rand.s

arm嵌入式系统开发--软件设计与优化随书源代码。开发环境c。基本运算优化部分。

;// ____________________________________________________________________
;//
;// Copyright (c) 2003, Andrew N. Sloss, Dominic Symes, Chris Wright
;// All rights reserved.
;// ____________________________________________________________________
;// 
;// NON-COMMERCIAL USE License
;// 
;// Redistribution and use in source and binary forms, with or without 
;// modification, are permitted provided that the following conditions 
;// are met: 
;//
;// 1. For NON-COMMERCIAL USE only.
;// 
;// 2. Redistributions of source code must retain the above copyright 
;//    notice, this list of conditions and the following disclaimer. 
;//
;// 3. Redistributions in binary form must reproduce the above 
;//    copyright notice, this list of conditions and the following 
;//    disclaimer in the documentation and/or other materials provided 
;//    with the distribution. 
;//
;// 4. All advertising materials mentioning features or use of this 
;//    software must display the following acknowledgement:
;//
;//    This product includes software developed by Andrew N. Sloss,
;//    Chris Wright and Dominic Symes. 
;//
;//  THIS SOFTWARE IS PROVIDED BY THE CONTRIBUTORS ``AS IS'' AND ANY 
;//  EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 
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;// Section 7.8: Random number generators

        AREA    ch07_8, CODE, READWRITE

        EXPORT  rand_32

;=================================================================////
;// return a random number between 0 and n

n       RN 0
r       RN 1
a       RN 2
t       RN 3
        
        EXPORT  rand_32
rand_32
        ; load state
        LDR     a, = 0x91E6D6A5
        LDR     r, seed
        ; generate next random number
        MLA     r, a, r, a      ; r = r*a+a      
        UMULL   t, n, r, n      ; n = (r*n)>>32
        ; save state and return
        STR     r, seed
        MOV     pc, lr
        
seed    DCD 0x12345678  ; initial random seed

        END