How do you generate a random number in Assembly language using the FASM compiler?

Question

Hey guys, I'm really new to assembly and I'm trying to create a simple program. For this I need to generate a random number.

Anybody know how I can do this with the FASM compiler?

Thanks, Sam

Answer 1

Take a look at this Wikipedia page, pick an algorithm, and implement it.

Edit: Or you could take the easy route. Use your OS's C runtime and call their rand functions.

Answer 2

You could use a Linear-Congruential Algorithm. Its the most common psuedo-random number algorithm.

Basically, you have a seed value. And then once you start generating random numbers each number becomes the seed for the new request.

The numbers are generated by

x = (a * s + b) MOD m

Where m, a and b are picked for the algorithm. There are some popular sets of these values used. Its a lot easier if you make m a power of 2, especially 2^32 for 32 bit machines. Then the mod step is done automatically by the machine.

Check out the wikipedia, they have popular sets of a, b and M and a lot more information.

There are more complicated things can be done with seeds as well (setting the seed based on the current time, for instance)

Answer 3

I'm a big fan of R250, being much faster to execute than LCG. http://www.ddj.com/184408549?pgno=7

Shows a significant increase in speed in the old assembly code I used to write back in the day.

Answer 4

random number

This is a slightly ambiguous question.

Most of the posters so far are probably right; they're explaining how to generate a pseudo-random number and that's probably what you need. Seed the algorithm with the current time (you'll have to ask the OS for that, or read it from the clock chip). That'll give you "random" numbers that are good enough for games and other simple uses.

But please don't use those "random numbers" for any security application (encryption, key generation, etc). For security applications you need a really good cryptographically-secure random number generator. Writing one of those is really hard. (Netscape got it wrong so early versions of Netscape Navigator had an easily-hackable HTTPS implementation; Debian very recently got it wrong leading to loads of easily-hackable SSH and HTTPS/SSL keys).