To insert pipeline stages, describe the necessary registers in your HDL code and place them after any multipliers, then set the “Multiplier Style (MULT_STYLE)” constraint to pipe_lut. If the target is a Virtex-4 or Virtex-5 device, and implementation of a multiplier requires multiple DSP48 blocks, XST can pipeline this implementation as well. Set “Multiplier Style (MULT_STYLE)” for this instance to pipe_block