Block Diagram of Personal Computer System

Block Diagram of various components in a Personal Computer System:

To function properly the computer needs both hardware and software. The hardware (H/W) consists of the mechanical and electronic devices, which we can see and touch. The software (S/W) consists of programs, the operating systems and the data that resides in the memory and the storage devices.

The input unit accepts data and instructions from outside through various input devices such as Key-boards, Mouses etc.
The processor (CPU) performs the arithmetical and logical operations and also controls the entire operation of the computer system.
The output (O/P) unit communicates the results of processing to the outside world through various output devices such as monitor, printer etc.
The storage unit stores the information in storage devices like hard-disk, floppy disk etc.

A block diagram of a computer which shows the interaction of various components is as shown in figure.
Block Diagram of Personal Computer

System Unit :

It includes the CPU, storage units such as RAM (Random Access Memory), ROM (Read Only Memory), hard disk drive, CD Drive, floppy disk drive etc.

CPU :

The C.P.U of a computer performs all the arithmetic operations, takes logical decisions and coordinates activities of all the other units of a computer. The C.P.U is thus rightly called the heart and nerve centre of a computer. To perform these actions C.P.U has the following sub units.

1) Arithmetic and Logic Unit (ALU)
2) Memory unit
3) Control unit

ALU:

The arithmetic and logic unit (ALU) performs all the arithmetical and logical operations. Thus when two numbers are to be added, these are sent from memory to ALU, where the addition takes place and the result is put back into the memory. The same way other arithmetical operations like subtraction, multiplication, division etc. are performed.



For logical operations also, the numbers to be compared (whether they are equal, whether one is less than the other, whether one is greater than the other etc.) are sent from memory to the ALU, where the comparison takes place and the result is returned to the memory. The result of a logical operation is either True or False. Thus if we compare the numbers 5 and 7 as 5 > 7 ?, the result is false. If we check the relation 5 < 7, the result is True. Such logical operations provide the capability of decision making to the computer.

Control Unit:

The control unit is the most important part of the CPU as it controls and co-ordinates the activities of all other units, such as ALU, memory unit, input and output units. This unit interprets instructions and transfers data from main memory to the ALU for processing.

Under the direction of a program, the control unit performs four basic operations.

1. Fetch : Getting the next program instruction from the computer's memory.
2. Decode : Figuring out what the program is telling the computer to do.
3. Execute : Performing the requested action, such as adding two numbers, deciding which one of them is larger etc.
4. Write back : Writing the results to an internal register (a temporary storage location) or to memory.

The four steps form a 'machine cycle' or a processing cycle and consist of two phases. - The Instruction cycle (fetch and decode) and the Execution cycle (execute and write back). Today's microprocessors can go through these four steps millions of times per second.

Memory Unit:

The memory unit stores all the instructions and data which are needed at the time of processing. It is also known as primary storage or main storage or intermediate access memory.

The main memory or primary memory of a computer is made of semi conductors. It is used to store data and instructions needed immediately by the CPU, to store intermediate results of processing. The memory can be thought of as 'Cells'. Each of these cells is further broken down into smaller parts known as 'bits'. A bit is a binary digit which is either 0 or 1.

Measure of Memory:

A group of 8 bits is called a Byte. One byte can store one character. The smallest addressable unit of memory is a byte. Bigger units are kilo Byte (KB), Mega Byte (MB), Giga Byte (GB) etc.
1KB = 2'° = 1024 bytes
1MB = 210 KB = 1024 x 1024 Bytes
1GB = 210 MB = 1024 x 1024 x 1024 Bytes.
The memory is usually organized into words of fixed lengths. Each word has the same number of bits, called the word length. Small machines may have word size of 1 or 2 bytes. Large machines have word sizes 4 or more bytes.

The memory addresses are consecutive numbers starting with address 0, 1, 2,   Thus at address 0, we find the first word, at address 1, the second word and so on.

A memory with 4096 locations with each location having 16 bits is called a 4096 word 16 - bit memory or 4K 16 bit memory (4K = 4 x 2'° = 4 x 1024 = 4096).

Random Access Memory (RAM):

The read and write memory of a computer is called RAM. Users can either read from it or write information into it. It can be accessed at random. However it is a volatile memory, since its contents will be erased when power goes off.

RAM is further classified into Static and Dynamic RAM.

Static RAMs store the information as long as power is on. Dynamic RAMs lose the information in a very short time (within milli sec.) even if power supply is on. So they need refreshing circuitry to periodically refresh its contents.



Static and dynamic RAMs use MOS (Metal Oxide Semi conductor) technology. Now-a-days CMOS (Complementary MOS) technology is used.

Read Only Memory (ROM):

We can only read from this memory, but we can not write new data into it. The contents of the ROM is decided by the manufacturer at the time of manufacturing. They are generally used to store fixed programs like monitor, assembly, debugging 111. programs etc. They are non volatile. That is they retain the information even if the power supply goes. off.

There are different types of ROMs.

1. Programmable Read Only Memory (PROM): They can be programmed (data recorded into it )by the user, but only once it can be written.

2. Erasable Programmable Read Only Memory (EPROM): In this case, the data recorded can be erased and new data can be written into it. The stored data is erased by exposing it to high intensity ultraviolet light for about 20 minutes.

3. Electrically Erasable PROM (EEPROM): The contents of such a chip can be erased and reprogrammed on the board itself on a byte by byte basis. Intel 2815A is a 16K (2K x 8) EEPROM. It is also used as a back up to RAM whose contents may be lost in a power failure. When power returns EEPROM memory can be used to replace the lost contents of the RAM.

Secondary Storage Devices:

Primary storage is expensive because each bit is represented )y a high speed device, such as a semi conductor which is costly. often it is necessary to store large volumes of data (Millions, billions or trillions of bytes). Therefore slower and less expensive storage units are used for computer systems. These are called secondary storage devices. Magnetic tapes, floppy disks, hard disks, optical disks etc. are used as secondary storage devices. Although both disks and tapes are widely used, the trend is towards disk storage for active files and magnetic tape for back up and archival storage.

Data are stored in them in the same binary codes as in main storage and are made available to main storage when needed, at the time of processing.

Data stored in the primary memory is volatile and its contents will be lost when power goes off. Secondary storage devices are cheaper and also the data stored in it is not volatile.

Characteristics of Secondary storage devices:

Non Volatile: The data recorded in the secondary storage devices like magnetic disk, magnetic tape, compact discs (CD-ROM) etc. are permanent and not volatile. That is, once recorded, it is not lost due to a power failure as in the case with random access memory (RAM) used in the main memory of the computer.

Massive capacity: Very large amounts of data can be stored in these devices. Thus for example a CD ROM can store 650 Mega Bytes (MB) or more of data in a single disc. Magnetic tapes, hard disks etc. can also store very huge volumes of data. Organizations using large data bases need secondary storage. They can also be used to store historical data or archival data.

Cost effective: The cost of secondary storage devices like floppy disk, hard disk, CD ROM etc. is reducing as a result of mass production techniques employed in their manufacture. They are very cheap storage devices.

Modular Expandability: There is no restriction in the amount of data being recorded as secondary storage is easily expandable. Data scan be categorized and stored on different tapes, CD ROMs, floppy disks etc. Whenever necessary the data contained in the secondary storage (disk or tape) can be loaded into the main memory by using appropriate discs or tapes.

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