Mainframes were the large cabinets housing the central processing unit (CPU) and main memory of early computers. The term persists to describe and differentiate these larger computers, known for their considerable size and amount of storage, processing power, and reliability, from smaller counterparts such as servers, minicomputers, workstations, and personal computers (PCs). While mainframe is a generic term, most people instantly associate these computing workhorses with IBM and their System Z, the most popular and widely used models. The Z15 is the latest model.