j'ai l'air un peu c.. avec ma "graine"
LMAO you don't want to know the meaning of the Quebec expression 'Graine'

a) So, when the statement is RANDOM RandomVar, the "output" value will be stored in RandomVar, no?
Indeed

b) R_V0 = rvalue & $03 will keep only the 4 LSB of the result, no?
Nope, 3=%00000011 so only the 2 LSBs

c) still don't understand why my code in post#1 doesn't work. Must I specify it is in a loop? It is and doesn't work.
I guess you should use something like
rvalue=R_V0 before using Random again...