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#1 NickDMax   User is offline

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Random Number Generator

Posted 10 May 2009 - 01:21 AM

Description: To use the function you would normally first seed the generator with some unpredictable value (such as the system clock value). Note that this is NOT a cryptographically secure generator! It is generally best only only actually use a few bits from an LCG.A Basic LCG example. The function uses the C calling convention (so it can integrate with C/C++).
PUBLIC _RandomGen, _RandomGenSeed

; A basic Linear Congruential Generator capable of generating
; pseudorandom numbers -- As you can see, implemented in assembly
; using a modulus of a power of 2 (the machine word size), the
; program is very simple.

MODEL flat, C
_DATA	segment dword public use32 'DATA'
    CurrentValue    dd  0   ; Since LCG's are iterative we need to be able to save the last value.
_DATA	ends

_TEXT    segment dword public use32 'CODE'

; Our equation is X(i+1) = A * X(i) + C mod 2^32
; These two constants used for A and C 
AValue  EQU     41C64E6Dh    ;Value for A in ASCII C's rand() function
CValue  EQU     03039h

_RandomGen proc near                   ; extern "C" unsigned int RandomGen();
    push    ebp                        
    mov     ebp, esp                   ; set base pointer to stack pointer
    push    ebx
        mov     eax, CurrentValue      ;initialize with previous value
        mov     ecx, AValue            ; We will be using the formula:
        mov     ebx, CValue            ; ebx*eax + ebx and for mod we will just
                                        ;ignore the value in edx.
        mul     ecx
        add     eax, ebx
        mov     CurrentValue, eax
        ROR     eax, 4                  ;Using rotate to mask hifrequency patterns.
    pop     ebx
    pop     ebp
_RandomGen endp

_RandomGenSeed proc near
    push    ebp                            
    mov     ebp, esp
        mov     eax, [ebp + 8]
        mov     CurrentValue, eax
    pop     ebp
_RandomGenSeed endp



; Example useage:
;#include <stdio.h>
;extern int RandomGen(void);
;extern void RandomGenSeed(int);
;int main() {
;    int i;
;    unsigned int a;
;    RandomGenSeed(10);  //Seed the random number generator with some value
;    for(i = 0; i < 100; ++i) {
;        a = RandomGen();
;        printf("Random Value: %un", a);
;    }
;    return 0;

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