1/* $NetBSD: random.S,v 1.2 1999/11/11 01:53:46 thorpej Exp $ */ 2 3/* 4 * Copyright (c) 1990,1993 The Regents of the University of California. 5 * All rights reserved. 6 * 7 * Redistribution and use in source and binary forms, with or without 8 * modification, are permitted provided that: (1) source code distributions 9 * retain the above copyright notice and this paragraph in its entirety, (2) 10 * distributions including binary code include the above copyright notice and 11 * this paragraph in its entirety in the documentation or other materials 12 * provided with the distribution, and (3) all advertising materials mentioning 13 * features or use of this software display the following acknowledgement: 14 * ``This product includes software developed by the University of California, 15 * Lawrence Berkeley Laboratory and its contributors.'' Neither the name of 16 * the University nor the names of its contributors may be used to endorse 17 * or promote products derived from this software without specific prior 18 * written permission. 19 * THIS SOFTWARE IS PROVIDED ``AS IS'' AND WITHOUT ANY EXPRESS OR IMPLIED 20 * WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF 21 * MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. 22 * 23 * Here is a very good random number generator. This implementation is 24 * based on ``Two Fast Implementations of the "Minimal Standard" Random 25 * Number Generator'', David G. Carta, Communications of the ACM, Jan 1990, 26 * Vol 33 No 1. Do NOT modify this code unless you have a very thorough 27 * understanding of the algorithm. It's trickier than you think. If 28 * you do change it, make sure that its 10,000'th invocation returns 29 * 1043618065. 30 * 31 * Here is easier-to-decipher pseudocode: 32 * 33 * p = (16807*seed)<30:0> # e.g., the low 31 bits of the product 34 * q = (16807*seed)<62:31> # e.g., the high 31 bits starting at bit 32 35 * if (p + q < 2^31) 36 * seed = p + q 37 * else 38 * seed = ((p + q) & (2^31 - 1)) + 1 39 * return (seed); 40 * 41 * The result is in (0,2^31), e.g., it's always positive. 42 */ 43#include <machine/asm.h> 44 45 .data 46ASLOCAL(randseed) 47 .long 1 48 49ENTRY(random) 50 movl #16807, %d0 51 mulsl _ASM_LABEL(randseed), %d1:%d0 52 lsll #1, %d0 53 roxll #2, %d1 54 addl %d1, %d0 55 moveql #1, %d1 56 addxl %d1, %d0 57 lsrl #1, %d0 58 movl %d0, _ASM_LABEL(randseed) 59 rts 60