1. A packet modification system comprising: a packet slicer to receive a complete packet at the packet modification system and slice the complete packet into a plurality of packet portions; a transmit-in-data buffer to receive and store the plurality of packet portions from the packet slicer; the transmit-in-data buffer to pass, in parallel, the plurality of packet portions to a modification engine of the packet modification system; a packet modification engine to effect a modification to one or more of the plurality of packet portions via a programmable processor core coupled with the packet modification engine by implementing at least the following operational stages at the programmable processor core: (i) a command fetch stage to fetch one or more commands to effect the modification on the one or more packet portions, (ii) a data fetch stage to fetch insertion and/or replacement data for use by the one or more commands, and (iii) an address and mask generation stage to generate four pointers and associated masks during each command cycle of the programmable processor core, the four pointers comprising: (a) a first pointer to packet data indicating a location where an operation is to occur among the plurality of packet portions, (b) a second pointer to insertion data which is to be inserted into one or more of the packet portions, (c) a third pointer to copy data which is to be copied from one of the packet portions and into another of the packet portions, and (d) a fourth pointer to residual packet data which is to come after the location where the operation is to occur among the plurality of packet portions as specified via the first pointer; wherein the data fetch stage follows the first, second, third, and fourth pointers to retrieve the data specified for use in completing the modification to the one or more packet portions via the programmable processor core; and a packet assembler to re-assemble the plurality of packet portions, including the one or more packet portions having been modified, into a modified complete packet for egress transmission from the packet modification system.