avx or sse get random numbers.

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Roman

Joined: 21 Apr 2012
Posts: 1995

Roman 19 Jul 2025, 18:44

fasmw 1.73
How get random numbers ?
I want generate Brownian noise.

19 Jul 2025, 18:44

AsmGuru62

Joined: 28 Jan 2004
Posts: 1728
Location: Toronto, Canada

AsmGuru62 19 Jul 2025, 22:16

- 32-bit or 64-bit? (there is a good and small x64 generator just discovered, passes Big Crush tests)
- in order to get good results:
1. generate unsigned 32-bit value
2. divide this value by 0x100000000 using FPU -- you will get a value in range from 0.0000 to 1.0000.
3. Using the value from step #2 you can now generate the values in any integer range (pseudocode, just use FPU for most of the code):

Code:

int InRange (int LOW, int HIGH)
{
    int     EXTENT = HIGH - LOW;
    int     PART   = ROUND (EXTENT * RANDOM_FROM_0_TO_1);

    return  PART + LOW;
}

Any generator will require a seed.
I usually use the function GetSystemTimeAsFileTime, it gives you a 64 bit value (FILETIME),
but the higher bits of the value almost never change, so you can use GetTickCount and XOR the returning EAX into higher bits of FILETIME.
Or, if your generator is 32 bit -- just use lower bits of FILETIME.

19 Jul 2025, 22:16

AsmGuru62

Joined: 28 Jan 2004
Posts: 1728
Location: Toronto, Canada

AsmGuru62 19 Jul 2025, 22:24

There are also instructions RDRAND/RDSEED.

19 Jul 2025, 22:24

Roman

Joined: 21 Apr 2012
Posts: 1995

Roman 19 Jul 2025, 22:47

RDTSC good Idea?

19 Jul 2025, 22:47

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20723
Location: In your JS exploiting you and your system

revolution 19 Jul 2025, 23:10

Roman wrote:

RDTSC good Idea?

It depends upon the quality of randomness needed.

In general RDTSC is awful for randomness, but for a simple unimportant task it can be perfectly fine to use RDTSC.

19 Jul 2025, 23:10

Roman

Joined: 21 Apr 2012
Posts: 1995

Roman 20 Jul 2025, 14:41

Code:

;RandSeed dd 0

add eax,287123
imul  edx,[RandSeed],08088405H
inc edx
mov   [RandSeed], edx
mul edx
;in eax=random number

20 Jul 2025, 14:41

AsmGuru62

Joined: 28 Jan 2004
Posts: 1728
Location: Toronto, Canada

AsmGuru62 20 Jul 2025, 18:51

Probably, some testing of the sequence is needed to verify the "randomness".

20 Jul 2025, 18:51

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