An operating system is software that enables programs to use computer hardware. It organizes and controls hardware resources and acts as an interface between programs and hardware. Operating systems perform functions like making computers more convenient to use, allowing efficient use of resources, and enabling computers to evolve without interruption. The structure of an operating system consists of four layers - hardware, the operating system software, system programs, and application programs. Early operating systems used batch processing where jobs were submitted in batches to be executed together, while later systems used multiprogramming and time sharing to allow simultaneous execution of multiple programs. Operating systems can also be classified as single-user or multi-user depending on how many users they support at once.

1. Operating SystemsOperating Systems
AUGUSTNE C THEVERKUNNEL
S2 M.TECH (IE & M)
ROLL NO: 02
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2. What is OS?
 Operating System is a software
 It is the software that enables all the programs we use.
 The OS organizes and controls the hardware.
 OS acts as an interface between the application
programs and the machine hardware.
 Examples: Windows, Linux, Unix, etc.,
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3. FUNCTIONS OF OPERATING SYSTEM
 Convenience: An OS makes a computer more
convenient to use.
 Efficiency: An OS allows the computer system
resources to be used in an efficient manner.
 Ability to Evolve: It helps in the proper
functioning of the computer without interruption
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4. Structure of Operating System:
Application Programs
System Programs
Software (Operating System)
HARDWARE
(Contd…)
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5. Structure of Operating System
(Contd…):
 The structure of OS consists of 4 layers:
1. Hardware
Hardware consists of CPU, Main memory, I/O
Devices, etc,
2. Software (Operating System)
Software includes process management
routines, memory management routines, I/O
control routines, file management routines.
(Contd…)
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6. Structure of Operating System
(Contd…):
3. System programs
This layer consists of compilers, Assemblers,
linker etc.
4. Application programs
This is dependent on users need. Ex. Railway
reservation system, Bank database
management etc.,
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7. Batch Processing:
 In Batch processing same type of jobs batch (BATCH- a
set of jobs with similar needs) together and execute at a
time.
 The OS was simple, its major task was to transfer
control from one job to the next.
 The job was submitted to the computer operator in form
of punch cards. At some later time the output appeared.
 The OS was always resident in memory. (Ref. Fig. next
slide)
 Common Input devices were card readers and tape
drives.
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8. Batch Processing (Contd…):
 Common output devices were line printers, tape drives,
and card punches.
OS
User
program
area
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9. Multiprogramming:
 Multiprogramming is a technique to execute number of
programs simultaneously by a single processor.
 In Multiprogramming, number of processes reside in
main memory at a time.
 The OS picks and begins to executes one of the jobs in
the main memory.
 If any I/O wait happened in a process, then CPU
switches from that job to another job.
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10. Multiprogramming (Contd…):
OS
Job 1
Job 2
Job 3
Job 4
Job 5
• Figure dipicts the layout of
multiprogramming system.
• The main memory consists of 5
jobs at a time, the CPU executes
one by one.
Advantages:
•Efficient memory utilization
•CPU is never idle, so
performance increases.
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11. Time Sharing Systems:
 Time sharing, or multitasking, is a logical extension of
multiprogramming.
 Multiple jobs are executed by switching the CPU
between them.
 In this, the CPU time is shared by different processes, so
it is called as “Time sharing Systems”.
 Examples: Multics, Unix, etc.,
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12. Types of OS:
Operating System can also be classified as,-
 Single User Systems
 Multi User Systems
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13. Single User Systems:
 Provides a platform for only one user at a
time.
 They are popularly associated with Desk Top
operating system which run on standalone
systems where no user accounts are
required.
 Example: DOS
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14. Multi-User Systems:
 Provides regulated access for a number of users by
maintaining a database of known users.
 Refers to computer systems that support two or more
simultaneous users.
 Another term for multi-user is time sharing.
 Ex: All mainframes and are multi-user systems.
 Example: Unix
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15. UNIX OPERATING SYSTEM
 Unix is a multitasking, multi-user computer operating
system
 It is written in C language
 UNIX can be used in a wide variety of computers
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16. MICROKERNEL BASED OPERATING
SYSTEM
 A set of independent system servers using the
primitive, generic services of a microkernel.
 The micro-kernel provides a virtual machine for
processor use, memory allocation and
communication between operating system
components
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17. MICROKERNEL BASED OPERATING SYSTEM
 In order to make an OS portable it is necessary
to make the machine dependent part as small as
possible
 The microkernel was developed in the early
1990s to overcome the problems concerning
portability, extensibility, and reliability of kernels
 Microkernel is a very small core of an operating
system
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