I am wondering why I got xxxxx at the output of this primitive when I try to simulate with modelsim. I don't get any error in the compilation and it looks it is going to work well. However I got always the xxx at the output....
Notice: with the ise simulator I got the same result.....
Extremely hard to say what is going on without your code.
X's usually mean you didn't reset or initialize something or didn't connect some important input signal or if it is pipelined didn't run for long enough to flush out the garbage.