Central Processing Unit

The central processing unit (CPU) coordinates the operation of the machine. The CPU is the `brains' of a computer that contains the circuits that decode and execute instructions. It works with data in discrete form--i.e., expressed directly as the digits of the binary code. A 1 or a 0 is a BInary digiT (BIT). It counts, lists, compares, and rearranges these binary digits, or bits, of data in accordance with very detailed program instructions stored within its memory. The results of these arithmetic and logic operations are translated into characters, numbers, and symbols that can be readily understood by the human operator or into signals intelligible to a machine controlled by the computer.

The circuits necessary to create a CPU for a personal computer are fabricated on a single integrated circuit called a microprocessor. Besides a single CPU, a Von Neumann design might have several coprocessors-that are subordinate processors for arithmetic and fast video displays.

Most CPU chips and microprocessors have five functional sections:

The Arithmetic and Logical Unit, or ALU

An arithmetic-logic unit (ALU) performs simple addition, multiplication, division, and logic operations (i.e., operations, such as OR and AND, known in mathematical logic). The ALU is the device that performs elementary operations such as arithmetic operations (addition, subtraction, and so on), logical operations (AND, OR, NOT), and comparison operations (for example, comparing the contents of two "slots" for equality). This unit is where the "real work" is done.

Registers

Some registers (temporary storage locations), hold data, instructions, or the results of calculations;

The Control Unit

The control section times and regulates the computer system and translates data in the registers into activities such as add, move, or compare data; The CU keeps track of which slot contains the current instruction that the computer is performing, telling the ALU what operation to perform and retrieving the information (from the memory) that it needs to perform it, and transfers the result back to the appropriate memory location. Once that occurs, the control unit goes to the next instruction (typically located in the next slot, unless the instruction is a jump instruction informing the computer that the next instruction is located elsewhere).

The Internal Bus

a network of communication lines that links internal CPU elements and provides several different data paths between elements of the computer system. The bus is the electronic backbone connecting the components of the computer together. The more powerful the computer, the bigger the bus, where bus size refers to the number of bits which can be communicated at one time. The bus width in personal computers has moved from 8 to 16 to 32 and is moving to 64 bits buses currently. As we move towards multimedia, the need to move large blocks of data very quickly will increase.

Clock

The clock synchronizes the components of a computer to work together. A faster clock enables the computer to execute more instructions a second. Since the 1970s, clocks on personal computers have increased from 1 to over 1,000 million cycles a second. The faster the clock the more heat is generated and the more steps must be taken to keep the electronic devices cool. Personal computers currently have two clocks: one for the CPU and one for the bus with the CPU clock running up to 10 times faster than the bus clock. The bus clock speed must be increased as we move into processing dynamic images.

Modern computers put the ALU and control unit into a single integrated circuit known as the Central Processing Unit or CPU. Typically, the computer's memory is located on a few small integrated circuits near the CPU. The overwhelming majority of the computer's mass is either ancilliary systems (for instance, to supply electrical power) or I/O devices.

The CPU is the key part of a computer because its operational speed predominantly determines the speed of the computer as a whole. The CPU and fast memories are implemented with transistor circuits.

Some larger computers differ from the above model in one major respect - they have multiple CPUs and control units working simultaneously. Additionally, a few computers, used mainly for research purposes and scientific computing, have differed significantly from the above model, but they have found little commercial application.