Proposed is an action machine for assembling response packets in a network processor. The action machine comprises: a first register array adapted to store data for entry into fixed-length fields of differing response packets, a fixed-length field having the same length in the differing response packets; and a second register array adapted to store data for entry into variable-length fields of differing response packets, a variable-length field having different values or lengths in the differing response packets. The action machine is adapted to assemble a response packet by combining data stored in the first register array with data stored in the second register array.