This document discusses operating systems and CPU scheduling algorithms. It begins by defining an operating system and providing examples of common operating systems. It then describes different types of operating systems including mainframe, batch processing, multiprogramming, time-sharing/multitasking, multiprocessor, distributed, and desktop systems. The document also discusses various CPU scheduling algorithms such as first-come first-served, priority-based, round robin, and shortest-job-first scheduling. Examples are provided to calculate average wait times for processes under different scheduling algorithms.

1. TOPIC
TYPES OF OPERATING SYSTEM &CPU
SCHEDULING ALGORITHM
Name:-Aman patra
Subject:-Operating system
Roll;-16
Lecturer:-sudha madam
College:-Sr degree and pg college

2. WHAT IS AN OPERATINGSYSTEM

Aprogramthatactsasanintermediarybetweenau
serofacomputerandthecomputerhardware.
 AnoperatingSystemisacollectionofsystemprogramst
hattogethercontroltheoperationsofacomputer
system.
 Some examples of operating systems are UNIX,
Mach, MS-DOS, MS-Windows, Windows/NT,
Chicago, OS/2,MacOS, VMS, MVS, and VM

3. TYPES OF OPERATING SYSTEM
 MainframeSystems
 Reduce setup time by batching similar jobs Automatic job sequencing – automatically
transfers control from one job to another. First rudimentary
 operatingsystem.
 BatchProcessingOperatingSystem:
 This type of OS accepts more than one jobs and these jobs are batched/ grouped
together according to their similar requirements. This is done by computer operator.
Whenever the computer becomes available, the batched jobs are sent for execution
and gradually the output is sent back to the user.
 Itallowedonlyoneprogramatatime.
 MultiprogrammingOperatingSystem:
 This type of OS is used to execute more than one jobs simultaneously by a single
processor. it increases CPU utilization by organizing jobs so that the CPU always
has one job to execute.
 Theconceptofmultiprogrammingisdescribedasfollows:
 Allthe jobsthatenter the systemarestoredinthe job pool( indisc).The
operatingsystemloadsaset of jobs from job pool into main memory and begins to
execute.

4. TYPES OF OPERATING SYSTEM
 Time-Sharing/multitaskingOperatingSystems
 Timesharing(ormultitasking)OSisalogicalextensionofmultiprogram
ming.Itprovidesextrafacilitiessuch as:
 Fasterswitchingbetweenmultiplejobstomakeprocessingfaster.
 Allowsmultipleuserstosharecomputersystemsimultaneously.
 Theuserscaninteractwitheachjobwhileitisrunning.
 These systems use a concept of virtual memory for effective
utilization of memory space. Hence, in this OS, no jobs are
discarded. Each one is executed using virtual memory concept. It
uses CPU scheduling, memory management, disc management
and security management. Examples: CTSS, MULTICS, CAL,
UNIX etc.
 MultiprocessorOperatingSystems
 Multiprocessor operating systems are also known as parallel OS
or tightly coupled OS. Such operating systems have more than
one processor in close communication that sharing the computer
bus, the clock and sometimes memory and peripheral devices. It
executes multiple jobs at same time and makes the processing
faster.

5. TYPES OF OPERATING SYSTEM
 DistributedOperatingSystems
 Indistributedsystem,thedifferentmachinesareconnectedinanet
workandeachmachinehasitsown processor and own local
memory.
 In this system, the operating systems on all the machines
work together to manage the collective network resource.
 DesktopSystems/PersonalComputerSystems
 ThePCoperatingsystemisdesignedformaximizinguserconvenie
nceandresponsiveness.Thissystemis neither multi-user nor
multitasking.
 ThesesystemsincludePCsrunningMicrosoftWindowsandtheAp
pleMacintosh.TheMS-DOSoperating system from Microsoft
has been superseded by multiple flavors of Microsoft
Windows and IBM hasupgraded MS-DOS to the OS/2
multitasking system.

6. CPU SCHEDULING
 Selects from among the processes in memory that are ready to
execute, and allocates the CPU to one of them
 CPU scheduling decisions may take place when a process:
1. Switches from running to waiting state
2. Switches from running to ready state
3. Switches from waiting to ready
4. Terminates

7. FIRST-COME, FIRST SERVED
 ( FCFS) same as FIFO
 Simple, fair, but poor performance. Average queueing
time may be long.
 What are the average queueing and residence times
for this scenario?
 How do average queueing and residence times
depend on ordering of these processes in the queue

8. CPU SCHEDULING
5:
CPU-Scheduling
8
EXAMPLE DATA:
Process Arrival Service
Time Time
1 0 8
2 1 4
3 2 9
4 3 5
0 8 12 21 26
P1 P2 P3 P4
FCFS
Average wait = ( (8-0) + (12-1) + (21-2) + (26-3) )/4 = 61/4 = 15.25
Scheduling
Algorithms
Residence Time
at the CPU

9. CPU SCHEDULING
5:
CPU-Scheduling
9
PRIORITY BASED SCHEDULING:
 Assign each process a priority. Schedule highest priority first. All
processes within same priority are FCFS.
 Priority may be determined by user or by some default mechanism. The
system may determine the priority based on memory requirements, time
limits, or other resource usage.
 Starvation occurs if a low priority process never runs. Solution: build
aging into a variable priority.
 Delicate balance between giving favorable response for interactive jobs,
but not starving batch jobs.
Scheduling
Algorithms

10. ROUND ROBIN:
 Use a timer to cause an interrupt after a predetermined time. Preempts if task
exceeds it’s quantum.
 Train of events
Dispatch
Time slice occurs OR process suspends on event
Put process on some queue and dispatch next
 Use numbers in last example to find queueing and residence times. (Use quantum
= 4 sec.)
 Definitions:
 Context Switch Changing the processor from running one task (or process)
to another. Implies changing memory.
 Processor Sharing Use of a small quantum such that each process runs
frequently at speed 1/n.
 Reschedule latency How long it takes from when a process requests to
run, until it finally gets control of the CPU.
10
CPU SCHEDULING Scheduling
Algorithms

11. CPU SCHEDULING
5:
CPU-Scheduling
11
EXAMPLE DATA:
Process Arrival Service
Time Time
1 0 8
2 1 4
3 2 9
4 3 5
0 8 12 16 26
P2 P3 P4 P1
Round Robin, quantum = 4, no priority-based preemption
Average wait = ( (20-0) + (8-1) + (26-2) + (25-3) )/4 = 74/4 = 18.5
P1
4
P3 P4
20 24 25
P3
Scheduling
Algorithms
Note:
Example violates rules for
quantum size since most
processes don’t finish in
one quantum.

12. SHORTEST-JOB-FIRSTSCHEDULING

 A different approach to CPU scheduling is the shortest-
job-first (SJF) scheduling algorithm. This algorithm
associates with each process the length of the latter's
next CPU burst.When the CPU is available, it is
assigned to the process that has the smallest next CPU
burst. If two processes have the same length next CPU
burst, FCFS scheduling is used to break the tie. Note
that a more appropriate term would be the shortest next
CPU burst,becausetheschedulingis
donebyexaminingthelengthof thenextCPUburstof
aprocess,ratherthanits total length. We use the term SJF
because most people and textbooks refer to this type of
scheduling discipline as SJF.

13. THANKYOU