1.1. Definition and Characteristics of Computer
A computer is an electronic device that has the ability to store, retrieve, and process data, and can
be programmed with instructions that it remembers. The physical parts that make up a computer
(the central processing unit, input, output, and memory) are called hardware. Programs that tell a
computer what to do are called software.
A set of instructions that perform a particular task is called a program, software program, or
software. Peripherals are any hardware device connected to a computer, any part of the computer
outside the CPU and working memory. Some examples of peripherals are keyboards, the mouse,
monitors, printers, scanners, disk and tape drives, microphones, speakers, joysticks, plotters, and
cameras. Computer is an advanced electronic device that takes raw data as input from the user and
processes these data under the control of set of instructions (called program) and gives the result
(output) and saves output for the future use. It can process both numerical and non-numerical
(arithmetic and logical) calculations.
Definition of a Computer
A computer is an electronic device that operates under the control of a set of instructions
that is stored in its memory unit.
A computer can be more accurately defined as an electronic device that takes data as input,
stores and processes it and displays the output according to the given instructions.
Characteristics of Computer
1. Speed
The computer can process data very fast, at the rate of millions of instructions per second.
Some calculations that would have taken hours and days to complete otherwise, can be
completed in a few seconds using the computer. For example, calculation and generation of
salary slips of thousands of employees of an organization, weather forecasting that requires
analysis of a large amount of data related to temperature, pressure and humidity of various
places, etc.
2. Accuracy
Computer provides a high degree of accuracy. For example, the computer can accurately give
the result of division of any two numbers up to 10 decimal places.
3. Diligence
When used for a longer period of time, the computer does not get tired or fatigued. It can
perform long and complex calculations with the same speed and accuracy from the start till the
end.
4. Storage Capability
Large volumes of data and information can be stored in the computer and also retrieved
whenever required. A limited amount of data can be stored, temporarily, in the primary
memory. Secondary storage devices like floppy disk and compact disk can store a large amount
of data permanently.
5. Versatility
Computer is versatile in nature. It can perform different types of tasks with the same ease. At
one moment user can use the computer to prepare a letter document and in the next moment
user may play music or print a document.
1.2.Block diagram of Computer
1.2.1. Input Unit
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 devices. Other commonly used
input devices are the Mouse, Scanner, Microphone 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.
1.2.2. 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.
1.2.2.1.Primary Storage (also known as Main memory or Internal memory)
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. Examples: RAM, ROM, Cache
memory etc
1.2.2.2.Secondary Storage
Secondary storage is used like an archive. It stores several programs, documents, data bases etc.
The programs that user 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.,
1.2.2.3.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.
1.2.3. Output Unit
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 Speaker, Headphone, Projector etc.
1.2.4. Central Processing Unit
The Control Unit (CU) and Arithmetic Logic Unit (ALU) of the computer are together known
as the Central Processing Unit (CPU). The CPU is like brain performs the following functions:
• It performs all calculations.
• It takes all decisions.
•It controls all units of the computer.
1.2.4.1.Arithmetic Logical Unit
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 sent to the output unit for displaying results.
1.2.4.2.Control Unit
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 it’s working.
Difference Between Primary Memory and Secondary Memory
Primary memory Secondary memory
Primary memory is temporary. Secondary memory is permanent.
Primary memory is directly accessible by
Processor/CPU.
Secondary memory is not directly accessible
by the CPU.
Nature of Parts of Primary memory varies,
RAM- volatile in nature. ROM- Non-volatile.
It’s always Non-volatile in nature.
Primary memory devices are more expensive
than secondary storage devices.
Secondary memory devices are less
expensive when compared to primary
memory devices.
The memory devices used for primary
memory are semiconductor memories.
The secondary memory devices are
magnetic and optical memories.
Primary memory is also known as Main
memory or Internal memory.
Secondary memory is also known as
External memory or Auxiliary memory.
Examples: RAM, ROM, Cache memory,
PROM, EPROM, Registers, etc.
Examples: Hard Disk, Floppy Disk,
Magnetic Tapes, etc.
1.3. Compiler
Compiler is the software that translates the program written in a high-level language to machine
language. The program written in high-level language is referred to as the source code (Source
program) and compiled program is referred as the object code (Target program).
1. Source program
It is normally a program written in a high-level programming language. It contains a set of
rules, symbols, and special words used to construct a computer program.
2. Target program
It is normally the equivalent program in machine code. It contains the binary representation of
the instructions that the hardware of computer can perform.
3. Error Message
A message issued by the compiler due to detection of syntax errors in the source program.
Each programming language has its own compiler. Some languages that use a compiler are C++,
COBOL, Pascal, and FORTRAN. In some languages, compilation using the compiler and linking
using the linker are required for creating the executable object code. The compilation process
generally involves two parts:
1. breaking down the source code into small pieces and creating an intermediate
representation.
2. constructing the object code for the intermediate representation.
The compiler also reports syntax errors, if any, in the source code.
1.4. Interpreter
The interpreter is used to convert the high-level language program into computer-understandable
form. However, the interpreter functions in a different way than a compiler. Interpreter performs
line-by-line execution of the source code during program execution. Interpreter reads the source
code line-by-line, converts it into machine understandable form, executes the line, and then
proceeds to the next line. Some languages that use an interpreter are BASIC and Python.
Difference between Compiler and Interpreter
Compiler Interpreter
1 Compiler works on the complete program at
once. It takes the entire program as input.
Interpreter program works line‐by‐line. It
takes one statement at a time as input
2 Compiler generates intermediate code,
called the object code or machine code.
Interpreter does not generate intermediate
object code or machine code.
3 Compiler executes conditional control
statements (like if‐else and switch‐case) and
logical constructs faster than interpreter.
Interpreter execute conditional control
statements at a much slower speed.
4 Compiled programs take more memory
because the entire object code has to reside
in memory.
Interpreter does not generate intermediate
object code. As a result, interpreted
programs are more memory efficient.
5 Compile once and run anytime. Compiled
program does not need to be compiled every
time.
Interpreted programs are interpreted line‐
by‐ line every time they are run.
6 Errors are reported after the entire program
is checked for syntactical and other errors.
Error is reported as soon as the first error is
encountered. Rest of the program will not
be checked until the existing error is
removed.
7 A compiled language is more difficult to
debug.
Debugging is easy because interpreter stops
and reports errors as it encounters them.
8 Compiler does not allow a program to run
until it is completely error‐free.
Interpreter runs the program from first line
and stops execution only if it encounters an
error.
9 Interpreter runs the program from first line
and stops execution only if it encounters an
error.
Interpreted languages are less efficient but
easier to debug. This makes such languages
an ideal choice for new students.
10 Examples of programming languages that
use compilers: C, C++, COBOL
Examples of programming languages that
use interpreters: BASIC, Visual Basic,
Python, Ruby, PHP, Perl, MATLAB, Lisp
1.5. Linker
A program that takes as input the object files of one or more separately compiled program modules,
and links them together into a complete executable program, resolving reference from one module
to another.
Linkers are of two types:
1. Linkage Editor
2. Dynamic Linker
The major task of a linker is to search and locate referenced module/routines in a program and to
determine the memory location where these codes will be loaded, making the program instruction
to have absolute references.
1.6. Loader
A program that takes an input an executable program, loads it into main memory, and causes
execution to being by loading the correct starting address into the computer register. When the
program finished, control must somehow be returned to the operating system. Following are the
functions:
1. Allocation: Allocates space in memory for the program.
2. Linking: Symbol Resolution between object modules.
3. Relocation: Adjust addresses dependent locations of address constants i.e., assign load
addresses to different parts of a program.
4. Loading: Store machine instructions and data into memory.
Following are the types of loaders:
1. Absolute Loaders
2. Relocating Loaders
3. Direct Linking Loaders
4. Bootstrap Loaders
Differences between Linker and Loader are as follows:
Linker Loader
The main function of Linker is to generate
executable files.
Whereas main objective of Loader is to load
executable files to main memory.
The linker takes input of object code
generated by compiler/assembler.
And the loader takes input of executable files
generated by linker.
Linking can be defined as process of
combining various pieces of codes and
source code to obtain executable code.
Loading can be defined as process of loading
executable codes to main memory for further
execution.
Linkers are of 2 types: Linkage Editor and
Dynamic Linker.
Loaders are of 4 types: Absolute, Relocating,
Direct Linking, Bootstrap.
