MINIMIZATION OF CONTROL STORE WIDTH IN DIGITAL-SYSTEMS

The high level of complexity and the high degree of parallelism involv ed in several modem microprogrammed systems imply the design of contro l units having an extremely large microcode. Consequently, the necessi ty of limiting the size of the control store pushed research in two di rections: microcode compaction, for reducing the number of words, and microcode bit minimisation, for reducing the number of control bits in a single word. In the paper, a new method for the optimisation of mic roword length in large microprogrammed systems is addressed. Microcode bit optimisation is formulated as a covering problem applied to a cho ice of maximal compatibility classes, collecting operations that can s hare the same bit field in the microcode. Comparisons with most of the published approaches are given using some benchmarks: results obtaine d show that the proposed method saves microcode bits in several cases, while in the largest examples it grants a significant reduction in co mputation time.