QuestaSim/ModelSim simulation gives me unkown value in wave window. However I get them as X and x, what is the difference between the two?
IEEE Std 1364-1995 &sect; 14.1.1.4 "Unknown and high impedance values" and 
IEEE Std 1800-2012 &sect; 21.2.1.4 "Unknown and high-impedance values":


  
  If all bits in a group are at the unknown value, a lowercase x is displayed for that digit.
  If all bits in a group are at a high-impedance state, a lowercase z is printed for that digit.
  If some, but not all, bits in a group are unknown, an uppercase X is displayed for that digit.
  If some, but not all, bits in a group are at a high-impedance state, then an uppercase Z is displayed for that digit, unless there are also
  some bits at the unknown value, in which case an uppercase X is
  displayed for that digit.
  


Example:
8'b1111_xxxx => displays as 8'hFx
8'b00x0_1001 => displays as 8'hX9
8'b1010_zzzz => displays as 8'hAz
8'b0z00_0110 => displays as 8'hZ6
8'b0zx0_1010 => displays as 8'hXA (unknown has higher display priority over high impedance)