Ok.. Thank you for all your comments. You're all right, I need to study a bit... But one last thing please:

I want to write on the whole P3 port. (Let's say to drive 8 LEDs)
I read that I must set the P3 latch to 00 (since it's at FF after reset which means input)
Then I can write the output I want on the port P3.

Am I right?

How to write to the P3 latch? Actually I don't see the difference in the assembly code between writing to the latch or writing to the port.

to do this: MOV P3,#00h
Is it writing to the port or the latch?

Thank you agian for your help, I hope I'll get less confused.

Julius