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