The central processing unit (CPU) is the part of the computer system where the manipulation of symbols, numbers, and letters occurs, and it controls the other parts of the computer system. Located near the CPU is primary storage (sometimes called primary memory or main memory), where data and program instructions are stored temporarily during processing. Buses provide pathways for transmitting data and signals between the CPU, primary storage, and the other devices in the computer system. The characteristics of the CPU and primary storage are very important in determining a computer’s speed and capabilities.

The Arithmetic-Logic Unit and Control Unit

The CPU consists of an arithmetic-logic unit and a control unit. The arithmetic-logic unit (ALU) performs the computer’s principal logical and arithmetic operations. It adds, subtracts, multiplies, and divides, determining whether a number is positive, negative, or zero. In addition to performing arithmetic functions, an ALU must be able to determine when one quantity is greater than or less than another and when two quantities are equal. The ALU can perform logic operations on letters as well as numbers.

The control unit coordinates and controls the other parts of the computer system. It reads a stored program, one instruction at a time, and directs other components of the computer system to perform the program’s required tasks. The series of operations required to process a single machine instruction is called the machine cycle. Older computers and PCs have machine cycle times measured in microseconds (millionths of a second). More powerful machines have machine cycle times measured in nanoseconds (billionths of a second) or picoseconds (trillionths of a second). Another measure of machine cycle time is by MIPS, or millions of instructions per second.

Primary Storage

Primary storage has three functions. It stores all or part of the software program that is being executed. Primary storage also stores the operating system programs that manage the operation of the computer. Finally, the primary storage area holds data that the program is using. Internal primary storage is often called RAM, or random access memory. It is called RAM because it can directly access any randomly chosen location in the same amount of time.

Primary memory is divided into storage locations called bytes. Each location contains a set of eight binary switches or devices, each of which can store one bit of information. The set of eight bits found in each storage location is sufficient to store one letter, one digit, or one special symbol (such as $). Each byte has a unique address, similar to a mailbox, indicating where it is located in RAM. The computer can remember where the data in all of the bytes are located simply by keeping track of these addresses.

Computer storage capacity is measured in bytes. One thousand bytes (actually 1,024 storage positions) is called a kilobyte. One million bytes is called a megabyte, one billion bytes is called a gigabyte, and one trillion bytes is called a terabyte.

Primary storage is composed of semiconductors, which are integrated circuits made by printing thousands and even millions of tiny transistors on small silicon chips. There are several different kinds of semiconductor memory used in primary storage. RAM is used for short-term storage of data or program instructions. RAM is volatile: Its contents will be lost when the computer’s electric supply is disrupted by a power outage or when the computer is turned off. ROM, or read-only memory, can only be read from; it cannot be written to. ROM chips come from the manufacturer with programs already burned in, or stored. ROM is used in general-purpose computers to store important or frequently used programs.