Chapter 3 - The Processor

Abstract

This chapter discusses the evolution of computer processors. The central processing unit consists of three logical units: the arithmetic and logic unit (ALU), main storage or the storage unit, and the control unit. The ALU performs the arithmetic operations of addition, subtraction, multiplication, and division. Main storage stores information that arrives via the input unit so that this information is available to the ALU when actual calculations are to be performed. The storage unit also retains the results of the ALU's calculations until they can be presented to the user via the output unit. The control unit coordinates the operations of the other two units. Main storage is used to retain active information, but because it is relatively expensive, most information is kept in the less costly secondary storage, or auxiliary storage. Most computers have instructions that can input data from outside the computer into the computer's main storage, output data from main storage, perform simple arithmetic calculations, move data between main storage locations, edit data, perform comparisons, and handle many other functions. The processor reads the instructions in a computer program and performs these instructions one at a time in the proper sequence. Machine language instruction formats vary widely among the different types of computers.