1. A data processing system comprising a central processor unit including means responsive to a macroinstruction for decoding said macroinstruction to produce a sequence of one or more microinstructions; and processing logic means responsive to said microinstructions for executing said microinstructions to provide one or more data processing operations; said system further comprising one or more external microcode control units, each including means responsive to a macroinstruction received from said central processor unit for decoding said macroinstruction to produce a sequence of one or more microinstructions; means responsive to said decoding means for transmitting said one or more microinstructions to said central processor unit for execution thereby; and said decoding means further including means responsive to a macroinstruction received from said central processor unit for determining whether said macroinstruction is one which is to be decoded by a selected one of said external microcode control units or one which is to be decoded by said central processor unit.