Gameboy Development Forum

Could you share a code snippet?
Is it possible that you set the random seed in the same loop you generate the random numbers in?

If you set it to the same value always, the numbers that will generate rand() will always be the same.

You need to add something like a loop to wait for the user to press and read from register DIV, for example. That's a sort of reasonable unpredictable value, so it can be used as random seed. But only if you wait for the user input!

I think it is pretty well explained in this blogpost:
http://imrannazar.com/GameBoy-Emulation … pt:-Timers

and partly in the Gameboy CPU manual (http://marc.rawer.de/Gameboy/Docs/GBCPUman.pdf) at page 38.

You have a timer on the gameboy that is incremented 16384 times per second (that's the DIV register you mention). If you extract a value from that register (0xFF04) at random intervals, you basically get a random number. In other words, despite that that counter is not random, your time intervals are and thus you get pseudorandom numbers.

Hope it helps. :-)

I am not really sure when using C; so far I have only programmed Gameboys using assembly, so it is a bit of guess. Sorry for that.

Here goes. In C, you can refer to a memory location using a pointer. The value of that pointer is then equal to the memory address. So to create a pointer to the DIV register, you type:

unsigned char *ptr_div_reg = 0xFF04;    // set pointer to memory address

To get the value residing at the memory address of the pointer, you use the dereference operator "*", like so:

unsigned char random_number = *(ptr_div_reg);    // get value at memory address

Finally, to get a random number within a certain range from that you could use a modulus. To get a random number between 0 and 5, inclusive (i.e. a dice roll), you could use:

unsigned char dice_roll = random_number % 6;

If you read from that register twice without enough time in between, you will probably read the same value, so be careful when using it. If you use it once or twice per frame, in different part of the code, you'll be fine. If you want to use it a lot, you'll need other system.

The ideal thing is to wait for user input or something, then read the value and use it as seed. For example, if your game has a title screen, you could read its value after the user closes the screen, or something like that. DIV starts always at the same value, you can't even use it as a seed if srand is called always at the same time in your boot sequence.

Well, `randomthing % 30` will return a number from 0 through 29, and then you're adding 10 to that, so that will give you a range of 10 through 39.

If you want 10 through 30 (including 30), you want: (randomthing % 21) + 10
If you want 10 until 30 (excluding 30), you want: (randomthing % 20) + 10

Last edited by DonaldHays (2017-02-23 12:03:58)

(randomthing % 2)+1 should give you either 1 or 2.

If you try to read randomthing only, what kind of value do you obtain? Only odd values?

This reminds me of an earlier post http://gbdev.gg8.se/forums/viewtopic.php?id=288 
I dont know what you did differently that caused it not to produce random negative numbers.

Last edited by MrElephant (2017-03-28 20:18:14)