Hans thank you for your help.
What I'm doing is an electronic sign base on a matrix of leds of 7 columns and 64 files.
One effect needs to show a message for a random period of time. Thats why I was needing a pseudo random generator. You program the sign with a PC via a RS232 ( the only input, no push button or any asyncronic event from the outside).
I call this routine ( rand8) and compare the output ( rand8reg) with zero.

JUMP:
LCALL Rand8
MOV A, Rand8reg
JNZ A, JUMP

The problem was that I was always inside the loop.

I studied this routine in detail and found that it works but the only number that don't give as an output is 0.
So I modified the above code

JUMP:
LCALL Rand8
MOV A, Rand8reg
DEC A
JNZ A, JUMP

And now is OK.

Thank you.

Daniel