Control memory word width optimization using multiple-valued circuits

The optimization of read-only memory (ROM) digit dimension for microprogrammed digital computers is considered. The design of integrated injection logic (I²L) multiple valued ROMs is reviewed and designs for corresponding digit-line decoders are presented. Results are presented for determining microoperation groupings that achieve the lower bound digit dimension for multivalued encodings, thus extending to arbitrary radix the theory for the binary case. To illustrate the applicability of these results, the microprogram specifications for previously published optimization examples, as well as that for the Digital Equipment Corporation EDP-9, are shown to have optimal encodings using either three- or four-valued ROMs and either ternary- or quaternary-to-binary decoders, resulting in dramatic savings in device count for the control memory portion of the corresponding machine.