Characteristics of computers

Computers can execute millions of instructions in a second. The results produced after processing the data are very accurate, but computers do not have adequate knowledge or intelligence to interpret the results. They only carry out instructions like an obedient servant. The computer gives correct results only if the data and instructions given are correct. The term Garbage In Garbage Out (GIGO) is used to mean this feature. That is, if a wrong input is given to the computer, it will give a wrong output. Look at Table 1.2 and identify the advantages and limitations of computer


Speed: A computer can perform millions of operations in a second or in fraction of second. It can do in a minute, as much work as a man do taking months and years.

Accuracy: A computer can perform arithmetic operations with a very high degree of accuracy. By accuracy, we mean fewer errors in the output and precision with which computations are performed. 

Diligence: Since computer is a machine, it can operate for long hours untiringly. Unlike human beings, it will not show any emotion or disobey you. Hence computers are best suited for routine jobs. 

Versatility: Computer can be used to perform many different kinds of processing tasks. It is a general purpose data processing machine. 

Huge memory: Computer has enormous memory capacity. Huge volume of data can be stored in its memory for processing. The storage capacity can also be increased as per requirement.


Lack of IQ: Many people think that computer has super human capabilities. However this is not true. A computer does not have natural intelligence as humans have. 

Lack of decision making power: Computer cannot decide on its own and it does not possess intuitive capabilities like human beings

Number system

A number system has a unique base, which depends upon the number of symbols. The number of symbols used in a number system is called base or radix of a number system.

Decimal number system

The decimal number system involves ten symbols 0, 1, 2, 3, 4, 5, 6, 7, 8 and 9 to form a number. Since there are 10 symbols in this number system, its base is 10. Therefore,the decimal number system is also known as base-10 number system. The weight of a digit depends on its relative position. Such a number system is known as positional number system

2 Binary number system 

A number system which uses only two symbols 0 and 1 to form a number is called binary number system. Bi means two. Base of this number system is 2. So it is also called base-2 number system. We use the subscript 2 to indicate that the number is in binary. Each digit of a binary number is called bit. A bit stands for binary digit. The binary number system is also a positional number system where place value of each binary digit is power of 2

Importance of binary numbers in computers 

Binary number system is based on two digits 1 and 0. The electric state ON can be represented by 1 and the OFF state by 0 as shown in Figure 1.8 Because of this, computer uses binary number system as the basic number system for data representation.

.3 Octal number system 

A number system which uses eight symbols 0, 1, 2, 3, 4, 5, 6 and 7 to form a number is called octal number system. Octa means eight, hence this number system is called octal. Base of this number system is 8 and hence it is also called base-8 number system.

4 Hexadecimal number system 

A number system which uses 16 symbols 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E and F to form a number is called hexadecimal number system. Base of this number system is 16 as there are sixteen symbols in this number system. Hence this number system is also called base-16 number system. 

Importance of octal and hexadecimal number systems

Digital hardware uses the binary number system for its operations and data. Representing numbers and operations in binary form requires too many bits and needs lot of effort. With octal, the bits are grouped in threes (because 23 = 8), and with hexadecimal, the binary bits are grouped in fours (because 24 = 16) and these groups are replaced with the respective octal or hexadecimal symbol. This conversion processes of binary numbers to octal and hexadecimal number systems and vice versa are very easy. This short-hand notation is widely used in the design and operations of electronic circuits.


Instructions and results temporarily or permanently. Memory can be classified into two: primary memory and secondary memory. Primary memory holds data, intermediate results and results of ongoing jobs temporarily. Secondary memory on the other hand holds data and information permanently.

Binary Digit = 1 Bit

1 Nibble = 4 Bits 

1 Byte = 8 Bits 

1 MB (Mega Byte) = 1024 KB 

1 GB (Giga Byte) = 1024 MB 

1 TB (Tera Byte) = 1024 GB 

1 KB (Kilo Byte) = 1024 Bytes 

1 PB (Peta Byte) = 1024 TB

Primary storage 

Primary memory is a semiconductor memory that is accessed directly by the CPU. It is capable of sending and receiving data at high speed. This includes mainly three types of memory such as RAM, ROM and Cache memory.

Random Access Memory (RAM) 

RAM refers to the main memory that microprocessor can read from and write to. Data can be stored and retrieved at random from anywhere within the RAM, no matter where the data is. Data or instructions to be processed by the CPU must be placed in the RAM The contents of RAM are lost when power is switched off. Therefore, RAM is a volatile memory

The speed of a RAM refers to how fast the data in memory is accessed. It is measured in Mega Hertz (MHz). When a computer is in use, its RAM contains the following:

 1. The operating system software. 

2. The application software currently being used.

 3. Any data that is being processed

 Read Only Memory (ROM)

 ROM is a permanent memory that can perform only read operations and its contents cannot be easily altered. ROM is non-volatile; the contents are retained even after the power is switched off. ROM is used in most computers to hold a small, special piece of 'boot up' program known as Basic Input Output System (BIOS).This software runs when the computer is switched on or 'boots up'. It checks the computer's hardware and then loads the operating system.

There are some modified types of ROM that include: 

1. PROM - Programmable ROM which can be programmed only once. PROMs are programmed at the time of manufacture. 

2. EPROM - Erasable Programmable ROM that can be erased using ultra violet radiation and can be programmed using special electronic circuits. 

3. EEPROM - Electrically Erasable Programmable ROM which can be erased and rewritten electrically.

iii. Cache memory 

Cache memory is a small and fast memory between the processor and RAM (main memory). Frequently accessed data, instructions, intermediate results, etc. are stored in cache memory for quick access. When the processor needs to read from or write to a location in RAM, it first checks whether a copy of that data is in the cache. If so, the processor immediately reads the cache, which is much faster than reading from RAM. Cache is more expensive than RAM, but it is worth using it in order to maximise system performance. Commonly used cache memories and their capacities are Level 1 Cache (128 KB), Level 2 Cache (1 MB), Level 3 Cache (8 MB) and Level 4 Cache (128 MB)

Secondary or Auxiliary memory 

Secondary memory is of permanent nature. Unlike the contents of RAM, the data stored in these devices does not vanish when power is turned off. Secondary memory is much larger in size than RAM, but is slower. It stores programs and data but the processor cannot access them directly. Secondary memory is also used for transferring data or programs from one computer to another. It also acts as a backup. The major categories of storage devices are magnetic, optical and semiconductor memory. 

i. Magnetic storage devices 

Magnetic storage devices use plastic tape or metal/plastic disks coated with magnetic materials. Data is recorded magnetically in these devices.

Magnetic Tape

Magnetic tape is a backup device that can store huge volume of data. The cost per unit volume of a magnetic tape is less compared to other magnetic storage devices.

 Hard disk 

The hard disk consists of metal disks coated with magnetic material concealed in dust free containers.

ii. Optical storage devices 

Optical disk is a data storage medium which uses low-powered laser beam to read from and write data into it. It consists of an aluminum foil sandwiched between two circular plastic disks.

Compact Disk (CD)

Digital Versatile Disk (DVD)

Blu-ray DVD

USB flash drive 

Flash memory cards

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