Modern operating systems contain features from batch, multiprogramming, time-sharing, real-time, and distributed operating systems. A key aspect of operating systems is managing programs, resources, and security/protection to achieve high system performance and good user service. The document discusses several fundamental concepts related to operating systems, including the CPU state, memory hierarchy, I/O organization, interrupts, and system calls.

1. Chapter 2: Overview of
Operating Systems
Dhamdhere: Operating Systems—
A Concept-Based Approach , 2 ed
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Copyright © 2008
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2. Chapter 2: Overview of
Operating Systems
Dhamdhere: Operating Systems—
A Concept-Based Approach , 2 ed
Slide No: ‹#›
Copyright © 2008
Overview of operating systems
• A modern OS contains a very large number of features
– Many of these features were first introduced in different classes
of operating systems, namely
* Batch Processing operating systems
* Multiprogramming operating systems
* Time sharing operating systems
* Real time operating systems
* Distributed operating systems
– These features are used in a modern OS as well
* We study these features in the context of the relevant OS
* Study their influence on system performance and user service

3. Chapter 2: Overview of
Operating Systems
Dhamdhere: Operating Systems—
A Concept-Based Approach , 2 ed
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Recap from Chapter 1
• The fundamental goals of an operating system
– Obtain
* High system performance
* Good user service
within a computing environment
– A computing environment consists of a computer system, its
interfaces with other systems, and user computations
* Measures of system performance and user service depend on the
computing environment

4. Chapter 2: Overview of
Operating Systems
Dhamdhere: Operating Systems—
A Concept-Based Approach , 2 ed
Slide No: ‹#›
Copyright © 2008
Recap from Chapter 1
• Fundamental tasks of an OS
– Management of Programs
* Organize their execution by sharing the CPU
* Ensure good user service
– Management of Resources
* Fast allocation and de-allocation without constraining user programs
* Efficient use of resources
– Security and Protection
* Ensure absence of interference with programs and resources by
entities within and outside the OS

5. Chapter 2: Overview of
Operating Systems
Dhamdhere: Operating Systems—
A Concept-Based Approach , 2 ed
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Introduction
In this chapter, we shall first study:
• Fundamentals of OS operation
– Features of a computer that are important for an OS
– How the OS controls execution of programs
– How a program interacts with an OS
• Efficiency, system performance and user convenience
– Measures of system performance
– User service as a measure of user convenience

6. Chapter 2: Overview of
Operating Systems
Dhamdhere: Operating Systems—
A Concept-Based Approach , 2 ed
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OS and the Computer System
• In this module, we study
– Fundamental features of computer systems that are important to
an OS
* Memory hierarchy
* Interrupt structure
* I/O organization
– How an OS uses these features to control operation of an OS
– Fundamentals of how a program interacts with an OS

7. Chapter 2: Overview of
Operating Systems
Dhamdhere: Operating Systems—
A Concept-Based Approach , 2 ed
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Memory utilization during operation of an OS
• The kernel is the core of the OS; it is memory resident at all times
• Non-kernel programs are loaded in the transient area when needed
• Rest of the memory is shared between user programs

8. Chapter 2: Overview of
Operating Systems
Dhamdhere: Operating Systems—
A Concept-Based Approach , 2 ed
Slide No: ‹#›
Copyright © 2008
Model of a computer system
• The CPU and the I/O subsystem have independent paths to memory
• The memory management unit plays a part in implementing virtual memory

9. Chapter 2: Overview of
Operating Systems
Dhamdhere: Operating Systems—
A Concept-Based Approach , 2 ed
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Copyright © 2008
The program status word (PSW)
• The CPU contains two kinds of registers
– User accessible registers
* Used for arithmetic or logical operations, addressing, indexing, etc.
These are called general purpose registers, or simply, CPU
registers
– Control registers
* Condition code register (also called the flags register), program
counter, etc.
* Their contents govern operation of the CPU
• Control registers are collectively called the program
status word (PSW)
– Individual control registers are considered as fields of the PSW

10. Chapter 2: Overview of
Operating Systems
Dhamdhere: Operating Systems—
A Concept-Based Approach , 2 ed
Slide No: ‹#›
Copyright © 2008
Fields of the Program Status Word (PSW)
• The ‘privileged mode’ field indicates whether the CPU is in the privileged
mode, which is used by the OS, or user mode
* Some instructions can be executed only when CPU is in the privileged mode
• Condition code and Program counter registers were mentioned before
• Other registers are discussed later

11. Chapter 2: Overview of
Operating Systems
Dhamdhere: Operating Systems—
A Concept-Based Approach , 2 ed
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Copyright © 2008
Privileged mode of CPU
• The CPU can operate in two modes. The privileged
mode (P) field of PSW controls the CPU mode
– Privileged mode (`P’ field contains 1)
* Certain sensitive instructions can be executed only when the CPU is
in this mode
 For example, initiation of an I/O operation, setting protection
information for a program
 These instructions are called privileged instructions
– User mode
* Privileged instructions cannot be executed when the CPU is in this
mode

12. Chapter 2: Overview of
Operating Systems
Dhamdhere: Operating Systems—
A Concept-Based Approach , 2 ed
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Copyright © 2008
State of the CPU
• The state of the CPU consists of
– The PSW and contents of the general purpose registers of the
CPU
– It indicates
* What the CPU is engaged in doing at any moment
* What the CPU can do, in terms of privileges to perform certain
operations
– The OS keeps track of the state of the CPU to know what the
program currently executing on the CPU is doing

13. Chapter 2: Overview of
Operating Systems
Dhamdhere: Operating Systems—
A Concept-Based Approach , 2 ed
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State of the CPU
(a) An assembly language program
(b) CPU state of the program after executing the COMPARE instruction
– 0 in the P field indicates that the CPU is in the user mode
– 150 in the PC field indicates that the CPU is about to execute the
instruction with address 150

14. Chapter 2: Overview of
Operating Systems
Dhamdhere: Operating Systems—
A Concept-Based Approach , 2 ed
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Copyright © 2008
Memory hierarchy
• The memory hierarchy is a cost-effective method of
obtaining a large and fast memory
– It is an arrangement of several memories with different access
speeds and sizes
* The CPU accesses only the fastest memory; i.e., the cache
* If a required byte is not present in the memory being accessed, it is
loaded there from a slower memory

15. Chapter 2: Overview of
Operating Systems
Dhamdhere: Operating Systems—
A Concept-Based Approach , 2 ed
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Memory hierarchy
• Cache memory is the fastest and disk the slowest in the hierarchy
• The CPU accesses only the cache memory
• If required data or instruction is not present in the cache, it is loaded there
from memory (if not present in memory, it is first loaded from disk)

16. Chapter 2: Overview of
Operating Systems
Dhamdhere: Operating Systems—
A Concept-Based Approach , 2 ed
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Memory hierarchy
• Cache memory
– Organization
* A cache block or cache line is loaded from memory when some
byte in it is referenced
* A ‘write-through’ arrangement is typically used to update memory
– The cache hit ratio (h) indicates what percentage of accessed
bytes were already present in cache
* The cache hit ratio has high values because of
 Temporal locality
 Spatial locality
* Effective memory access time =
h x access time of cache memory +
(1 – h) x (time to load a cache block + access time of cache memory)

17. Chapter 2: Overview of
Operating Systems
Dhamdhere: Operating Systems—
A Concept-Based Approach , 2 ed
Slide No: ‹#›
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Memory hierarchy
• Main memory
– Memory protection prevents access to memory by an
unauthorized program
* Memory bound registers indicate bounds of the memory allocated to
a program (see next slide)
• Virtual memory
– The part of memory hierarchy consisting of the main memory
and a disk is called virtual memory
* A program and its data are stored on the disk
* Required portions of the program and its data are loaded in memory
when accessed

18. Chapter 2: Overview of
Operating Systems
Dhamdhere: Operating Systems—
A Concept-Based Approach , 2 ed
Slide No: ‹#›
Copyright © 2008
Memory protection using bound registers
• The lower bound register (LBR) and upper bound register (UBR) contain
addresses of first and last bytes allocated to the program
• LBR, UBR are stored in the memory prot info (MPI) field of the PSW
• The CPU raises an interrupt if an address is outside the LBR–UBR range

19. Chapter 2: Overview of
Operating Systems
Dhamdhere: Operating Systems—
A Concept-Based Approach , 2 ed
Slide No: ‹#›
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Input / Output organization
• An I/O operation slows down a program’s execution due
to mismatch of CPU and I/O speeds
– Involvement of the CPU in I/O operations should be the minimum
possible
* CPU should be free to execute instructions while I/O operations are
in progress
– Different I/O modes
* Programmed I/O
* Interrupt I/O
* Direct memory access (DMA)

20. Chapter 2: Overview of
Operating Systems
Dhamdhere: Operating Systems—
A Concept-Based Approach , 2 ed
Slide No: ‹#›
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Input / Output modes
• Details of the Input/output modes
– Programmed I/O
* Data transfer between memory and an I/O device takes place
through the CPU
 CPU cannot perform any other operation until I/O completes
– Interrupt I/O
* An I/O instruction starts an I/O operation and frees the CPU to
execute other instructions
 An interrupt is raised every time a unit of data is to be
transferred between memory and the I/O device
 An interrupt processing program in the kernel actually transfers
the data

21. Chapter 2: Overview of
Operating Systems
Dhamdhere: Operating Systems—
A Concept-Based Approach , 2 ed
Slide No: ‹#›
Copyright © 2008
Input / Output modes
• Details of the Input/output modes (contd)
– Direct memory access (DMA)
* An I/O instruction indicates the operation to be performed and the
number of bytes of data to be transferred. Its execution starts the
I/O operation
 Data transfer is coordinated by the DMA controller; it does not
involve the CPU
 When the I/O operation completes, the DMA controller raises
an I/O interrupt to indicate its completion (interrupts are
discussed later)
 CPU is free to execute other instructions while an I/O operation
is in progress

22. Chapter 2: Overview of
Operating Systems
Dhamdhere: Operating Systems—
A Concept-Based Approach , 2 ed
Slide No: ‹#›
Copyright © 2008
Interrupts
• An interrupt signals the occurrence of an event to the
CPU
– An event is a situation that requires OS intervention
– At an interrupt, the interrupt action in the hardware diverts the
CPU to execution of an interrupt processing routine (IPR)
* The IPR is a routine in the OS kernel
* It handles the situation that caused the interrupt
* After the IPR, kernel switches the CPU to execution of a user
program
– Different classes of interrupts convey occurrences of different
kinds of events

23. Chapter 2: Overview of
Operating Systems
Dhamdhere: Operating Systems—
A Concept-Based Approach , 2 ed
Slide No: ‹#›
Copyright © 2008
Classes of interrupts
• Three important classes of interrupts are
– Program interrupt
* Caused by conditions within the CPU during execution of an
instruction; e.g.,
 Occurrence of an arithmetic overflow, addressing exception, or
memory protection exception
 Execution of the software interrupt instruction
– I/O interrupt
* Indicates completion of an I/O operation
– Timer interrupt
* Indicates that a specified time interval has elapsed

24. Chapter 2: Overview of
Operating Systems
Dhamdhere: Operating Systems—
A Concept-Based Approach , 2 ed
Slide No: ‹#›
Copyright © 2008
The interrupt action
1. The interrupt code is formed depending on the cause of the interrupt;
it is stored in the interrupt code (IC) field of the PSW
2. The PSW is saved in a memory location (or in the stack)
3. Contents of an appropriate interrupt vector are loaded in the PSW;
new contents of the PSW now control CPU operation

25. Chapter 2: Overview of
Operating Systems
Dhamdhere: Operating Systems—
A Concept-Based Approach , 2 ed
Slide No: ‹#›
Copyright © 2008
Steps in interrupt action
• The interrupt action consists of three steps
1. Set interrupt code in the PSW
* The code describes the situation that caused the interrupt
* It is stored in the interrupt code field of the PSW
2. Save the PSW
* PSW is saved in a memory location
* Interrupt code gets saved as a part of the PSW
3. Load interrupt vector
* Interrupt vector of the occurred interrupt is chosen
 It contains address of the interrupt processing routine (IPR)
* It is loaded in the PSW
 Control is transferred to the IPR

26. Chapter 2: Overview of
Operating Systems
Dhamdhere: Operating Systems—
A Concept-Based Approach , 2 ed
Slide No: ‹#›
Copyright © 2008
Interrupt driven operation of a kernel
• At the occurrence of an interrupt, the CPU is diverted to the interrupt
processing routine (IPR), which is a part of the kernel
• The IPR handles the interrupt by performing appropriate actions
• The IPR passes control to the scheduler, which selects a program
for execution

27. Chapter 2: Overview of
Operating Systems
Dhamdhere: Operating Systems—
A Concept-Based Approach , 2 ed
Slide No: ‹#›
Copyright © 2008
Servicing of an I/O
interrupt and return to
the same user
program
User program
→ IPR for I/O interrupt
→ Scheduler
→ User program

28. Chapter 2: Overview of
Operating Systems
Dhamdhere: Operating Systems—
A Concept-Based Approach , 2 ed
Slide No: ‹#›
Copyright © 2008
Nested interrupts
• An interrupt may occur while a previous interrupt is being
processed
– The interrupt action would divert the CPU to handle the new
interrupt. On completion of its processing, processing of the
previous interrupt would be resumed
* It may delay processing of the previous interrupt
– The interrupt mask (IM) indicates whether a new interrupt should
be allowed to occur while a previous interrupt is being serviced
* Each bit in the mask controls masking of one class of interrupts
 If an interrupt is masked off: it remains pending
 If an interrupt is enabled: it can occur

29. Chapter 2: Overview of
Operating Systems
Dhamdhere: Operating Systems—
A Concept-Based Approach , 2 ed
Slide No: ‹#›
Copyright © 2008
Simple and nested interrupt processing
(a) A single interrupt occurs and is serviced
(b) A nested interrupt occurs. Processing of the previous interrupt
is suspended until the new interrupt gets handled

30. Chapter 2: Overview of
Operating Systems
Dhamdhere: Operating Systems—
A Concept-Based Approach , 2 ed
Slide No: ‹#›
Copyright © 2008
System Call
• A computer has a special instruction called a ‘software
interrupt’ instruction
– Its sole purpose is to cause a program interrupt
– Its operand becomes the interrupt code
* A program uses the software interrupt instruction to make a request
to the system
 The operand of the instruction indicates what kind of request is
being made
 Association between interrupt code and kind of request is
OS-specific
* This method of making a request is known as a system call

31. Chapter 2: Overview of
Operating Systems
Dhamdhere: Operating Systems—
A Concept-Based Approach , 2 ed
Slide No: ‹#›
Copyright © 2008
Types of system calls
• System calls are used to make diverse kinds of requests
– Resource related
* Resource request or release, checking resource availability
– Program related
* Execute or terminate a program, set or await timer interrupt
– File related
* Open or close a file, read or write a record
– Information related
* Get time and date, get resource information
– Communication related
* Send or receive message, setup or terminate connection

32. Chapter 2: Overview of
Operating Systems
Dhamdhere: Operating Systems—
A Concept-Based Approach , 2 ed
Slide No: ‹#›
Copyright © 2008
Efficiency, system performance and user
convenience
• The nature of a computing environment determines
– The nature of user computations
* Non-interactive computing environment
 Back-office applications
* Interactive computing environment
 Online applications
* Real time computing environment
 Time-critical applications
– Notions of efficiency, system performance and user service
* e.g., user service is
 Response time for interactive computing
 Deadline for real time computing

33. Chapter 2: Overview of
Operating Systems
Dhamdhere: Operating Systems—
A Concept-Based Approach , 2 ed
Slide No: ‹#›
Copyright © 2008
Computations in an OS
• We discuss four kinds of computations in an OS
– Program
* A set of functions or modules, including those from libraries
– Job
* A sequence of job steps
 Each job step is an execution of a program
 It is executed only if previous job steps executed successfully
– Process
* A process is an execution of a program
– Subrequest
* A subrequest is a computational requirement presented by a user to
a process. It produces a single response comprising of results or
actions

34. Chapter 2: Overview of
Operating Systems
Dhamdhere: Operating Systems—
A Concept-Based Approach , 2 ed
Slide No: ‹#›
Copyright © 2008
Measures of efficiency and user convenience
• We use the following measures
– Efficiency of use
* CPU efficiency
 Percent utilization of CPU
– System performance
* Throughput
 Amount of work done per unit of time
– User service
* Turn-around time
 Time between submission and completion of a job or process
* Response time
 Time between issuing and completion of a subrequest

35. Chapter 2: Overview of
Operating Systems
Dhamdhere: Operating Systems—
A Concept-Based Approach , 2 ed
Slide No: ‹#›
Copyright © 2008
Efficiency and user convenience in different
OS classes
• Service is a user-centric consideration
• Efficiency is a system-centric consideration
• Different OSs trade them off differently

36. Chapter 2: Overview of
Operating Systems
Dhamdhere: Operating Systems—
A Concept-Based Approach , 2 ed
Slide No: ‹#›
Copyright © 2008
Efficiency and user convenience in different
OS classes
• Explanation of the previous chart:
– A multiprogramming OS provides better efficiency than a batch
processing OS because it services many processes in an
overlapped manner
* However, user convenience does not change
– A time sharing system provides better response times than a
multiprogrammed system
* Hence it provides the better user convenience
– A real time OS does not provide as much efficiency as a time
sharing OS because it may reserve some resources for
exclusive use of a process

37. Chapter 2: Overview of
Operating Systems
Dhamdhere: Operating Systems—
A Concept-Based Approach , 2 ed
Slide No: ‹#›
Copyright © 2008
Batch processing system
In early operating systems, the computer operator had to
manually set-up execution of every job
• Aim of batch processing:
– To reduce operator’s intervention in processing of user jobs
* A batch is a sequence of jobs
* The operator sets up processing of a batch, rather than processing
of individual jobs
 It saves valuable time spent in human actions

38. Chapter 2: Overview of
Operating Systems
Dhamdhere: Operating Systems—
A Concept-Based Approach , 2 ed
Slide No: ‹#›
Copyright © 2008
Batch processing system
• The operator forms a batch of jobs and inserts ‘start of batch’ and
‘end of batch’ cards
• Operator initiates processing of a batch
• The batch monitor, which is a primitive OS, performs transition
between individual jobs

39. Chapter 2: Overview of
Operating Systems
Dhamdhere: Operating Systems—
A Concept-Based Approach , 2 ed
Slide No: ‹#›
Copyright © 2008
Turn-around time in a batch processing system
• ‘Turn-around time’ of a job is the time between the submission of a job
and obtaining of its results
• If results are printed after entire batch is processed, turn-around time
depends on execution time of all jobs in the batch

40. Chapter 2: Overview of
Operating Systems
Dhamdhere: Operating Systems—
A Concept-Based Approach , 2 ed
Slide No: ‹#›
Copyright © 2008
Batch processing
• Control statements are used to facilitate batch
processing
– Users insert control statements in their jobs to specify
* Start and end of a job, end of program or data
* Requirements concerning execution of a program
– The batch monitor processes the control statements to take
appropriate actions
* Initiate or terminate processing of a batch
* Initiate or terminate processing of a job
* Protect adjoining programs in the batch against interference
 If a program tries to read more data than provided, the end-of-
data card alerts the batch monitor
 Batch monitor terminates the program (see next slide)

41. Chapter 2: Overview of
Operating Systems
Dhamdhere: Operating Systems—
A Concept-Based Approach , 2 ed
Slide No: ‹#›
Copyright © 2008
Control statements in IBM 360/370 systems
• The // JOB and /& statements are start-of-job and end-of-job statements
• The // EXEC statement indicates which program is to be executed
• The /* card indicates end of the program or end of its data

42. Chapter 2: Overview of
Operating Systems
Dhamdhere: Operating Systems—
A Concept-Based Approach , 2 ed
Slide No: ‹#›
Copyright © 2008
Multiprogramming Systems
In a batch processing system, the CPU remained idle while
a program performed I/O operations
• Aim of multiprogramming:
– Achieve efficient use of the computer system through overlapped
execution of several programs
* While a program is performing I/O operations, the OS schedules
another program

43. Chapter 2: Overview of
Operating Systems
Dhamdhere: Operating Systems—
A Concept-Based Approach , 2 ed
Slide No: ‹#›
Copyright © 2008
Operation of a multiprogramming system
(a) Program 1 performs I/O and Program 2 executes on the CPU
(b) Program 2 initiates I/O, so OS switches the CPU to Program 3
(c) Program 1’s I/O operation completes, so OS switches CPU to its
execution

44. Chapter 2: Overview of
Operating Systems
Dhamdhere: Operating Systems—
A Concept-Based Approach , 2 ed
Slide No: ‹#›
Copyright © 2008
Architectural support for multiprogramming
• The computer’s architecture must contain following
features to support multiprogramming
– DMA
* To provide parallel operation of the CPU and I/O
– Interrupt hardware
* To implement the interrupt action, which passes control to the OS
– Memory protection
* To prevent corruption or disruption of a program by other programs
– Privileged mode of CPU
* CPU must be in privileged mode to execute sensitive instructions
* It is in the user mode, i.e., non-privileged mode, while executing
user programs

45. Chapter 2: Overview of
Operating Systems
Dhamdhere: Operating Systems—
A Concept-Based Approach , 2 ed
Slide No: ‹#›
Copyright © 2008
Functions of a multiprogramming kernel
• The kernel of a multiprogramming OS performs the
following functions
– Resource allocation and protection
* It uses memory protection to avoid interference between programs
– Keeping track of start and end of I/O operations through interrupt
processing, so that it can
* Initiate I/O operations
* Perform scheduling

46. Chapter 2: Overview of
Operating Systems
Dhamdhere: Operating Systems—
A Concept-Based Approach , 2 ed
Slide No: ‹#›
Copyright © 2008
Concepts and techniques of multiprogramming
• Three key concepts are
– Use a suitable program mix of CPU-bound and I/O-bound
programs
* A CPU-bound program performs computations most of the time, and
I/O operations seldom
* An I/O bound program performs I/O operations frequently
– Assign suitable priorities to programs
* Decide which program should be favoured for execution on the
CPU—an I/O-bound program or a CPU-bound program?
– Use a suitably high degree of multiprogramming
* Degree of multiprogramming = No of programs in memory
* Facilitates parallel operation of CPU and I/O system

47. Chapter 2: Overview of
Operating Systems
Dhamdhere: Operating Systems—
A Concept-Based Approach , 2 ed
Slide No: ‹#›
Copyright © 2008
Priority in a multiprogramming OS
• Priority is a tie-breaking rule used when more than one
program is ready for execution
– At any moment, the highest priority program that is not waiting
for an I/O operation to complete is chosen for execution
– Preemptive priority
* Definition: Preemption is forcible removal of a program from the
CPU
* A low priority program executing on the CPU is preempted when a
high priority program becomes ready for execution

48. Chapter 2: Overview of
Operating Systems
Dhamdhere: Operating Systems—
A Concept-Based Approach , 2 ed
Slide No: ‹#›
Copyright © 2008
Program priority in Multiprogramming OSs
The priority assignment rule for achieving better performance:
“I/O bound programs should have higher priorities than
CPU bound programs”
We shall discuss different priority assignments to I/O-bound and CPU-
bound programs, study their influence on performance, and conclude
accordingly

49. Chapter 2: Overview of
Operating Systems
Dhamdhere: Operating Systems—
A Concept-Based Approach , 2 ed
Slide No: ‹#›
Copyright © 2008
Timing chart when a CPU-bound program has
higher priority
• At time 0, the CPU-bound program is selected for execution
• It starts an I/O operation at t11, hence progiob is selected for execution
• progcb is scheduled at t13 when its I/O operation completes
progiob: I/O-bound
program
progcb : CPU-bound
program

50. Chapter 2: Overview of
Operating Systems
Dhamdhere: Operating Systems—
A Concept-Based Approach , 2 ed
Slide No: ‹#›
Copyright © 2008
A CPU-bound program has higher priority
• Observations
– CPU utilization is reasonable
– I/O utilization is poor
– Periods of concurrent CPU and I/O activities are rare
Q: Can I/O utilization be improved?
A: No! Adding an I/O-bound program does not help much because it
has a low priority

51. Chapter 2: Overview of
Operating Systems
Dhamdhere: Operating Systems—
A Concept-Based Approach , 2 ed
Slide No: ‹#›
Copyright © 2008
Timing chart when an I/O bound program has
higher priority
• progiob is selected at time 0
• It starts an I/O operation at t21, hence progcb is scheduled
• progcb is preempted when progiob’s I/O operation is completed

52. Chapter 2: Overview of
Operating Systems
Dhamdhere: Operating Systems—
A Concept-Based Approach , 2 ed
Slide No: ‹#›
Copyright © 2008
An I/O-bound program has higher priority
• Observations:
– CPU utilization is reasonable
– I/O utilization is reasonable
– Periods of concurrent CPU and I/O activities are frequent
Q: Can CPU and I/O utilization be improved?
A: Yes
* Addition of a CPU-bound program improves CPU utilization
* Addition of an I/O-bound program improves I/O utilization

53. Chapter 2: Overview of
Operating Systems
Dhamdhere: Operating Systems—
A Concept-Based Approach , 2 ed
Slide No: ‹#›
Copyright © 2008
Performance and user service in
multiprogramming
– Performance is measured as throughput
* Throughput is the number of programs serviced by the system in a
unit of time
* For high throughput, I/O-bound programs should have higher
priorities than CPU-bound programs
– User service is measured as turn-around time of a job
* Turn-around time of a job is the elapsed time between submission
of a job and its completion

54. Chapter 2: Overview of
Operating Systems
Dhamdhere: Operating Systems—
A Concept-Based Approach , 2 ed
Slide No: ‹#›
Copyright © 2008
Improving throughput in a multiprogramming
system
• How to improve throughput?
– Add a CPU-bound program, give it the lowest priority
* It uses the CPU when it would have been idle, e.g., the interval
t26-t27
* Its presence does not affect progress made by other programs
– Add an I/O-bound program, give it a high priority
* Its presence improves I/O utilization
* It may affect progress of the CPU-bound program marginally

55. Chapter 2: Overview of
Operating Systems
Dhamdhere: Operating Systems—
A Concept-Based Approach , 2 ed
Slide No: ‹#›
Copyright © 2008
Variation of throughput with degree of
multiprogramming
Q: Why does the throughput stagnate at high values of m?

56. Chapter 2: Overview of
Operating Systems
Dhamdhere: Operating Systems—
A Concept-Based Approach , 2 ed
Slide No: ‹#›
Copyright © 2008
Time sharing systems
• In interactive computing, users desire good response
times to their subrequests
– Response time to a subrequest is the time since the subrequest
is made to the time its results are obtained
– A time sharing system provides good response times to
subrequests through
* Round-robin processing of programs
* Using a time slice (δ) to prevent a program from using too much
CPU time in one go

57. Chapter 2: Overview of
Operating Systems
Dhamdhere: Operating Systems—
A Concept-Based Approach , 2 ed
Slide No: ‹#›
Copyright © 2008
A schematic of round-robin scheduling
with time slicing
• The OS maintains a list of programs that wish to execute on the CPU
• The scheduler selects the first program in the list
• If the time slice elapses before the scheduled program completes its
operation, it is preempted and put back into the scheduling list

58. Chapter 2: Overview of
Operating Systems
Dhamdhere: Operating Systems—
A Concept-Based Approach , 2 ed
Slide No: ‹#›
Copyright © 2008
Operation of a time slicing scheduler
(a) The program is preempted when the time slice elapses; i.e.,
after it has consumed δ seconds
(b) The program gives up the CPU when it starts an I/O operation;
i.e., before it has consumed δ seconds; another program is now
scheduled

59. Chapter 2: Overview of
Operating Systems
Dhamdhere: Operating Systems—
A Concept-Based Approach , 2 ed
Slide No: ‹#›
Copyright © 2008
Time sharing systems
• Relation between time slice and response time
– Choice of the time slice depends on two factors
* Response time to programs
* Overhead of switching between programs (σ)
– If every program consumes δ CPU seconds before producing a
response,
* response time is = na x (δ + σ), where na is the number of active
programs
* CPU efficiency is = δ / (δ+σ)

60. Chapter 2: Overview of
Operating Systems
Dhamdhere: Operating Systems—
A Concept-Based Approach , 2 ed
Slide No: ‹#›
Copyright © 2008
Choice of δ
• Influence of δ on response time
– If δ is very small
* A program may need more CPU time than δ to satisfy a request
 Hence a program may have to be scheduled several times
before a response can be obtained
* Efficiency is low if δ and σ are comparable
* Cache performance may be poor
– If δ is large
* Response time would be better than na x ( δ+σ )
* Efficiency would be better than δ / (δ+σ) because programs may
not use the complete time slice

61. Chapter 2: Overview of
Operating Systems
Dhamdhere: Operating Systems—
A Concept-Based Approach , 2 ed
Slide No: ‹#›
Copyright © 2008
Swapping
• Swapping is the technique of moving inactive programs
out of the memory temporarily
– It increases the number of programs that can be processed by
the time sharing system
Q: What are the conditions under which swapping is most effective?

62. Chapter 2: Overview of
Operating Systems
Dhamdhere: Operating Systems—
A Concept-Based Approach , 2 ed
Slide No: ‹#›
Copyright © 2008
A schematic of swapping
• The OS swaps out a program that is not likely to be executed immediately
in future; it frees the memory occupied by the program
• It swaps in a new program in the newly freed memory
• The swapped-out program is loaded back before its turn for execution

63. Chapter 2: Overview of
Operating Systems
Dhamdhere: Operating Systems—
A Concept-Based Approach , 2 ed
Slide No: ‹#›
Copyright © 2008
Real time Operating system
• A real time OS is used to service time critical
applications
– A time critical application is one that malfunctions if it does not
receive a ‘timely response’
* Timeliness of the response depends on the nature of the application
* A real time OS focuses on providing a timely response to an
application

64. Chapter 2: Overview of
Operating Systems
Dhamdhere: Operating Systems—
A Concept-Based Approach , 2 ed
Slide No: ‹#›
Copyright © 2008
Real time Operating system
• Two kinds of real time operating systems
– Hard real time system: Meets the response requirement of an
application under all conditions (including error recovery actions,
if any)
* Used in command and control applications
* A computer system may have to be dedicated to an application
– Soft real time system: Meets the response requirement of an
application in a probabilistic manner
* Used in applications such as multimedia and reservation systems

65. Chapter 2: Overview of
Operating Systems
Dhamdhere: Operating Systems—
A Concept-Based Approach , 2 ed
Slide No: ‹#›
Copyright © 2008
Real time operating systems
• Features of a real time OS
– Permits creation of multiple processes within an application
* Overlapped operation of processes speeds up the application
– Permits priorities to be assigned to processes
– Permits a programmer to define interrupts in an application’s
domain and provide interrupt processing routines
* Enables the application to respond readily to events
– Uses priority driven or deadline oriented scheduling
* Helps in meeting the application’s time constraints
– Provides fault tolerance and graceful degradation
* Such that essential services can be performed in a timely manner

66. Chapter 2: Overview of
Operating Systems
Dhamdhere: Operating Systems—
A Concept-Based Approach , 2 ed
Slide No: ‹#›
Copyright © 2008
A satellite data logging application
• A satellite sends data to an earth station periodically
– A computer application in the earth station receives the data
* The computer stores the data in the signal in a file
* Storing of data should be completed before the next signal arrives

67. Chapter 2: Overview of
Operating Systems
Dhamdhere: Operating Systems—
A Concept-Based Approach , 2 ed
Slide No: ‹#›
Copyright © 2008
Tasks in the satellite data logging application
• Process 1 copies the data
• Process 2 writes the data into a file
• Process 3 performs statistical analysis on the data

68. Chapter 2: Overview of
Operating Systems
Dhamdhere: Operating Systems—
A Concept-Based Approach , 2 ed
Slide No: ‹#›
Copyright © 2008
Distributed operating system
• Definition: a distributed computer system
– A distributed computer system consists of a number of computer
systems, each having its own memory and performing some of
the control functions of the OS, interacting through the network
– Each computer system is called a host or node, and the place
where it is located is called a site

69. Chapter 2: Overview of
Operating Systems
Dhamdhere: Operating Systems—
A Concept-Based Approach , 2 ed
Slide No: ‹#›
Copyright © 2008
A distributed system
• A WAN connects geographically distant nodes
• A LAN connects nodes within an office, laboratory or building

70. Chapter 2: Overview of
Operating Systems
Dhamdhere: Operating Systems—
A Concept-Based Approach , 2 ed
Slide No: ‹#›
Copyright © 2008
Distributed systems
• Benefits of a distributed system
– Resource sharing
* An application can use resources located in other computers
– Reliability
* Provides availability of resources despite faults
– Computation speed-up
* Parts of a computation can be executed simultaneously in different
computers
– Communication
* Users in different computer systems can communicate
– Incremental growth
* Cost of enhancing capabilities of a system is proportional to the
desired enhancement

71. Chapter 2: Overview of
Operating Systems
Dhamdhere: Operating Systems—
A Concept-Based Approach , 2 ed
Slide No: ‹#›
Copyright © 2008
Features of Distributed Operating Systems
• Parts of a distributed operating system execute in
different nodes of a distributed system. Its salient
features are:
– The OS provides support for distributed computations
* Remote procedure call (RPC)
* Distributed file systems
– The OS uses distributed control techniques because
* Several computers participate in a decision
* Control data may be distributed across nodes in the system

72. Chapter 2: Overview of
Operating Systems
Dhamdhere: Operating Systems—
A Concept-Based Approach , 2 ed
Slide No: ‹#›
Copyright © 2008
Classical OS concepts in Modern OSs
• A modern OS supports different kinds of users, hence it
has to fulfill diverse requirements
– It uses many (most?) of the classical concepts discussed so far
to meet different requirements
* We will see details of these in the next slide
– It uses adaptive techniques so that it can select the techniques
that best suit a specific program
* This way it can handle diverse workloads effectively

73. Chapter 2: Overview of
Operating Systems
Dhamdhere: Operating Systems—
A Concept-Based Approach , 2 ed
Slide No: ‹#›
Copyright © 2008
Classical OS concepts in Modern OSs
• A modern OS uses the following classical concepts
– Batch processing concepts
* Database queries and back-office processing are batched
– Priority-based preemptive scheduling
* To provide better service to high priority applications
* To obtain efficient use of resources
– Time slicing
* To provide good response times
– Swapping
* To increase the number of processes serviced
– Creating multiple processes in an application
* To provide timely response to real time applications
– Resource sharing across boundaries of computers