1. A microprogram computer having a control element for executing a series of control steps in response to a machine language instruction word stored in a memory, said control element comprising: a control storage element connected to access data from said memory and storing microinstructions defining plural sequences of steps for decoding portions of said machine language words to produce first output data partially defining control steps for said computer; a secondary storage element, connected to access data from said memory and responsive to said machine language words for decoding a different portion of said machine language words to produce second output data partially defining control steps for said computer, and for selecting one of said plural sequences of said control storage element in response to said machine language words; means for merging said first and second output data of said control storage element and said secondary storage element to produce third output data defining said series of control steps for said computer; and a control unit connected to said merging means and producing output timing and controlling data to said computer for execution in accordance with said third output data.