Abstract: A digital computer system is described as comprising an input data circuit, a processor for executing a program stored in a memory to process the inputted data, a depletable energy source (e.g., a battery) for energizing the elements of the computer system, and a power control circuit for selectively controlling the drain imposed upon the energy source in accordance with the data input into the system and the program executed by the processor. In an illustrative embodiment of this invention, the power control circuit is operative in at least three modes of operation, namely off/rest, power down, and operating. In the off/rest mode, the power control circuit imposes a minimum power drain upon the depletable energy source. An operator may manipulate a switch or actuation means to apply a transition signal to the power control, whereby the computer system is disposed from its off/rest mode to its power down mode.