MASCO

Abstract

Two methods are used to design the control unit of a processor. The first is the hardwire method, in which each machine instruction is applied directly to an array of decoders. Since changes to the decoder array are difficult and expensive to make once the processor has gone into production, the function of each machine instruction is effectively fixed. The second method is the more flexible microprogramming approach in which each machine language instruction is performed by executing a series of microinstructions retrieved from a control store (usually distinct from main memory) and applied to a small network of decoders, which generate the control signals. Thus, the function of a particular machine instruction is determined solely by the series of microinstructions it retrieves from the control store. A microinstruction routine is sometimes referred to as firmware because of its intermediary relationship to hardware and software. Show more