1 Chapter 6 Operating Systems. 2 Learning outcomes Describe the functions of the Operating System and its components. Explain where the operating system.

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Chapter 6

Operating Systems

2

Learning outcomes Describe the functions of the Operating System and its

components. Explain where the operating system is usually stored and how it

gets started. Explain the concept of process

• Process control block? Explain how the long-term, short-term and medium-term

schedulers operate. Explain how the operating system manages memory

• Partitioning, swapping, paging, demand paging Explain how the operating system manages I/O devices

• Programmed I/O, Interrupt-driven I/O, DMA Explain the concept of Interrupt

• Describe how interrupts are caused.

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Additional Reading

Essential Reading• Stalling (2003): Chapter 7 and 8

Further Reading• Brookshear (2003): Chapter 3.1 – 3.4

• Flynn and McHoes (2001): Chapters 1,2,3,4,5 and 7.

• Silberschatz et al. (2002): Chapters 1,4,6,9,10, and 13

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What is an operating system? A program that runs on the hardware and supports

• Resource Abstraction• Resource Sharing

Abstracts and standardises the interface to the user across different type of hardware• Virtual machine hides the messy details witch must be performed.

Manages the hardware resources• Each program gets time with the resource• Each program gets space on the resource

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Introduction (1)

The aims of an operating system are:• User convenience

• System performance• Number of requests serviced per unit time, etc

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Introduction (2) Fundamental tasks of an operation system

• Management of Programs

• Organize their execution by sharing the CPU

• Ensure good user service and efficient use

• Management of Resources

• Efficient allocation/de-allocation without constraining user programs

• Security and Protection

• Ensure absence of interference with programs and resources by entities within and outside the operating system

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Operating Systems Application programs needs to access the devices

connected to a computer. Operation System: (system program – slide-19, chapter 2).

• is a software layer between the hardware and the user.

• provides a consistent application program interface (API).

• first program that runs when the computer boots up.

• is a program that is always running when the machine is on.

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Main functions of an operating system

1. User/computer interface:• Provides an interface between the user and

the computer

2. Resource manager: • manages all computer’s resources.

• Process manager

• Memory manager

• Device manager

• File manager, etc.

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Operation System

User command interface

Resource management

Process Manager

Memory manager

Device Manager

File manager

Network manager

A model of an operation System

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Operating system as a user/computer interface

A user command such as open, save or print would correspond a sequence of machine-code instructions.

The user does not need to provide these sequences of instructions.

Operating system translates these commands to a machine-code instructions.

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Operating system as a resource manager

Resource management

Process Manager:• Next program to be executed? • Time to be given to each program?

Memory manager:• Best use of the memory to run as many programs as possible

I/O Device (e.g.printer) Manager:• Which program should use a particular I/O

device?Network manager:

• which computer should execute a particular program?

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Type of operating systems

Multi-programming• Operating system can handle several programs at

once.

Time-sharing• Operating system allows many user to share the

same computer and interact with it.

• Or, in case of a single-user computer (e.g. PC), the user can work on several programs at the same time.

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How the operating system get started?

Main memory has a small section of permanent read only memory (ROM)

ROM contains a program, bootstrap.

At the start the CPU runs bootstrap. Which directs the CPU to load the operation system from disk and transfer control to it.

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OperatingSystem

Main memory

Bootstrapprogram

Main memory

BootstrapProgram

Operating System

Disk storageROM

ROM

RAM

RAM

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Operating system as a process manager Coordinates the occupation of the main memory by

different processes and their data.

At any time the operation system may be dealing with many processes.

e.g. a process may be executed or allowed to wait in main memory, or swapped out of the main memory.

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Processes

Definition of a process Process Scheduling Operations on Processes Cooperating Processes

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What is a process

Process – a program in execution; process execution must progress in sequential fashion.

A process includes:

• program counter

• stack

• data section

• heap

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The Stack and the Heap

An important thing to understand about variable storage is the difference between the program stack and the heap.

Memory in the stack is of a fixed size. Each function has its own set of variables that

reside in the stack storage, which are pushed on and popped off the program stack as the program moves in and out of functions.

When you declare a simple variable, you are making room for it on the program stack.

int myInteger = 10;int myResult = myInteger * 100;

This is possible because we know the exact amount of memory each of these variables require. (These variables can also reside in registers, so simple arithmetic operations can be executed very quickly.)

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The Stack and the Heap

Memory in the heap must be allocated, that is, it must be requested and assigned by the operating system. This is a slower operation because it typically requires more operations than simply writing or reading a single value from a memory location. The amount allocated can be determined at run time.

For example, creating a variable that holds a string, the size of which we can’t know before hand, requires an allocation from the heap:

string s = Console.ReadLine();

Instead of storing the entire string on the stack, a reference to the string is stored on the stack, and the string itself is stored on the heap.

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Process State

As a process executes, it changes state• new: The process is being created.

• running: Instructions are being executed.

• waiting: The process is waiting for some event to occur.

• ready: The process is waiting to be assigned to a process.

• terminated: The process has finished execution.

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Process Control Block (PCB)

Information associated with each process. Identifier Process state Program counter CPU registers CPU scheduling information Memory-management information Accounting information I/O status information

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CPU Switch From Process to Process

The PCB is saved when a process is removed from the CPU and another process takes its place (context switch).

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Process Scheduling Queues

Job queue – set of all processes in the system.

Ready queue – set of all processes residing in main memory, ready and waiting to execute.

Device queues – set of processes waiting for an I/O device.

Process migration between the various queues.

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Schedulers

Long-term scheduler (or job scheduler) – selects which processes should be brought into the ready queue.

Short-term scheduler (or CPU scheduler) – selects which process should be executed next and allocates CPU.

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Medium Term Scheduling

Time sharing Operating systems may introduce a medium term scheduler Removes processes from memory (and thus CPU contention) to reduce the

degree of multiprogramming – swapping Swapping may be needed to improve the process mix or to free up memory if it

has become overcommitted

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Intermediate queue

Job queue

CPU

I/O

I/O

I/O

I/O

ReadyqueueProcess

request

End

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Scheduling Criteria

CPU utilization – keep the CPU as busy as possible Throughput – # of processes that complete their execution per

time unit Turnaround time – amount of time to execute a particular

process • waiting to get into memory + waiting in the ready queue +

executing on the CPU + I/O Waiting time – amount of time a process has been waiting in the

ready queue Response time – amount of time it takes from when a request

was submitted until the first response is produced,

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Optimization Criteria

Max CPU utilization Max throughput Min turnaround time Min waiting time Min response time In most cases we optimize the average measure

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Scheduling AlgorithmsFirst-Come, First-Served (FCFS)

Process Burst TimeP1 24 P2 3 P3 3

Suppose that the processes arrive in the order: P1 , P2 , P3 The Gantt Chart for the schedule is:

Waiting time for P1 = 0; P2 = 24; P3 = 27 Average waiting time: (0 + 24 + 27)/3 = 17

P1 P2 P3

24 27 300

CPU–I/O Burst Cycle – Process execution consists of a cycle of CPU execution and I/O wait.

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FCFS Scheduling (Cont.)

Suppose that the processes arrive in the order P2 , P3 , P1 The Gantt chart for the schedule is:

Waiting time for P1 = 6; P2 = 0; P3 = 3 Average waiting time: (6 + 0 + 3)/3 = 3 Much better than previous case. Average waiting time is generally not minimal and may vary

substantially if the process CPU-burst times vary greatly

P1P3P2

63 300

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FCFS Scheduling (Cont.)

FCFS is non-preemptive Not good for time sharing systems where each user needs to

get a share of the CPU at regular intervals Short process(I/O bound) wait for one long CPU-bound process

to complete a CPU burst before they get a turn

• lowers CPU and device utilization1. I/O bound processes complete their burst and enter ready queue – I/O

devices idle and I/O bound processes waiting

2. CPU bound process completes CPU burst and moves to I/O device

3. I/O bound processes all quickly complete their CPU bursts and enter I/O queue – now CPU is idle

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Shortest-Job-First (SJR) Scheduling

Associate with each process the length of its next CPU burst. Use these lengths to schedule the process with the shortest time (on a tie use FCFS)

Two schemes: • nonpreemptive – once CPU given to the process it cannot be

preempted until completes its CPU burst.

• preemptive – if a new process arrives with CPU burst length less than remaining time of current executing process, preempt.

• This scheme is known as the shortest-Remaining-Time-First (SRTF).

SJF is optimal – gives minimum average waiting time for a given set of processes.

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Process Arrival Time Burst Time

P1 0.0 7

P2 2.0 4

P3 4.0 1

P4 5.0 4 SJF (non-preemptive)

Average waiting time = (0 + 6 + 3 + 7)/4 = 4

Example of Non-Preemptive SJF

P1 P3 P2

73 160

P4

8 12

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Example of Preemptive SJF

Process Arrival Time Burst Time

P1 0.0 7

P2 2.0 4

P3 4.0 1

P4 5.0 4

SJF (preemptive)

Average waiting time = (9 + 1 + 0 +2)/4 = 3

P1 P3P2

42 110

P4

5 7

P2 P1

16

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Priority Scheduling A priority number (integer) is associated with each process The CPU is allocated to the process with the highest priority

(smallest integer highest priority).• Can be preemptive (compares priority of process that has

arrived at the ready queue with priority of currently running process) or non-preemptive (put at the head of the ready queue)

SJF is a priority scheduling where priority is the predicted next CPU burst time.

Problem Starvation – low priority processes may never execute.

Solution Aging – as time progresses increase the priority of the process.

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Round Robin (RR) Each process gets a small unit of CPU time (time

quantum), usually 10-100 milliseconds. After this time has elapsed, the process is preempted and added to the end of the ready queue.

If there are n processes in the ready queue and the time quantum is q, then each process gets 1/n of the CPU time in chunks of at most q time units at once. No process waits more than (n-1)q time units.

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Example of RR with Time Quantum = 20

Process Burst Time

P1 53

P2 17

P3 68

P4 24 The Gantt chart is:

Typically, higher average turnaround than SJF, but better response.

P1 P2 P3 P4 P1 P3 P4 P1 P3 P3

0 20 37 57 77 97 117 121 134 154 162

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Memory Management

When a process is executed it has to be in main memory as the main memory can be accessed quicker.

An efficient use of the main memory is an important task of the operation system.

Different memory management techniques are used for this purpose.

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Memory partition

How processes are arranged in the main memory before been executed?

Fixed-sized partitions Variable-sized partitions

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Fixed-sized partitions

OS 8M

8M

8M

8M

8M

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Variable-sized partitions

OS 8M

2M

4M

8M

18M

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Swapping

I/O operations are slow If a running process requires an I/O operation.

The CPU will move to another process in the main memory.

Suppose the main memory is full of processes waiting on I/O.

CPU becomes idle To solve this problem Swapping technique is

used.

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Operation System

Operation System

disk

Long-termqueue

Long-termqueue

Medium-term

Completedprocesses

Completedprocesses

No Swapping

With Swapping

Main memory

Main memory

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os os

P1

os

P1

p2

os

P1

p2

p3

os

P1

P3

os

P1

P4

P3

os

P4

p3

os

p2

P4

p3

adcb

ehgf

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Fragmentation Memory is divided into partitions Each partition has a different size Processes are allocated space and later freed After a while memory will be full of small holes!

• No free space large enough for a new process even though there is enough free memory in total

If we allow free space within a partition we have internal fragmentation

Fragmentation:

• External fragmentation = unused space between partitions

• Internal fragmentation = unused space within partitions

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Problems with swapping

Swapped process are I/O output processes.

I/O processes are slower. The swapping process is slow as well. Solution:

• Reduce the amount of codes that needs to be swapped.• Paging

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Paging

A program is divided into small fixed-sized chunks(pages).

Main memory is divided into small fixed-sized chunks (frames).

A page is stored in one frame.

A program is stored in a set of frames. These frames do not need to be continuous.

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disk

Process A

page 0page 1page 2page 3

In use

In use

In use

disk

Process A

page 0page 1page 2page 3

In use

In use

In use page 3

of A

page 2 of A

page 1 of A

page 0 of A

141518

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A- page table

13

14

1516

17

18

19

20

13

14

1516

17

18

19

20

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Logical and physical address

disk

Process A

page 0page 1page 2page 3

In use

In use

In use page 3

of A

page 2 of A

page 1 of A

page 0 of A

141518

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A- page table

1314

1516

17

18

19

20

Page 1

I...

J(30)

Logical address(J)1:30

Physical address(J)

14:30

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simple paging is not efficient

Better than fixed and variable-sized partitions. OS - loads all pages of a process in the main

memory. However, not all pages of a process need to

be in the main memory in order to be executed.

OS - can still execute a process if only some of the pages are loaded

Demand paging.

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Demand paging Operating system – loads a page only when it is required

No swapping in or out of unused pages is needed.

Better use of memory.

CPU can access only a number of pages of a process at one time.

Then asks for more pages to be loaded.

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Virtual memory Demand paging gives rise the concept of virtual

memory. Only a small part of a process needs to be in

main memory at one time. Programs which require bigger memory that

main memory can still be executed. Impression of a bigger computer memory. This concept of the main memory is called

virtual memory. Demand paging and virtual memory are widely

used in today’s operation systems (wind-2000, XP).

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Interrupts

Definition of ‘Interrupt’

Event that disrupts the normal execution of a program and causes the execution of

special instructions

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Interrupts

Interrupt

Program

time t

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Interrupts

Program

time t

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Interrupts

Program

Interrupt Service Routine

Interrupt

Program

time t

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Interrupts fahrenheit = (cent * 9/5 )+ 32

Interrupt

Program

time t

mov R1, cent mul R1, 9 div R1, 5 add R1, 32 mov fahr, R1

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Interrupts

Program

Interrupt Service Routine

Interrupt

Program

time t

mov R1, cent mul R1, 9

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Interrupts

ProgramSave

Context Interrupt Service Routine

Restore Context

Interrupt

Program

time t

mov R1, cent mul R1, 9

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Interrupts

ProgramSave

Context Interrupt Service Routine

Restore Context

Interrupt

Program

time t

mov R1, cent mul R1, 9

eg push R1 eg pop R1

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CPU Switch From Process to Process

The PCB is saved when a process is removed from the CPU and another process takes its place (context switch).

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I/O devices

Called peripherals:• Keyboard

• Mouse

• Speakers

• Monitor

• scanner

• Printer

• Disk drive

• CD-drive. OS – manages all I/O operations and devices

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OS - I/O management

There are four main I/O operations.• Control: tell the system to perform some

action (e.g. rewind tape).

• Test: check the status of the device

• Read: read data from the device

• Write write data to the device.

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I/O modules

System bus

CPUMain

memory

I/O module I/O module

I/Odevice

I/Odevice

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Advantages of I/O modules They allow the CPU to view a wide range of devices in a

simple-minded format CPU does not need to know details of timing, format, or

electronic mechanics. CPU only needs to function in terms of a simple read and

write commands. They help the CPU to work more efficiently They are 3 ways in which I/O modules can work

• Programmed I/O

• Interrupt-driven I/O

• Direct memory access.

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Programmed I/O The CPU controls I/O device directly Via the I/O

modules.

The CPU sends an I/O command to the I/O module.

And waits until the I/O operation is completed before sending another I/O command.

The performance is poor as the CPU spends too much time waiting the I/O device.

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Programmed I/O

Issue Read to I/O module

Check status

Read word from I/O module

Write word To memory

done

yes

NO

Next instruction

Ready

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Interrupt-driven I/O

The CPU issues a command to the I/O module and then gets on with executing other instructions.

The I/O module interrupts the CPU when it is ready to exchange data with the CPU.

The CPU then executes the data transfer.

Most computer have interrupt lines to detect and record the arrival of an interrupt request.

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Interrupt-driven I/O

Issue Read to I/O module

Check status

Read word from I/O module

Write word To memory

done

yes

NO

Next instruction

Ready

CPU goes to do Other things

When the statusIs ready the I/O module sendsAn interrupt-signal

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How does I/O module send an interrupt to the CPU?

I/O module is linked to the control bus.

I/O module reads a word from the I/O device.

Puts the word in the data register which is linked to data bus.

Sends a interrupt signal to the CPU via control bus.

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How does CPU know Interrupt-signal?

The CPU executes an instruction cycle. An interrupt stage is added at the end of the cycle. At the end of an instruction cycle the CPU checks for

interrupts. The CPU hardware has a wire, interrupt-request line

that the CPU can sense. If no interrupt the CPU carries on executing next

instruction. Otherwise, it updates the process control block, save

it. Then process the interrupt. Resume the execution of the interrupted process.

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How does CPU process interrupts?

Interrupt detection. CPU executes Interrupt-handler program. Interrupt-handler program makes use of the

process control block save earlier. Interrupt-handler decides what to do with

interrupt. Then asks the CPU to resume the execution

interrupted.

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Disadvantages of Interrupt-driven I/O

CPU is responsible for managing I/O data transfer.

Every transferred word must go through the CPU.

Devices with large transfer, e.g. disk drive, the CPU wastes time dealing with data transfer.

Solution: Direct-memory-access(DMA).

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Direct-memory-access - DMA

Special-purpose processor. Handles data transfer. CPU issues to the DMA:

• starting address in main memory to read/write to.

• Starting address in the I/O device to read/write to.

• The number of words to be transferred.

DMA transfers data without intervention from the CPU.

DMA sends interrupt to the CPU when transfer is completed.

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DMA/CPU - bus system

DMA take care data transfer. CPU free to do other jobs. However, they can not use the bus at the

same time. DMA can use the bus only when the

CPU is not using it. Some times it has to force to CPU to free

the bus, cycles stealing.

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DMA/CPU

System bus

CPUMain

memory

I/O module

I/Odevice

DMA

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Summery

OS- memory manager• Fixed-sized partition: waist of memory.

• Variable-sized partition: fragmentation.

• Swapping. Time wasted in swapping the whole process

• Simple paging: process divided into pages and loaded into main memory(divided into frames).

• Demand paging: only the required pages are loaded to main memory.

OS- I/O manager• Programmed I/O: CPU waste waiting for I/O operation.

• Interrupt-driven I/O: CPU responsible for data transfer.

• DMA: takes care of data transfer instead the CPU.

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fin

Thank you and good luck