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