Unit 6 Operating Systems

Unit 6Operating Systems

Dr Damitha Karunaratna

University of Colombo school of computing

Topics to discuss• What is an operating system(OS)?

• Main objectives of an OS

• Main functions of an OS

• Evolution of OS.

• Process management.

• Context switching.

• Process state diagrams

Operating Systems 2

Components of a Computer System hardware—electronic, mechanical, optical

devices. software—programs.

Software saved on the storage media. Software burned into hardware – Firmware

Liveware – Computer Users

Operating Systems 3

What is an operating system(OS)? An operating system (OS) is a resource manager. What is the necessity for a resource manager?

Many processes are active at any given time and compete for resources.

An operating system provides orderly and controlled allocation of the resources among processes (jobs) that are competing for them.

Operating Systems 4

Different types of Software Application Software Systems Software

Operating system System utilities

The quality of system software also directly affects the application software

Operating Systems 5

Types of Software

Operating Systems 6

Utility software: system software designed to help analyze, configure, optimize or maintain a computer (Anti-virus, Backup software, Editors, Data compression, Disk cleaners ….).They are not essential to the running of the computer

Transient Component

What is an operating system(OS)?

Operating Systems 7

What is an operating system(OS)?

Operating Systems 8

Kernel : Part of the O/S that resides in the main memory all the time.

Execution of source programs

Operating Systems 9

Address mapping for re-locatable programs

Operating Systems 10

Different types of Oss(Based on the processor)•Windows/Linux – For personal computers

•Unix,z/OS, OS/390, VM – For mainframes

•MacOs – For Macs

• X Server, Windows Server – Server Operating Systems

•Symbian, Android – For mobile phones.


Different types of OSs (Based on the users)•Single User – Allows only a single user to use the

OS at any given time. The use may run several processes at the same time. • Example - DOS

•Multi User - Allows multiple users to access a computer system at the same time• Example UNIX, Time-sharing systems and Internet

servers.


Different types of OSs(Based on the number of tasks)•Single Task– Allows only one running program at

any given time.

•Multi Task - A multi-tasking operating system allows more than one program to be running at the same time.


Different types of OSs•Real Time – OS is designed to run applications with

very precise timing and with a high degree of reliability.• The main objective of real-time operating systems is

their quick and predictable response to events.• These types of OS are needed in situations where

downtime is costly or a program delay could cause a safety hazard.


Main objectives of an OS

Convenience: Make the computer more convenient to use◦ Provide easy to use interface for a normal users.◦ Hide the complexity of the hardware devices from the

application developer.

Efficiency: Monitor and manage resources of the computer system efficiently◦ CPU◦ Main memory◦ Secondary Storages◦ Various devices connected to the computer


Services provided by an OS

Process Management Storage (external) management Memory management I/O device management Management of the File System Networking User Interface Protection


Services provided by an OS….

Error detection and response◦ Hardware errors: memory error or device failure◦ Software errors: arithmetic errors, access forbidden

memory locations

Accounting◦ collect statistics (billing)◦ monitor performance


Types of User Interfaces Command Line Interface(CLI)


Types of User Interfaces Graphical User Interface(GUI)


OS and Processors

Operating systems are software

Operating systems are designed and developed for a specific CPU family.

◦ Macintosh OS: Motorola 680xx, PowerPC Gx, Intel

◦ DOS: Intel CPUs◦ Windows 9x and XP: Intel 80386,

80486, and Pentium CPUs◦ Linux: Intel CPUs◦ MS NT & 2000: Intel CPUs


Can any OS run on any processor?

Compatibility of Software

QuestionWill software developed for one operating system work on another? For example will MS Word for Macintosh run on a PC with Windows XP?


Compatibility of Software

Question

Will software developed for one operating system work on another?

Answer

No (unless there is special emulation software or hardware). The software is typically developed separately for each operating system.


Evolution of Operating systemsSerial Processing (1940 – mid 1950s)◦ Single user system.◦ Programmer/User acts as the

operator and interacted with the hardware.

◦ No operating system.◦ Machines run from a console with

display lights, toggle switches.◦ Paper Tapes or Punched cards for

the program and I/O.◦ Setup included loading the

compiler, source program, saving compiled program, and loading and linking


Disadvantages : Scheduling: Hardcopy sigh-up sheet for

reserving time User could reserve for 45 mins and finish in 30

mins => wastage of time User may not be able to finish in scheduled

time

Simple batch system Simple Batch Processing Systems

◦ Use of high-level languages ◦ Jobs are batched together by the

language.◦ Input/output is through punch

cards and magnetic tapes.◦ Software called the Monitor was

introduced to sequence the jobs.◦ Hardware support for the monitor

model◦ Memory protection: some memory

areas are accessible only to the monitor◦ Privileged mode instructions: only

accessible to the monitor◦ Interrupts (early machines did not have

this)


Simple batch processing system

• The user submits a job (written on cards or tape) to a computer operator.

• The computer operator place a batch of several jobs on an input device.

• A special program called the monitor, manages the execution of each program in the batch.

◦ “Monitor” is always in main memory.◦ Monitor reads and loaded programs sequentially and then (the utility programs when

needed) passed the control to the loaded program.◦ When a job terminates the control returns back to the monitor program.

◦ Alternate execution between user program and the monitor program.

• instructions for the monitor were given by using a special purpose language called Job Control Language (JCL)


Simple batch system …..• A user program executes in user mode, in

which certain areas of memory were protected from the user’s use, and user program is not allowed to execute certain instructions.

•The monitor executes in a system mode, or a kernel mode and it can execute privileged instructions and can access protected memory segments.

• Machine time alternates between monitor and the user programs.


Simple batch system …..

Disadvantages•A portion of the memory has to be allocated for the monitor

•A small portion of the machine time is consumed by the monitor.

Advantage of batch systems.•Reduce setup time by batching similar jobs.


Card Deck of a job


Simple batch system : Problems◦ During I/O operations CPU is not used.


Simple batch system : I/O ◦ I/O devices (Card Readers, Printers) slow when compared

to CPU.

Solution: Offline Operation (Satellite Computers)◦ Speed up computation by loading jobs into memory from

tapes while card reading and line printing is done off-line using smaller machines.


Multiprogramming Running multiple programs “at the same time”

◦ Requires sharing the CPU among multiple processes

Transfer of control is called a context switch


Firefox Word javac Firefox Word

Multiprogramming Why multiprogramming?

◦ Single user cannot keep CPU and I/O devices busy at all times.

Multiprogramming organizes jobs (code and data) so CPU always has one to execute.•A subset of total jobs in system is kept in

memory.•One job selected and CPU is give for that job.•When it has to wait, OS switches to another job.


Multiprogramming


Time Sharing Systems• Processor’s time is shared among multiple users

• Multiple users simultaneously access the system through terminals.


Multiprogramming Vs Time Sharing Systems• Multiprogramming maximizes CPU utilization• Time-sharing minimizes user response time


Loading the Operating System• OS is also a software like any other, but has to be

loaded and run by the OS itself.

• The process of initializing the computer and loading the OS is known as bootstrapping or booting the system.• The bootstrapping program normally exist in non-

volatile memory and is executed automatically when the machine is turned on.

• The operating system software (kernel) copied into RAM, usually from the hard disk, during the boot-up.

• Once loaded the OS wait for an event to occur (eg: user typing a command) and process the event.

• OS is an event driven software.


Loading the Operating System• The kernel remains in RAM while the computer is

on and is in charge of the overall operation of the computer system.

• The kernel contains the “internal programs” for the most often used operations like file management, memory management, security.


Gaining Control The OS gets the control of the CPU when either an

external event or an internal event occurs. External Events Character typed at the console Completion of an I/O operation Timer quantum allowed for a process expires.

Internal Events Division by zero, System call issued by a program Page Fault Unauthorized memory access


Interrupts External events get the attention of the CPU

through Interrupts. For example when a disk driver has finished

transferring the requested data, it generates an interrupt to the OS to inform the OS that the task is over.

Interrupts occur asynchronously to the ongoing activity of the processor. Thus the times at which interrupts occur are unpredictable.


Interrupt Handlers Interrupt Handlers : Code that get executed

when an interrupt occurs. Associated with each type of interrupt

there is a specific program to handle that type of interrupts – Interrupt handler (Interrupt service routine)


Getting the services of OS

How do the user programs get the service of OS? User programs access the functionality of the OS through system calls –

privileged operations. Example : open(), close(), fork(),…….

The execution of system call change the execution mode of the CPU to supervisor mode.


Processes Process is a fundamental concepts in modern operating systems.

It was first introduced by the designers of Multics operating systems in the 1960s.

The programs that reside in main memory are absolutely different from their counter-parts the program files on hard disks or any other secondary storage devices.


Process and a program• A process is a program in execution• An instance of a program running on a computer.• The entity that can be assigned to and executed

on a processor

• A program is a static set of instructions


Process and a program• A process exists in a limited span of time. Two or

more processes could be executing the same program, each using their own data and resources.

• A program is a static entity made up of instructions. A program exists in the secondary storage till it is deleted. A program does not perform the action by itself.


Process creation and termination

• When a new process is created, the operating system builds the date structures that are used to manage the process and allocates space in main memory to the process.

• A process may terminates in a number of ways.• After completion of the instructions.• User terminates (kills) the process explicitly. For

example clicking on the cross button in the windows applications.

• A process may terminate due to abnormal condition.

• When a process finishes, the operating system will free the memory space it occupies and remove the data structures it allocated to manage the process.


Processes in a typical Linux system


Uniprocessor Scheduling

Type of processes• I/O bound processes• Processor bound processes


Uniprocessor Scheduling

Types of Scheduling

• Long-term scheduling(Job scheduling) : It determines which programs are admitted to the system for processing. Job scheduler selects processes from the queue and loads them into memory for execution. Process loads into the memory for CPU scheduling.

• Medium-term scheduling : Medium term scheduling is in charge of swapping processes between the main memory and the secondary storage.

• Short-term scheduling (low-level scheduling) : Determines which ready process will be assigned the CPU when it next becomes available.


Scheduling Policies◦ Non-preemptive

◦ Once a process is in the running state, it will continue until it terminates or blocks itself for I/O.

◦ Preemptive◦ Currently running process may be interrupted

and moved to the Ready state by the OS.◦ Allows for better service since any one process

cannot monopolize the processor for very long


Processor Scheduling Assigning the processor to the processes.

Turnaround time : Time required for a particular process to complete, from submission time to completion.

Response time : The time taken in an interactive program from the issuance of a command to the commence of a response to that command.

Throughput : Number of processes completed per unit time. May range from 10 / second to 1 / hour depending on the specific processes.

Waiting time : How much time a process spends in the ready queue waiting its turn to get on the CPU.


Long-term scheduling(Job scheduling)• Determines which processes are admitted to the

system for processing

• Controls the degree of multiprogramming• If more processes are admitted• better CPU usage• less likely that all processes will be blocked

• The long term scheduler may attempt to keep a mix of processor-bound and I/O-bound processes


Medium-Term Scheduling• Swapping decisions based on the need to manage

multiprogramming

• Done by memory management software


Logical view of swapping


Short-Term Scheduling• Determines which process is going to execute next

(also called CPU scheduling)

• The short term scheduler is known as the dispatcher• Dispatching the CPU to the process


Schedulers - ComparisonLong Term Scheduler Short Term Scheduler Medium Term Scheduler

Job Scheduler CPU scheduler Processes swapping scheduler

Selects processes from a pool and loads them into the memory for execution

Selects those processes which are ready to execute for dispatching

Swapped out/Re-introduces the processes into memory and execution can be continued.

Controls the degree of multiprogramming

Provides lesser control over the degree of multiprogramming

Controls the degree of multiprogramming

Speed is lesser than short term scheduler

Speed is fastest among other two

Speed is in between (short and long term schedulers)


Queuing Diagram for Scheduling


Seven state process transition Diagram


Typically, new processes are not in the main memory.

Seven state process transition Diagram


Long Term Scheduler

Short Term Scheduler

Medium Term Scheduler

Process Control Block (PCB)

All of the information needed to keep track of a process when switching states is kept in a data package called a process control block. The process control block typically contains:• An ID number that identifies the process

• Pointers to the locations in the program and its data where processing last occurred

• Register contents

• States of various flags and switches

• A list of files opened by the process

• The priority of the process

• The status of all I/O devices needed by the process


Context Switching• A context switch is the mechanism to store and restore the state or

context of a CPU in Process Control block so that a process execution can be resumed from the same point at a later time.

• Using this technique a context switcher enables multiple processes to share a single CPU. Context switching is an essential part of a multitasking operating system features.

• When the scheduler switches the CPU from executing one process to execute another, the context switcher saves the content of all processor registers for the process being removed from the CPU, in its process control block.

• Context switch time is pure overhead.

• Context switching can significantly affect performance as modern computers have a lot of general and status registers to be saved.


Context Switching


Short-term scheduling algorithms• First-come-first served

• Round Robin

• Shortest Process next


Objectives of Short-Term Scheduling

User-oriented◦ Response Time: Elapsed time from the

submission of a request to the beginning of response

◦ Turnaround Time: Elapsed time from the submission of a process to its completion

System-oriented◦ processor utilization◦ fairness◦ throughput: number of process completed per

unit time


First-come-first served scheduling• Each process joins the end of the Ready queue.

• When the current process ceases to execute, the process waited the longest time in the Ready queue is assigned the CPU.


Example - FCFS


Process Arrival Time

Service Time

Finish time

Turnaround time

A 0 3 3 3B 2 6 9 7C 4 4 13 9D 6 5 18 12E 8 2 20 12

Average Turnaround time = (3 + 7+ 9+ 12 + 12)/5= 8.60

Shortest Job First (Shortest Process Next)• Nonpreemptive policy

• Process with shortest expected processing time is selected next

• Short process jumps ahead of longer processes


Shortest Job First(Shortest Process Next)



Service Time

Finish time

Turnaround time

A 0 3 3 3B 2 6 9 7C 4 4 15 11D 6 5 20 14E 8 2 11 3


FCFS - Issues


A short process may have to wait a very long time before it can execute Favors CPU-bound processes◦I/O processes have to wait until CPU-bound process completes

Round Robin Scheduling• Clock interrupt is generated at periodic intervals.

• When an interrupt occurs, the currently running process is placed in the ready queue (preempted)

• Next process in the ready queue is assigned the CPU.

• Known as time slicing


Queuing Diagram


Round Robin Scheduling Scheduling Policy – Preemption, Time quantum for each process = 1



Service Time

Finish time

Turnaround time

A 0 3 4 4B 2 6 18 16C 4 4 17 13D 6 5 20 14E 8 2 15 7

0 5 10 15 20


Round Robin Scheduling Scheduling Policy – Preemption, Time quantum for each process = 4



Service Time

Finish time

Turnaround time

A 0 3 3 3B 2 6 17 15C 4 4 11 7D 6 5 20 14E 8 2 19 11

Average Turnaround time = ( 3+ 15+ 7+ 14 + 11)/5= 10.00

Gantt Chart ?

Priority Scheduling• Scheduler will always choose a process of higher

priority over one of lower priority

• Use multiple ready queues to represent multiple levels of priority

• Lower-priority may suffer starvation• Allow a process to change its priority based on its age or

execution history


Priority Queuing


Can be either preemptive or non-preemptive

Threads A thread is the smallest schedulable unit in a system that can be managed independently by an operating system. A thread can also be viewed as an execution streams within a single process. Generally a thread contained inside a process. It is possible to have processes with one threads or processes with multiple threads.

All threads of a process share common code, data, and other resources, including CPU registers.• Allows multiple tasks to be performed simultaneously in a single address

space.• Context switching generates less overhead.


Threads ….. Threads are very useful whenever a process has multiple tasks to perform independently of the others.

For example in a word processor, a background thread may check spelling of a document while a foreground thread processes user keystrokes, while another thread may automatically backs up the edited section periodically.


Von Neumann Architecture – Hardware components

The hardware for a Von Neumann machine consists of three principle components; processor, memory, and I/O facilities.


Both programs and data are stored in the memory

Unit 6 Operating Systems

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