Minimisation of control store width in digital systems

The high level of complexity and the high degree of parallelism involved in several modern microprogrammed systems imply the design of control units having an extremely large microcode. Consequently, the necessity of limiting the size of the control store pushed research in two directions: microcode compaction, for reducing the number of words, and microcode bit minimisation, for reducing the number of control bits in a single word. In this paper, a new method for the optimisation of microword length in large microprogrammed systems is addressed. Microcode bit optimisation is formulated as a covering problem applied to a choice of maximal compatibility classes, collecting operations that can share the same bit held in the microcode. Comparisons with most of the published approaches are given using some benchmarks: results obtained show that the proposed method saves microcode bits in several cases, while in the largest examples it grants a significant reduction in computation time