Mahdalina

EXPLAIN FUNCTIONAL UNIT OF A COMPUTER.

             A computer consists of five functionally independent main parts: input, memory, arithmetic and logic, output, and control units, as shown in Figure 1.1. The input unit accepts coded information from human operators, from electromechanical devices such as key boards, or from other computers over digital communication lines. The information received is either stored in the computer's memory for later reference or immediately used by the arithmetic and logic circuitry to perform the desired operations. The processing steps are determined by a program stored in the memory. Finally, the results are sent back to the outside world through the output unit. All of these actions are coordinated by the control unit. Figure 1.1 does not show the connections among the functional units. The arithmetic and logic circuits, in conjunction with the main control circuits, as the processor, input and output equipment is often collectively referred to as the input-output (I/O) unit.

Input Unit

             Computers accept coded information through input units, which read data. The well known input device is the keyboard. Whenever a key is pressed, the corresponding letter or digit is automatically translated into its corresponding binary code and transmitted over a cable to either the memory or the processor. Many other kinds of input devices are available, including joysticks, trackballs, and mouse, which can be used as pointing device. 

Memory Unit

             The function of the memory unit is to store programs and data. There are two classes of storage, called primary and secondary. Primary storage is a fast memory that operates at electronic speeds. Programs must be stored in the memory while they are being executed. The memory contains a large number of semi conductor storage cells, each capable of storing one bit of information. These cells are rarely read or written as individual cells but instead are processed in groups of fixed size called words. The memory is organized so that the contents of one word, containing n bits, can be stored or retrieved in one basic operation.

             To provide easy access to any word in the memory, a distinct address is associated with each word location. Addresses are numbers that identify successive locations. A given word is accessed by specifying its address and issuing a control command that starts the storage or retrieval process. The number of bits in each word is often referred to as the word length of the computer .Typical word lengths range from 16 to 64 bits. The capacity of the memory is one factor that characterizes the size of a computer.

             Programs must reside in the memory during execution. Instructions and data can be written into the memory or read out under the controller of the processor. It is essential to be able to access any word location in the memory as quickly as possible. Memory in which any location can be reached in a short and fixed amount of time after specifying its address is called random-access Memory (RAM). The time required to access one word is called the Memory access time. This time is fixed, independent of the location of the word being accessed. It typically ranges from a few nanoseconds (ns) to about 100 ns for modem RAM units. The memory of a computer is normally implemented as a Memory hierarchy of three or four levels of semiconductor RAM units with different speeds and sizes. The small, fast, RAM units are called caches. They are tightly coupled with the processor and are often contained on the same integrated circuit chip to achieve high performance. The largest and slowest unit is referred to as the main Memory.

             Although primary storage is essential, it tends to be expensive. Thus additional, cheaper, secondary storage is used when large amounts of data and many programs have to be stored, particularly for information that is access infrequently. A wide selection of secondary storage devices is available, including magnetic disks and tapes and optical disks

Arithmetic  and Logic Unit.

             Most computer operations are executed in the arithmetic and logic unit (ALU) of the processor. Consider a typical example: Suppose two numbers located in the memory are to be added. They are brought into the processor, and the actual addition is carried out by the ALU. The sum may then be stored in the memory or retained in the processor for immediate use.

             Any other arithmetic or logic operation, for example, multiplication, division, or comparison of numbers, is initiated by bringing the required operands into the processor, where the operation is performed by the ALU. When operands are brought into the processor, they are stored in high-speed storage elements called registers. Each register can store one word of data. Access times to registers are somewhat faster than access times to the fastest cache unit in the memory hierarchy.

             The control and the arithmetic and logic units are many times faster than other devices connected to a computer system. This enables a single processor to control a number of external devices such as keyboards, displays, magnetic and optical disks, sensors, and mechanical controllers.

Output Unit

             The output unit is the counterpart of the input unit. Its function is to send processed results to the outside world. The most familiar example of such a device is a printer. Printers employ mechanical impact heads, inkjet streams, or photocopying techniques, as in laser printers, to perform the printing. It is possible to produce printers capable of printing as many as 10,000 lines per minute. This is a tremendous speed for a mechanical device but is still very slow compared to the electronic speed of a processor unit.

             Monitors, Speakers, Headphones and projectors are also some of the output devices. Some units, such as graphic displays, provide both an output function and an input function. The dual role of input and output of such units are referred with single name as I/O unit in many cases. Speakers, Headphones and projectors are some of the output devices. Storage devices such as hard disk, floppy disk, flash drives are also used for input as well as output.

Control Unit

             The memory, arithmetic and logic, and input and output 'units store and process information and perform input and output operations. The operation of these units must be coordinated in some way. This is the task of the control unit. The control unit is effectively the nerve center that sends control signals to other units and senses their states.

             I/O transfers, consisting of input and output operations, controlled by the instructions of I/O programs that identify the devices involved and the information to be transferred. However, the actual timing signals that govern the transfers are generated by the control circuits. Timing signals are signals that determine when a given action is to take place. Data transfers between the processor and the memory are also controlled by the control unit through timing signals. It is reasonable to think of a control unit as a well-defined, physically separate unit that interacts with other parts of the machine. In practice, however, this is seldom the case. Much of the control circuitry is physically distributed throughout the machine. A large set of control lines (wires) carries the signals used for timing and synchronization of events in all units.

The operation of a computer can be summarized as follows:

1.   The computer accepts information in the form of programs and data through an input unit and stores it in the memory.

2.   Information stored in the memory is fetched, under program control, into an arithmetic and logic unit, where it is processed.

3.   Processed information leaves the computer through an output unit.

4.   All activities inside the machine are directed by the control unit.