Increasing performance by microcoding

Abstract A quantitative comparison between ways to implement an interpreter for the high level language Concurrent Pascal is given. Candidates are four variants on two computers with and without microprogramming: specifically implementation via 1. (i) machine instructions on PDP-11, 2. (ii) machine instructions on HP-21MX, 3. (iii) micro instructions on HP-21MX, 4. (iv) a version combining (ii) and (iii). After introducing the respective machine architectures the problems caused by the different implementation methods are discussed together with examples of selected interpreter instructions and a comparison of the performance and the programming effort. The conclusion is a performance gain of a factor of about 8 caused by matching the structures and a loss in programmer's convenience without numerical quantification.