Operating Systems

1
Operating Systems

• Chapter 6

2
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

3
Introduction

• The aims of an operating system are
• User convenience
• System performance
• Number of requests serviced per unit time, etc

4
Introduction

• 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

5
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.

6
Main functions of an operating system

• User/computer interface
• Provides an interface between the user and the
  computer
• Resource manager
• manages all computers resources.
• Process manager
• Memory manager
• Device manager
• File manager, etc.

7
A model of an operation System
User command interface
Process Manager Memory manager Device
Manager File manager Network manager
Operation System
Resource management
8
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.

9
Operating system as a resource manager

• 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?

Resource management
10
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.

11
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.

12
Main memory
Main memory
Disk storage
R O M
R O M
Bootstrap program
Bootstrap Program
Operating System
Operating System
R A M
R A M
13
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.

14
Processes

• Definition of a process
• Process Scheduling
• Operations on Processes
• Cooperating Processes

15
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

16
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.

17
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

18
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).

19
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.

20
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.

21
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

22
Intermediate queue
Job queue
CPU
Ready queue
End
Process request
I/O
I/O
I/O
I/O
23
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,

24
Optimization Criteria

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

25
Scheduling AlgorithmsFirst-Come, First-Served
(FCFS)

• Process Burst Time
• P1 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

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

26
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

27
FCFS Scheduling (Cont.)

• FCFS is non-preemptive
• Not good for time sharing systems where 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 utilization
• I/O bound processes complete their burst and
  enter ready queue I/O devices idle and I/O
  bound processes waiting
• CPU bound process completes CPU burst and moves
  to I/O device
• I/O bound processes all quickly complete their
  CPU bursts and enter I/O queue now CPU is idle
• CPU bound completes I/O and executes on CPU back
  to step 1

28
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 know as the shortest-Remaining-Tim
  e-First (SRTF).
• SJF is optimal gives minimum average waiting
  time for a given set of processes.

29
Example of Non-Preemptive SJF

• 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

30
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

31
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.

32
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.

33
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.

34
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.

35
Memory partition

• How processes are arranged in the main memory
  before been executed?
• Fixed-sized partitions
• Variable-sized partitions

36
Fixed-sized partitions
OS 8M
8M
8M
8M
8M
37
Variable-sized partitions
OS 8M
2M
4M
8M
18M
38
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.

39
disk
Main memory
Operation System
Long-term queue
Completed processes
No Swapping
Main memory
Long-term queue
Operation System
Completed processes
With Swapping
Medium-term
40
os
os
os
os
P1
P1
P1
p2
p2
p3
a
c
d
b
os
os
os
os
P1
P1
p2
P4
P4
P4
P3
P3
p3
p3
e
g
h
f
41
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

42
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

43
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.

44
disk
disk
13

page 0 of A
13
Process A
Process A

page 1 of A
14
page 0 page 1 page 2 page 3
page 0 page 1 page 2 page 3
14

page 2 of A
15
15
In use
In use
16
16
In use
In use
17
17

page 3 of A
A- page table
18
18
In use
In use
19
13
19
14


20
20
15
18
45
Logical and physical address
disk

page 0 of A
Page 1
13
Process A
page 1 of A
14
I . . . J(30)
page 0 page 1 page 2 page 3
Logical address(J)
page 2 of A
15
130
In use
16
In use
17
page 3 of A
A- page table
18
In use
19
13
14

20
Physical address(J)
15
18
1430
46
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.

47
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.

48
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 todays operation systems (wind-2000, XP).

49
Interrupts

• Definition of Interrupt
• Event that disrupts the normal execution of a
  program and causes the execution of special
  instructions

50
Interrupts
Interrupt
Program
time t
51
Interrupts
Program
time t
52
Interrupts
Interrupt
Program
Program
Interrupt Service Routine
time t
53
Interrupts
Interrupt
Program
mov R1, cent
mul R1, 9
div R1, 5
add R1, 32
mov fahr, R1
time t
54
Interrupts
Interrupt
Program
Program
Interrupt Service Routine
mul R1, 9
mov R1, cent
time t
55
Interrupts
Interrupt
Program
Program
Save Context
Restore Context
Interrupt Service Routine
mul R1, 9
mov R1, cent
time t
56
Interrupts
Interrupt
Program
Program
Save Context
Restore Context
Interrupt Service Routine
mul R1, 9
mov R1, cent
eg push R1
eg pop R1
time t
57
I/O devices

• Called peripherals
• Keyboard
• Mouse
• Speakers
• Monitor
• scanner
• Printer
• Disk drive
• CD-drive.
• OS manages all I/O operations and devices

58
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.

59
I/O modules
System bus
I/O module
I/O module
CPU
Main memory
I/O device
I/O device
60
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.

61
Programmed I/O

• The CPU controls I/O device directly Via the I/O
  modules.
• The CPU sends an I/O command 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.

62
Programmed I/O
Issue Read to I/O module
Check status
Ready
Read word from I/O module
Write word To memory
NO
done
yes
Next instruction
63
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.

64
Interrupt-driven I/O
Issue Read to I/O module
CPU goes to do Other things
Check status
When the status Is ready the I/O module sends An
interrupt-signal
Ready
Read word from I/O module
Write word To memory
NO
done
yes
Next instruction
65
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.

66
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.

67
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.

68
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).

69
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.

70
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.

71
DMA/CPU
System bus
DMA
CPU
Main memory
I/O module
I/O device
72
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.