Abstract.—In the area of human information processing an important concept is that of a limited capacity central processing system (CPS). Posner (1970) emphasized the distinction between a short‐term memory (STM) and an operational memory (OM). To evaluate this distinction, as well as to obtain data on the capacity limit of the CPS, and experiment was carried out, in which 14 subjects had to keep a running count of the number of occurrences of two or more predetermined items in a series of stimuli. Based on the experimental data a register model of the CPS was suggested which emphasized the distinction between the STM and the OM as one of two different memory functions carried out by a common memory system. The model also suggested that the capacity limit of the CPS was around eight items of verbally coded informat