KEY INFORMATION SECTION 2. The nerve centre, the brain and the heart of a PC is the central processing unitorCPU

The nerve centre, the brain and the heart of a PC is the central processing unitorCPU. This unit is built into a single microprocessor chip - an integrated circuit - which executes program instructions and supervises the computer's overall operation. In electronics, an integrated circuit (also known as IC, microcircuit, microchip, silicon chip, or chip) is a miniaturized electronic circuit (consisting mainly of semiconductor devices, as well as passive components) that has been manufactured in the surface of a thin substrate of semiconductor material. Integrated circuits are used in almost all electronic equipment in use today and have revolutionized the world of electronics.

A multi-core processor is simply a single chip containing more than one microprocessor core, effectively multiplying the potential performance with the number of cores (as long as the operating system and software is designed to take advantage of more than one processor). Because the cores are physically very close they interface at much faster clock rates, improving overall system performance.

The unit consists of three main parts:

1. The control unit, which examines the instructions in the user's program, interprets each instruction and causes the circuits and the rest of the components -disk drives, monitor, etc. - to be activated to execute the functions specified, selects instructions and data from memory, and controls the flow between main storage and the arithmetic-log­ical unit.

The control unit has the following components:

· a counter that selects the instructions, one at a time, from memory;

· a regi­ster that temporarily holds the instructions read from memory while they are being executed;

· a decoder that takes the coded instruc­tion and breaks it down into individual commands necessary to carry it out;

· a clock, which produces marks at regular intervals.

The sequence of control unit operations is as follows:

· The next instruction to be executed is read out from primary stor­age into the storage register.

· The instruction is passed from the storage register to the instruction register.

· The operation part of the instruction is decoded so that the proper arithmetic or logical operation can be performed.

· The address of the op­erand is sent from the instruction register to the address regis­ter.

· At last the instruction counter register provides the address register with the address of the next instruction to be executed.

2. The arithmetic logic unit (ALU), which performs mathematical calculations (addition, subtraction, multiplication, division and exponentiation, etc.) and logical operations (and, or, etc.).

Data enter the ALU and return to main storage through the storage register. The accumulator serving as a register holds the results of processing operations. The results of arithmetic ope­rations are returned to the accumulator for transfer to main stor­age through the storage register. The comparer performs logical comparisons of the contents of the storage register and the ac­cumulator. At the core of the ALU is a very high-speed binary adder, which is used to carry out at least the four basic arithmetic functions.

3. The registers, which are high-speed units of memory used to store and control information. One of these registers is the program counter (PC) which keeps track of the next instruction to be performed in the main memory. Another is the instruction register (IR) which holds the instruction that is currently being executed.

The CPU coordinates all the activities of the various com­ponents of the computer. It determines which operations should be carried out and in what order. The CPU controls the opera­tion of the entire system by issuing commands to other parts of the system and by acting on responses. When required it reads information from the memory, interprets instructions, performs operations on the data according to the instructions, writes the results back into the memory and moves information between memory levels or through the input-output ports.

The programs and data which pass through the central processor must be loaded into the main memory (also called the internal memory) in order to be processed. Thus, when the user runs an application, the microprocessor looks for it on secondary storage devices (disks) and transfers a copy of the application into the RAM area. RAM (random access memory) is temporary, i.e. its information is lost when the computer is turned off. However, the ROM section (read only memory) is permanent and contains instructions needed by the processor. The BIOS (basic input/output system) uses ROM to control communication with peripherals, e.g. disk drives. The amount of RAM determines the number of programs you can run simultaneously and how fast they operate.

Most of today's computers have internal expansion slots that allow users to install adapters or expansion boards, which provide extra functions. Popular adapters include high-resolution graphics boards, memory expansion boards, and internal modems.

The RAM capacity can sometimes be expanded by adding extra chips. These are usually contained in small circuit boards called single in-line memory modules (SIMMs). Modern Pentium processors also accept dual in-line memory modules (DIMMs) which allow for a wider data path. One area where microprocessors differ is in the amount of data -

the number of bits - they can work with at a time. There are 16, 32 and 64-bit processors now and the computer's internal architecture is evolving very quickly.

The power and performance of a computer is partly determined by the speed of its microprocessor. The speed of a processor is measured in gigahertz(GHz). Thus, a CPU running at 4 GHz can make about four thousand million calculations a second. An internal clock sends out signals at fixed intervals to measure and synchronize the flow of data.

The main circuit board is known as the motherboard. This contains the CPU, the memory chips, expansion slots and controllers for peripherals, connected by internal buses, or paths, that carry electronic signals. For example, the front side bus carries all data that passes from the CPU to other devices.