Computer Structure: Central Processing Unit

CPU

It is also known as a processor or microprocessor. The Central Processing Unit (CPU) was first developed by Intel in 1974. The computer CPU is responsible for handling all instructions and calculation it receives from other hardware components in the computer and software programs running on the computer. The CPU performs the valid processing of data, the data it processes is obtained via the system bus, from the main memory. Results from the CPU are then sent back to main memory via the system bus. The CPU controls and co-ordinates the operation of the other major components. The CPU has three main components which are

The Main Memory (Primary Memory)
The Control Unit
The Arithmetic/Logical Unit (ALU)

The Main Memory (Primary Memory)

The memory of a computer can hold program instructions, data values, and the intermediate results of calculations. All the information in memory is encoded in fixed size cells called bytes. A byte can hold a small amount of information, such as a single character or a numeric value between 0 and 255.
Main Memory is the working memory of the CPU, with fast access and limited numbers of bytes being transferred. The main memory of the computer is also known as RAM, which stands for Random Access Memory (Main Memory). It is constructed from integrated circuits and needs to have electrical power in order to maintain its information. When power is lost, the information is lost as well. It can be directly accessed by the CPU.

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Main memory is expensive compared to external memory so it has limited capacity. The CPU will normally transfer data to and from the main memory in groups of two, four or eight bytes, even if the operation it is undertaking only requires a single byte.

External Memory (Secondary Memory)

External memory is for the long term storage of information. Data from external memory will be transferred to the main memory before the CPU can operate on it. Access to the external memory is much slower, and usually involves groups of several hundred bytes. External memory which is sometimes called backing store or secondary memory, allows the permanent storage of large quantities of data. The capacity of external memory is high, usually measured in hundreds of megabytes or gigabytes even in tetrabyte at present. External memory has the important property that the information stored is not lost when the computer is switched off. The most common form of external memory is a hard disc which is permanently installed in the computer and will typically have a capacity of hundreds of megabytes. 

The Control Unit

This controls the fetching of instructions from the main memory and the subsequent execution of these instructions. Among other tasks carried out are the control of input and output devices and the passing of data to the Arithmetic/Logical Unit for computation.

Arithmetic-Logic Unit (ALU)

An arithmetic-logic unit (ALU) is part of a computer processor (CPU) that carries out arithmetic and logic operations on the operands in computer instruction words. In some processors, the ALU is divided into two units, an arithmetic unit (AU) and a logic unit (LU).

Arithmetic Logic Unit

Typically, the ALU has direct input and output access to the processor controller, main memory (random access memory or RAM in a personal computer), and input/output devices. Inputs and outputs flow along an electronic path that is called a bus. Generally the ALU includes storage places for input operands, operands that are being added, the accumulated result (stored in an accumulator), and shifted results. The flow of bits and the operations performed on them in the subunits of the ALU is controlled by gated circuits. The gates in these circuits are controlled by a sequence logic unit that uses a particular algorithm or sequence for each operation code. In the arithmetic unit, multiplication and division are done by a series of adding or subtracting and shifting operations. There are several ways to represent negative numbers. In the logic unit, one of 16 possible logic operations can be performed – such as comparing two operands and identifying where bits don't match.

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