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Compiled by :
S. Agarwal, Lecturer & Systems Incharge
St. Xavier’s Computer Centre,
St. Xavier’s College
Kolkata.
March-2003
WHAT IS PROGRAM
A program is a sequence of instructions that a
computer can interpret and execute. It is developed
using high level computer languages.
WHAT IS SOFTWARE
Software is computer program or a set of programs,
which provides the instructions which enable the
computer hardware to work. Two main types of
software are system software (operating systems),
which control the workings of the computer, and
applications, such as word processing programs,
spreadsheets, and databases.
Types of Languages Used to
Write a Software
Machine Language
 Assembly Language
 High Level Language

Machine Language
• A set of machine instructions written using binary code that a processor
can understand.
• A processor interprets and translates machine instructions into HW
signals.
Example:
the instruction to add 2 numbers stored in registers A and B might look
like this:
00000011 11000011
Problems:
 Binary representation of instructions and data are not easily generated,
manipulated, or understood by humans.
 The languages are machine dependent.
 Writing program in machine language requires the knowledge of the
internals of the computer.
Assembly Language
• A symbolic form of machine language
• Allows alphabetic mnemonics for operation codes and
storage locations
• There exists one-to-one correspondence between
assembly instructions and machine instructions.
Example:
M. L. Instruction:
Equivalent A.L. Instruction:
00000011 11000011
ADD A, B
Advantages ….
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Ease: Because of the use of mnemonics it is
more understandable and easy to remember
than a machine language code..
Speed: programs in AL run faster than those
written in high level languages.
Compactness: routines include only code that
programmers want to include.
Versatility: anything that can be done with a
computer can be done with AL
Problems ….
Big programs in assembly language : harder
to write and error prone.
 The languages are machine dependent.
 The programmer must still be aware of the
internal architecture of the target machine.

High Level Language
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Use English like codes that are easier to remember.
Generally machine (architecture) independent.
Provides language constructs for specifying and
manipulating complex data structures.
Supports a variety of programming styles (i.e. linear,
object oriented)
Hides the low level architectural details from the
programmer i. e. dose not normally require the
knowledge of the internal architecture of the computers.
Example : C, C++, Pascal, COBOL, Java
Binary codes
Some binary codes used
to represent information
in a digital computer:
1) BCD Code
2) ASCII Code
3) Gray Code
4) EBCDIC Code
5) Excess-3 Code
BCD CODE

BCD stands for Binary-Coded Decimal.

A BCD number is a four-bit binary group
that represents one of the ten decimal digits 0
through 9.
Example:
Decimal number 4926
4
9
2
6
8421 BCD coded number 0100 1001 0010 0110
THE
ASCII
CODE

ASCII is acronym for American Standard Code for
Information Interchange

Represents numbers, letters, punctuation marks
and control characters

Standard ASCII is a 7-bit code (127 characters)

Extended ASCII (IBM ASCII), an 8-bit code, is
also very popular

Extended ASCII adds graphics and math symbols
to code (total of 256 symbols)
ASCII : Example …..
BIT PATTERN
01000001
01100001
00110001
00101011
00100000
00101101
00011100
00101100
ASCII CODE
65
97
49
43
32
45
28
44
CHARACTER
A
a
1
+
(
,
0x000000
00100111101111011111111111100000
0x000004
10101111101111110000000000010100
0x000008
10101111101001000000000000100000
0x00000c
10101111101001010000000000100100
0x000010
10101111101000000000000000011000
0x000014
10101111101000000000000000011100
 Used to translate other
0x000018
10001111101011100000000000011100
forms
of languages00000001110011100000000000011001
into
0x00001c
0x000020
00000000000000000111100000010010
machine
language.
0x000024
10001111101110000000000000011000
0x000028
00000011000011111100100000100001
0x00002c
10101111101010000000000000011100
0x000030
00100101110010000000000000000001
 Three
types of commonly
0x000034
10101111101110010000000000011000
00101001000000010000000001100101
used0x000038
translators
are
:
0x00003c
00010100001000001111111111110111
1) Assembler 00111100000001000001000000000000
0x000040
0x000044
00110100100001000000010000110000
2)
Compiler
0x000048
10001111101001010000000000011000
3) Interpreter00001100000100000000000011101100
0x00004c
0x000050
00000000000000000001000000100001
0x000054
10001111101111110000000000010100
0x000058
00100111101111010000000000100000
0x00005c
00000011111000000000000000001000
Translators
Assembler
• A program that converts assembly code into machine
code
• After translation creates object module i.e. the machine
language representation of a program
How does an assembler work ?
Compiler
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Translates high-level code
into machine language code.
Produces object code
(Translated Instructions
ready for computer ) by
translating the source code (
high-level language
instruction ).
Object code is stored in the
machine and can be used
repeatedly.
SOURCE CODE
PROGRAM
TRANSLATION
PROCESS
COMPILER
OBJECT CODE
LINKAGE
EDITOR
LOAD MODULE
OTHER OBJECT
CODE MODULES
Digital Computer as a Multilevel Machine
Applications Level
Translation (Compiler)
Assembly Language Level
Machine code
Operating System Level
Machine code
Translation (Assembler)
Partial Interpretation,
Partial Pass-through
Interpretation (microprogram)
Microprogram Level
Directly executed by hardware
Hardware
Machine Language Level
Digital Logic Level
Software
High Level Language Level
Interpreter
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Like compiler translates a high level language into machine language.
Unlike compiler translates the program at the time of executing the
program instruction by instruction.
The translated code is not stored permanently in computer’s memory,
hence before each execution interpretation becomes necessary.
As object code is not stored in the computer, execution of the program
becomes slower because of the need for interpretation before each
execution.
Normally it is easier to design an interpreter than a compiler.
Generations of Languages
1st. Since 1940s. MACHINE LANGUAGE: binary code.
2nd. Since early ’50s. ASSEMBLY LANGUAGE:
mnemonics for numeric code.
3rd. Since mid ‘50s. HIGH-LEVEL LANGUAGES:
procedural in nature.
e.g. C, C++, COBOL
4th. Since late ‘70s. MODERN APPLICATION PACKAGES:
non-procedural in nature.
e.g. Structured Query Language (SQL)
SYSTEM SOFTWARE
The system softwares are programs specially
designed for controlling the computer
hardware, ensuring the proper utilization of the
computer resources and providing a convenient
environment to the users of the computer to
work with.
Example: Operating system, Compiler,
Interpreter, Loader, Linker
APPLICATION SOFTWARE
These are programs developed by the users of the computer to
perform a specific task. They can be of two types:
a) Ready Made:
These are programs developed by software companies for general
purpose applications. These programs can be bought and can be
installed for performing some predefined job.
Examples : MS-WORD, FACT, TALLY, FOXPRO etc.
b) Customized or Tailor made :
These are programs developed for SPECIFIC USER REQUIREMENT
within an organization. These programs are developed by programmers
as per the user requirements.
Example: Library Management System, Hotel Management System,
Railway Reservation System
SOFTWARE LAYERS
APPLICATION SOFTWARE
OPERATING SYSTEM:
SYSTEM SOFTWARE
SCHEDULED COMPUTER EVENTS
ALLOCATES COMPUTER
RESOURCES
HARDWARE
MONITORS EVENTS
LANGUAGE TRANSLATORS:
INTERPRETERS
COMPILERS
UTILITY PROGRAMS:
ROUTINE OPERATIONS
PROGRAMMING LANGUAGES:
MANAGE DATA
ASSEMBLY LANGUAGE; FORTRAN;
COBOL; PL / 1; QBASIC; PASCAL; C; C++;
FOURTH GENERATION LANGUAGES
Operating System
A system software that acts as an intermediary between the
user and the computer and works as
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Interface manager
• Human interaction made easy
• interfacing, abstraction, control
and sharing
Resource manager
• Efficient use of resources
System and data security and
protection provider.
Control program that prevents the
system from improper use and
takes care of error situations
Jobs of an O.S. : Providing User
Interface ……
Operating system provides these facilities for the user:
• Program creation : editors, debuggers, other
development tools.
• Program execution : load, files, IO operations.
• Access to IO devices: Read and writes.
• Controlled access to files: protection mechanisms,
abstraction of underlying device.
• System access: Controls who can access the
system.
Jobs of an O.S. : As a Resource
Manager…….
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Processors : Allocation of processes to processors,
preemption, scheduling.
Memory: Allocation of main memory.
IO devices : when to access io devices, which ones
etc.
Files: Partitions, space allocation and maintenance.
Process : Applications, Data, objects.
Jobs of an O.S. : Providing Protection
and Security ……..
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When sharing resources, protection of the systems and
user resources from intentional as well as inadvertent
misuse.
Protection generally deals with access control. Ex:
Read only file
Security deals usually with threats from outside the
system that affects the integrity and availability of the
system and information with the system.
Example: username, password to access system. Data
encryption to protect information.
Evolution of Operating System…
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Serial Processing
Batch Processing
Multiprogramming
Time Sharing
Parallel System
Distributed System
Real Time System
Serial Processing
(Early 1950’s)
Typical setup:
• Users would sign up for a time slot (e.g., 2am-3am)
• During that slot, had exclusive use of the computer
• Must load program into memory space, overwriting old conten
Drawbacks:
• Inconvenient for user
• Wasteful of computer time (computer was idle during setup,
debugging
Batch Processing
( Mid 1950s)
Typical setup:
• Users would submit jobs on
cards/tape to computer operator
• Operator would combine multiple
jobs into a single batch job and load
into card/tape reader
• Each user job would be executed in
turn
• Users received output after all jobs
finished
the primitive operating system in
charge of executing the batch job was
called a resident monitor
it resided permanently in memory
it monitored the execution of each job in
succession
Multiprogramming
(Early 1960s)
Typical setup:
•Users submitted jobs to computer operator
•Jobs were loaded into separate memory partitions
•Computer would begin executing first job
•During idle period, could switch to another job
•User could receive output as soon as their job terminated
Advantages and Drawbacks …
Advantages:
• Interactive ness is restored.
• CPU is kept busy.
Disadvantages:
• Hardware and O.S. required
become significantly more complex
Content of Primary Memory in
Single/Multi program systems
PROGRAM 1
UNUSED MEMORY
MULTIPROGRAMMING
ENVIRONMENT
OPERATING SYSTEM
OPERATING SYSTEM
TRADITIONAL SINGLEPROGRAM SYSTEM
PROGRAM 1
PROGRAM 2
PROGRAM 3
UNUSED MEMORY
Time Sharing
(Mid 1960s)
Typical setup:
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7.11
Many users share large capacity CPU:
Time in CPU divided into slices ( e.g.,2 milliseconds)
Each user has access to CPU during slice.
If the job is not completed within the given time slice, it
is preempted and the next job gets the CPU.
The preempted jobs gets the CPU back after all other
jobs get a chance to get the CPU
New computer architectures  new demands
Multiprocessor Systems ( parallel systems or tightly
coupled systems)
• More than one processor in close communication, share bus
and clock, other resources
• Can increase throughput, save on shared resources, provide
greater reliability
Distributed systems ( loosely coupled systems)
 Distribute the computation among several processors, each
with its own memory & resources
 Processors communicate via communications lines, e.g.,
high-speed buses or phone lines
 Similar benefits as multiprocessor, but generally simpler,
cheaper, more scaleable but slower.
New architectures (cont.)
Real-time systems
Some systems impose well-defined, fixed-time constraints
where large number of events external to the system are
required to be taken care of.
e.g., control for scientific experiments, medical imaging
systems, industrial control systems.
Hard real-time:
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Critical tasks must be completed in specified time
Secondary storage limited or absent, data stored in short term
memory or ROM
Not compatible with timesharing
Soft real-time:
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Critical tasks are given priority over other tasks, but no
guarantees
Limited utility in industrial control of robotics