Block diagram of computer

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Block Diagram of Computer and its Explanation

        Block Diagram of Computer


A computer can process data, pictures, sound and graphics. They can solve highly complicated problems quickly and accurately.


Computers need to receive data and instruction in order to solve any problem. Therefore we need to input the data and instructions into the computers. The input unit consists of one or more input devices. Keyboard is the one of the most commonly used input device. Other commonly used input devices are the mouse, floppy disk drive, magnetic tape, etc. All the input devices perform the following functions.

• Accept the data and instructions from the outside world. • Convert it to a form that the computer can understand. • Supply the converted data to the computer system for further processing.

Storage Unit:

The storage unit of the computer holds data and instructions that are entered through the input unit, before they are processed. It preserves the intermediate and final results before these are sent to the output devices. It also saves the data for the later use. The various storage devices of a computer system are divided into two categories.

1. Primary Storage: Stores and provides very fast. This memory is generally used to hold the program being currently executed in the computer, the data being received from the input unit, the intermediate and final results of the program. The primary memory is temporary in nature. The data is lost, when the computer is switched off. In order to store the data permanently, the data has to be transferred to the secondary memory. The cost of the primary storage is more compared to the secondary storage. Therefore most computers have limited primary storage capacity.

2. Secondary Storage: Secondary storage is used like an archive. It stores several programs, documents, data bases etc. The programs that you run on the computer are first transferred to the primary memory before it is actually run. Whenever the results are saved, again they get stored in the secondary memory. The secondary memory is slower and cheaper than the primary memory. Some of the commonly used secondary memory devices are Hard disk, CD, etc.,

Memory Size:

All digital computers use the binary system, i.e. 0’s and 1’s. Each character or a number is represented by an 8 bit code.

The set of 8 bits is called a byte. A character occupies 1 byte space. A numeric occupies 2 byte space. Byte is the space occupied in the memory.

The size of the primary storage is specified in KB (Kilobytes) or MB (Megabyte). One KB is equal to 1024 bytes and one MB is equal to 1000KB. The size of the primary storage in a typical PC usually starts at 16MB. PCs having 32 MB, 48MB, 128 MB, 256MB memory are quite common.


The output unit of a computer provides the information and results of a computation to outside world. Printers, Visual Display Unit (VDU) are the commonly used output devices. Other commonly used output devices are floppy disk drive, hard disk drive, and magnetic tape drive.


All calculations are performed in the Arithmetic Logic Unit (ALU) of the computer. It also does comparison and takes decision. The ALU can perform basic operations such as addition, subtraction, multiplication, division, etc and does logic operations viz, >, <, =, ‘etc. Whenever calculations are required, the control unit transfers the data from storage unit to ALU once the computations are done, the results are transferred to the storage unit by the control unit and then it is send to the output unit for displaying results.


It controls all other units in the computer. The control unit instructs the input unit, where to store the data after receiving it from the user. It controls the flow of data and instructions from the storage unit to ALU. It also controls the flow of results from the ALU to the storage unit. The control unit is generally referred as the central nervous system of the computer that control and synchronizes its working.


The control unit and ALU of the computer are together known as the Central Processing Unit (CPU).TheCPUis like brain performs the following functions:

•Itperformsallcalculations. •Ittakesalldecisions. •Itcontrolsallunitsofthecomputer.

A PC may have CPU-IC such as Intel 8088, 80286, 80386, 80486, Celeron, Pentium, Pentium Pro, Pentium II, Pentium III, Pentium IV, Dual Core, and AMD etc.

1. Control Unit: It is responsible for directing and coordinating most of the computer system activities. It does not execute instructions by itself. It tells other parts of the computer system what to do. It determines the movement of electronic signals between the main memory and arithmetic logic unit as well as the control signals between the CPU and input/output devices.

2. Arithmetic logic Unit: ALU performs all the arithmetic and logical functions i.e. addition, subtraction, multiplication, division and certain comparisons. These comparisons include greater than, less than, equals to etc. The ALU controls the speed of calculations.

3. Registers: It is a special temporary storage location within the CPU. Registers quickly, accept, store and transfer data and instructions that are being used immediately (main memory hold data that will be used shortly, secondary storage holds data that will be used later). To execute an instruction, the control unit of the CPU retrieves it from main memory and places it onto a register. The typical operations that take place in the processing of instruction are part of the instruction cycle or execution cycle. The instruction cycle refers to the retrieval of the instruction from main memory and its subsequence at decoding. The process of alerting the circuits in CPU to perform the specified operation. The time it takes to go through the instruction cycle is referred to as I-time.

4. Bus: The term Bus refers to an electrical pathway through which bits are transmitted between the various computer components. Depending on the design of the system, several types of buses may be present. The most important one is the data bus, which carries the data through out the central processing unit. The wider the data bus, the more data it can carry at one time and thus the greater the processing speed of the computer. Ex: Intel 8088 processor uses a data bus of 8 bits wide. Some super computers contain buses that are 128 bits wide. WHAT IS RAM AND ITS FUNCTION>>

The main memory of the computer is called as Random Access Memory (RAM). The name derives from the fact that data can be stored in and retrieved at random, from anywhere in the electronic main memory chips in approximately the same amount of time, no matter where the data is. Main memory is in an electronic or volatile state. When the computer is off, main memory is empty, when it is on it is capable of receiving and holding a copy of the software instructions, and data necessary for processing.

Because the main memory is a volatile form of storage that depends on electric power can go off during processing, users save their work frequently on to non volatile secondary storage devices such as diskettes or hard disk.

The main memory is used for the following purposes:

1. Storage of the copy of the main software program that controls the general operation of the computer. This copy is loaded on to the main memory when the computer is turned on, and it stays there as long as the computer is on.

2. Temporary storage of a copy of application program instruction, to be received by CPU for interpretation and processing or execution.

3. Temporary storage of data that has been input from the key board, until instructions call for the data to be transferred in to CPU for processing.

4. Temporary storage of data, which is required for further processing or transferred as output to output devices such as screen, a printer, a disk storage device.

Classification of computers based on memory size In terms of capacity, price performance, computers can be broadly classified as follows.

• Main Frame Computers. • Mini Computers. • Micro Computers.

Main Frame Computers:

They are most expensive of all the computers, are very big in size and offer maximum computing power. A large number of peripherals can be attached to them. They are generally used in large networks and the mainframe works as central computer or node. All other smaller terminals are connected to central computer or server through satellite link.

A typical example is the air line reservation system. The main frame computer is placed at the head office, where all the information about all flights is stored and small computers are placed at various branches and are attached to the central data bank so that up to date information of all flights is always available.

The structural configuration of mainframe consists of • Data communication equipment. • Interface equipment. • Primary storage. • Secondary storage. • Central processors with multiprogramming facilities.


• These are large general purpose computers capable of handling all kinds of problems whether scientific or commercial.

• Can accept and transfer data from I/O devices at the rate of million bytes per second. • Can accept all type of high level languages. • Can support large number of terminals say up to 100 or more? • They have flexibility to operate automatically.

• They have high on line secondary storage capacity and can support a number of peripheral devices like magnetic tape drives, hard disk drives, visual display units etc.

• They have high cache memory, to process applications faster than mini or micro computers. Mini Computers:

These are the smaller versions of the mainframe. They offer same computing power as their bigger counterparts but a small/number of peripherals can be attached to the mini computer. It is comparatively cheaper in cost, small in size, very rugged and reliable. It does not require air conditioning and can be operated at room temperature. In business environment a mini system can be ‘tailor made’ for job specification.


• They can accept and transfer data from I/O devices at the maximum speed of 4 million bytes per second. • Can support up to a maximum of 20 terminals. • They usually employ microprocessors in the CPU for data storage and data manipulation. • They have faster processing speed. • In most mini computers, data and instructions are stored in fixed word locations. They have main memory ranging from 256kb to 12 Mb and word length of 16/32 bits and are designed for 4 to 8 users in time sharing models. Mini Computers can be sub classified in to • Mini minis • Midi minis • Maxi minis(super minis)

Micro Computers (also termed as miniaturized mini computer): They can carry input, storage, arithmetic, logic, control and output functions. They use silicon made micro processor chips which are of a size of 5mm square and 0.1 mm thick, with layers of etched and printed circuits-containing all elements required to process binary encoded data.

These systems use random access memory and read only memory as primary storage. They range from single board size to a system containing many circuit boards mounted in a cabinet. They are light enough to be moved easily and are designed to be used by one person at a time.

A typical microcomputer has a keyboard for input and user diskettes and floppy disk drives to enter data and programs and to receive processed outputs. They use magnetic tape and floppy discs as secondary storage devices.

A visual display unit and/or character printer is used to prepare output in a human readable form. All micro computers are byte addressable machines. They use micro processor chips viz. Features of Micro Computers:

They use very little power, require little air conditioning and does not require special wiring, raised floors.

They are stable and reliable. Personal Computers:

They are the one of the latest type of micro computers. They are called so because they are designed for personal use of individuals or small business units, office automation units or professionals.

Personal computers can be used for basic programming, fun and games, business and professional applications, tele communications; data base management, accounting, and word processing. What is a hard disk? What are its features

Hard disk is one of the storage devices, developed to store huge amount of data. The introduction of high capacity hard disks for micro computer system solved two serious problems related to the limited storage capacity of diskettes.

First, as a business begins to use micro computers extensively, the amount of software acquired and data collected tends to grow substantially. As a result the number of diskettes required, increases, dramatically second, the largest file that can be accessed at one time is limited to the capacity of the main memory and the storage medium. A hard disk can store huge amount of data in a most convenient way.

In hard disk system, data is stored in the same way as it is on diskettes. A series of tracks are divided in to sectors when the disk is formatted. Hard disk is made out of a rigid substance that is capable of storing a greater amount of data than the soft material used for diskettes. The hard disk drives for the micro computer can be internal or external.

In a disk pack, the access mechanism can position itself to access data from each of the 200 cylinder is a set of all tracks with the same distance from the axis about which the disk pack rotates. In this example there are 10 tracks in each cylinder.

The capacity of diskettes in wide use today ranges from 360KB to 144MB each. Microsoft hard disk capacity ranges from 10MB to 1GB or higher. Hard disks larger capacity allows the user to store larger files and larger programs than can be used with diskettes. Access time with the hard disk is much lower than that of diskette i.e., data retrieval is much faster with the hard disk than a diskette.[[File:CM-rie.

Characteristics of a Hard disk:

1. They are rigid metal platters connected to a central spindle. 2. The entire disk unit is placed in a permanently sealed container. 3. Air that flow through the container is filtered to prevent contamination. 4. The disks are rotated at a very high speed (usually around 3600 RPM) 5. These disk drives can have four or more disk platters in a sealed unit.

6. In most of the disk units, the read/ write head does not touch the surface of the disk. Instead they are designed to float from 0.5 to 1.25 millionth of an inch from the disk surface. (Flying head design). Because the heads float so close to the sensitive disks, any contamination such as dust particle or hair, cause a head crash or a disk crash, which destroys some or all the data on the disk. Therefore hard disks are handled under sterile conditions.

7. Hard disk technology was introduced by IBM (1970) since then they have become the most necessary memory hungry software

What is an operating system? What are its functions?

The operating system is a program that acts as an interface between the user and the hardware. It is a collection of programs that coordinates the operations of computer hardware and software.

Functions of an operating system:

1. Starting the Computer: In most computers, the BIOS and the diagnostic routines are contained in ROM. They are permanently available to other programs to check equipment and perform input/output tasks. In addition, ROM contains a bootstrap loader program that is used to start a computer. The remainder of the operating system is contained in secondary memory. When the computer is turned on, it automatically begins to execute the boot program. This program first runs diagnostic programs to check the status of RAM and of each of the attached system devices. The boot program then reads in the operating system executive program from the disk. The layout of the memory at this stage is shown in figure.

2. Running application programs: To run an application programs such as word processing program, the user types the name of the program after getting the Dos prompt on the screen. Ex C:\>WS

The operating system accepts the command WS and reads the program into RAM. When the application program is in RAM, the operating system executive program gives control of the computer system to the applications programs. The computer then executes the application program. When the application program ends, control is restored to the operating system executive program, which monitors the keyboard for the users’ next command. When the application program is running the operating system manages the allocation of memory. When a user requests that a program be run, the operating system allocates the memory required for the program instructions and for data.

3. Running utility programs: An operating system includes utility programs that give the user control over various features of the computer system. The utility programs are either residient or transient. Residient programs are always present in RAM; transient program must be read into RAM, as they are requested.

Ex. CON: name of the keyboard, or console. PRN: name of the printer. Disk Copy A: B: (copies the contents of the disk in drive A, onto the disk on drive B) 4. Managing files: Information on the disk is organized into files. A file is collection of Bytes. Example.jpg