The precise number of microcomputers marketed and sold to date is unknown. However, since their advent in the mid 1970's, the combined annual sales by all manufacturers is nearing or exceeds one million units. Without question, microcomputers have been widely accepted in a startlingly short period of time because of their low unit cost, low for both systems and software. This does not mean that microcomputers are currently a cost-effective way to replace existing machines, although they may be in some specific situations. Rather, their cost constitutes a low threshold, the point at which owning or using a computer becomes viable for an individual. This threshold is important to both the business and personal user and has changed workstation computation from concept to reality.