The proposed specification of the Sabrac digital computer was first discussed in papers presented to the International Conference on Information Processing (Lehman, 1959) and to the First Annual Conference of the British Computer Society. Since that time the specification of the machine has changed considerably. The revised design has produced a machine, now in operation, which despite its low cost and small size has many "Second Generation" features (Lehman, 1961a). The incorporation of a number of improvements became possible since external circumstances considerably extended the time available for designing the machine. This extension also resulted in a considerable gap between finalization of the logical design and availability of the main rack for commissioning purposes. Thus during the winter of 1960 the question arose whether it would not be possible to make use of this waitingtime to check out the logic of the machine, exposing the inevitable design-errors, by means of a simulation of the logic on another computer. In effect, this implies the continuous solution of a set of some six hundred timedependent, recursive, symbolic (Boolean) equations subject to some appropriate set of initial conditions. The latter arise from the initial behaviour (when applying voltages) of the machine circuit elements, from the state of any console switches, from the "initial contents" of the simulated store and, when relevant, from the "contents" of the input media.