807,%eax
                     54:        imull   _C_LABEL(_randseed)
                     55:        shld    $1,%eax,%edx
                     56:        andl    $0x7fffffff,%eax
                     57:        addl    %edx,%eax
                     58:        js      1f
                     59:        movl    %eax,_C_LABEL(_randseed)
                     60:        ret
                     61: 1:
                     62:        subl    $0x7fffffff,%eax
                     63:        movl    %eax,_C_LABEL(_randseed)
                     64:        ret