Computer Operating Systems
Single-user/Multi-user Operating Systems
Dr. E.C. Kulasekere
University of Moratuwa
IEE Sri Lanka
Chapter 1 Expectations
Define what an operating system is.
List at least four functions performed by
an operating system.
Briefly explain the difference between a
general purpose operating system and a
dedicated operating system.
classify software according to either
business, operating system, utility,
education or entertainment categories
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Dr. E.C. Kulasekere
IEE Sri Lanka
Chapter 1 Expectations (Cont …)
Identify input output devices.
Explain the difference between a singleuser operating system and a multi-user
operating system.
Identify some of the available operating
systems.
Classify common operating systems as
single-user or multi-user.
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Dr. E.C. Kulasekere
IEE Sri Lanka
What is an operating system?
is a collection of system programs that
together control the operation of a
computer system (?).
The OS is based on particular hardware.
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IEE Sri Lanka
Abstract view of system components
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What does an operating system do?
The OS controls the way the the Computer
Systems functions.
initialize the hardware of the computer
system.
provide basic routines for device control
provide for the management, scheduling and
interaction of tasks
maintain system integrity and handle errors.
Maintains system clean up functions.
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IEE Sri Lanka
Where are operating systems found?
The complexity of the OS depends on the
type of functions that they provide.
Some OSs are meant for managing networks.
Some are found in petrol pumps.
Some are found in electronic items in homes.
Some are found in cars.
Embedded devices are some examples as
well.
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IEE Sri Lanka
What are General purpose/Dedicated OSs?
Windows operating systems.
Many of the Unix operating systems.
Embedded systems are not GP OSs.
Firmware is also not GP.
The characteristic of a GP OS is its
ability to run many software on an
application platform.
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Dr. E.C. Kulasekere
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Some available operating systems.
MS-DOS
Windows 9x
Windows NT
Windows XP
Windows 200x
Unix – Solaris
Unix - Linux
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Example OS: A security control
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What support utilities are
required by an Operating
System to perform its
tasks?
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What are programs?
A set of instructions for a specific
platform?
Byte code? Machine code?
Programming languages (Compilers)
Application software
Special purpose software
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IEE Sri Lanka
What are input and output devices?
These are devices controlled by the
operating system.
Commonly found in most computer
systems.
Input devices: Keyboard, mouse tablet,
sensors.
Output devices: monitors, printers
speakers, actuators.
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Dr. E.C. Kulasekere
IEE Sri Lanka
What is a single user operating system?
All windows operating systems fall into
this category.
In essence, a single-user operating
system provides access to the computer
system by a single user at a time.
If another user needs to access the
computer resources, he/she has to wait
until the current user departs.
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Dr. E.C. Kulasekere
IEE Sri Lanka
What is a multi-user operating system?
It lets more than one user to access the
computer system resources at one time.
Access is provided via networks.
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IEE Sri Lanka
Multi-user ?
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Issues raised by multi-user OSs.
System manages multiple requests to
same resource concurrently.
Should avoid deadlocks.
Serial equipment (printers) hooked up
to the multi-user OS should be queued.
The OS simulates real time performance
by task switching.
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IEE Sri Lanka
Operating system utilities
The OS is stored in the hard disk.
The part relevant is loaded into RAM at the
time of execution.
Some of the utility programs of the OS are
Managing files and documents
Development of programs and software
Communicating with other computer
systems.
Managing user requirements for programs,
storage space and priority.
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Operating system interfaces
This can be a basic command
interpreter (SHELL) like the dos prompt
or a Unix command window.
Shell is also called the command line
interpreter (CLI).
The interface can be graphical as well.
X-Windows for Unix.
Windows GUI
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Advantages and problems of multi-user OSs
Advantages
Expensive hardware can be shared.
Better utilization of resources and less idle time for
each resource.
Problems
As number of users increase the performance
degrades.
Some resources such as HD space can be
expensive since a lot is needed.
The software is more complicated making it
expensive.
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IEE Sri Lanka
What is a multi-tasking OS?
Ability to run more than one program at
a time.
A requirement of a multi-user system.
Simulates real time (Video games)
Increases productivity but also
increases coordination complexity.
More resources required.
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What are application software?
Software is another word for program.
Category of software that runs on an
operating system platform.
The operating system and nay
associated software does not fall into
the category of application software.
Enhances the utility value of the
operating system. Types: Business,
Education, Entertainment, Utility.
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IEE Sri Lanka
Research
Artwork
Air control
Fishing, coastal data
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IEE Sri Lanka
Application Software (Cont …)
Hair dryer,
Washing machine,
Kettle, ...
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Summary of Definitions
Whether multiple programs can
run on it simultaneously: multitasking or Single-tasking if not
Whether it can take advantage of
multiple processors: multiprocessing
Whether multiple users can run
programs on it simultaneously:
multi-user
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IEE Sri Lanka
Summary of definitions (Cont …)
Whether it can reliably prevent
application programs from directly
accessing hardware devices:
protected
Whether it has built-in support for
graphics: GUI Based
Whether it has built-in support for
networks: Networked
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IEE Sri Lanka
More classifications
Real-Time Operating System (RTOS):
used to control machinery, scientific
instruments and industrial systems
Single-User, Single Task: to manage the
computer so that one user can
effectively do one thing at a time. Palm
OS is an example
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IEE Sri Lanka
More classifications (Cont …)
Single-User, Multi-Tasking: use on their
desktop and laptop operating systems
today. Examples are windows based OS
Multi-User: allows many different users
to take advantage of the computer's
resources simultaneously. Example can
be UNIX.
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Dr. E.C. Kulasekere
IEE Sri Lanka
It's important to differentiate here between multi-user
operating systems and single-user operating systems that
support networking.
Windows 2000 and Novell Netware can each support
hundreds or thousands of networked users, but the
operating systems themselves aren't true multi-user
operating systems
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Dr. E.C. Kulasekere
IEE Sri Lanka
Popular OS classification
Unix: multi-tasking, multi-processing, multiuser, protected, with built-in support for
networking but not graphics.
Windows NT: multi-tasking, multi-processing,
single-user, protected, with built-in support for
networking and graphics.
Windows 95/98: multi-tasking, multiprocessing, single-user, unprotected, with
built-in support for networking and graphics.
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Dr. E.C. Kulasekere
IEE Sri Lanka
Popular OS classification (Cont …)
Windows 3.x: single-tasking, singleprocessing, single-user, unprotected,
with built-in support for graphics but
not networking.
DOS: single-tasking, single-processing,
single-user, unprotected with no built-in
support for graphics or networking.
NetWare: multi-tasking, multiprocessing, single-user, unprotected,
with built-in support for networking but
not graphics.
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IEE Sri Lanka
UNIX: Most popular multi user OS
Solaris, from Sun Microsystems.
AIX, from International Business Machines (IBM).
Digital Unix, from Compaq (which purchased Digital
Equipment).
IRIX, from Silicon Graphics (SGI)
HPUX, from Hewlett-Packard.
SCO, from the Santa Cruz Operation.
FreeBSD, maintained by a group of individuals.
OpenBSD, maintained by a group of individuals with a single
coordinator.
NetBSD, maintained by a Foundation incorporated in the state
of Delaware.
Linux, maintained by a loosely coordinated group of
volunteers.
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Dr. E.C. Kulasekere
IEE Sri Lanka
NT Based: Most popular single user OS
Developed and promoted by Microsoft.
It is multi-tasking
Multi-processor
Network-aware
Used to provide:
Networked file system (NFS)
Printer sharing
Run shared application software.
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Dr. E.C. Kulasekere
IEE Sri Lanka
Chapter 2 Expectations
list four typical parts (modules) of an
operating system.
state three functions performed by a realtime executive.
explain the difference between pre-emptive
and co-operative switching.
discuss the relationship between system
performance and system overhead.
identify the essential operations that need to
be performed in order to switch from one
program to another.
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Dr. E.C. Kulasekere
IEE Sri Lanka
Chapter 2 Expectations (Cont…)
define the terms context switching,
system overhead, time-slice, quantum
period and scheduling.
distinguish between first-in first-out and
round robin scheduling
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Dr. E.C. Kulasekere
IEE Sri Lanka
What are the various parts of an OS
Real time executive (kernel) is the part of the
OS that is responsible for running and
coordinating programs.
Modularizing the OS helps coordination of
programs in a multi-user environment. Typical
modules:
Kernel
Process manager
Scheduler
File manager
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Dr. E.C. Kulasekere
IEE Sri Lanka
What are the various parts of an OS (Cont …)
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applications
software
utilities
systems
software
Operating system
hardware
components
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CPU
device
device
...
device
memory
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Dr. E.C. Kulasekere
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Kernel functions
Switching between programs.
Hardware device control and
programming.
Memory management.
Process management.
Scheduling.
Inter-process communication.
Processing of exceptions and interrupts.
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Dr. E.C. Kulasekere
IEE Sri Lanka
How do OSs run more than one
program at a time?
Single-user and multi-user OSs both can
be multi-tasking.
Single processor or multi processor
systems can be multi-tasking.
Simulating multi-tasking using
Co-operative switching.
Preemptive switching
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Dr. E.C. Kulasekere
IEE Sri Lanka
What is cooperative and preemptive switching?
Co-operative switching means that a
task that is currently running will
voluntarily give up the processor at
some time, allowing other processes to
run.
Preemptive switching means that a
running task will be interrupted (forced
to give up) and the processor given to
another waiting process.
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Dr. E.C. Kulasekere
IEE Sri Lanka
Issues related to task switching
co-operative switching: one process
could hang and thus deny execution of
other processes, resulting in no work
being done. Eg. Win3.1
Pre-emptive scheduling gives more
response to all processes and helps
prevent (or reduce the number of
occurrences of) the dreaded machine
lockup. Eg. Win NT.
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IEE Sri Lanka
Issues related to task switching (Cont …)
Only 32-bit programs in Windows 95 are
pre-emptive switched. 16-bit programs are
still co-operatively switched, which means
it is still easy for a 16-bit program to lock
up a Windows 95 computer.
Advantages gained out weigh the
disadvantages associated with switching.
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Dr. E.C. Kulasekere
IEE Sri Lanka
Chapter 3 Expectations
describe how an operating system is
initially loaded.
given a set of characteristics, determine
the type of operating system as batch,
real-time, time-sharing, interactive or
embedded.
state an example of a shell
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Dr. E.C. Kulasekere
IEE Sri Lanka
How does an OS load itself?
It can be loaded in two ways:
it is already present in ROM (so is
permanent, immediately accessible and
difficult to update). Eg. Petrol pumps,
industrial controllers. Best suited for small
and simple OS.
it is loaded from disk to RAM when the
computer is turned on. Eg. Win 9x. Easier
to update and manipulate.
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IEE Sri Lanka
What is the bootstrap process?
describes the task of initially loading the
operating system from disk into RAM.
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IEE Sri Lanka
What are the different types of OSs.
Batch
Inter-active
Time-sharing/multi-user
Multi-tasking
Real-time
Multi-processor
Embedded
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Dr. E.C. Kulasekere
IEE Sri Lanka
What is a shell?
A shell is a program that handles user
input and output
It provides routines for handling user
input from a keyboard or mouse
It provides routines for displaying
information on the terminal screen.
A shell also provides a mechanism to
interpret user commands and run
additional programs that users request
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Dr. E.C. Kulasekere
IEE Sri Lanka
Chapter 4 Expectations
define the terms file, cluster, inode, symbolic
link
describe a typical structure for the root file
system
list four typical entries for a root file entry
explain how free space might be managed
list the file systems supported by Windows
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Dr. E.C. Kulasekere
IEE Sri Lanka
Chapter 4 Expectations (Cont …)
describe what access or file control
permissions are and why they might be
needed
explain how the file space may become
fragmented, what effect this might have
on performance, and how the problem
might be corrected
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IEE Sri Lanka
File management
What is a file?
What are the typical operations
performed on files?
How does the above operations
different for single and multi user OS s?
What are the security risks associated
with file access?
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What are file control blocks?
FCB is also called the file descriptor is a
data structure.
The FCB consists of the following:
Filename
Location of file on secondary storage
Length of file
Date and time or creation or last access
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IEE Sri Lanka
Naming files
MS-DOS Uses eight character file names, a
dot, then a three-character extension that
denotes the type of file. Filenames are not
case-sensitive.
UNIX Filenames can be up to 254 characters
long and are case-sensitive
Windows Filenames can be up to 255
characters long and are not case-sensitive
File types? Doc,txt and associated programs.
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IEE Sri Lanka
How does an OS keep track of files?
Is it different for single user and multi
user systems?
How does hardware such as HDs play a
role here?
As files are created, free sectors are
allocated to hold the file contents and
marked as allocated.
Tables will keep track of the used
sectors.
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IEE Sri Lanka
What is a root file system?
The root file system is a table of entries
like a directory
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IEE Sri Lanka
How is free space managed?
Free space is unused clusters.
Clusters can come in different sizes.
How does sector size affect the free
space available when the disk is in use?
What is a linked list of clusters?
What are FAT, FAT32 and NTFS file
systems?
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Access control and file permissions
In multi-user operating systems, files
may be accessed by multiple users
Permission rights associated with
folders (directories) and files are used
to protect or restrict access to files
In UNIX this is a rwx format while in
NTFS more permissions are available.
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Other information
What is a symbolic link or short cut?
How are they used in multi-user OS s?
What is file system integrity?
How can it be checked?
Are they the same for UNIX and
WINDOWS?
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IEE Sri Lanka
What is fragmentation?
An OS reads data from HD in contiguous clusters
more efficiently and faster than from non-contiguous
clusters.
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IEE Sri Lanka
Chapter 5 Expectations
List the system requirements for
Windows 95, Windows NT and Windows
2000 Professional
State the general features of Windows
operating systems
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Hardware requirements
System requirements for Windows 95
386DX or higher
4MB memory or higher [8MB recommended]
35-40MB disk space
3.5" floppy drive or CD-ROM
VGA or higher resolution graphics card
System requirements for Windows 98
486DX/66MHz or higher
16MB memory or higher
195MB disk space
CD-ROM
VGA or higher resolution graphics card
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IEE Sri Lanka
Hardware requirements (Cont …)
System requirements for Windows NT Workstation 4.0
16 MB RAM Recommended
486/25MHz or higher processor
110 MB available hard-disk space
VGA, Super VGA, or video graphics adapter
CD-ROM drive.
System requirements for Windows 2000 Professional
133MHz Pentium compatible processor
64MB RAM
2GB Hard disk with 650MB available free space
Single or dual processor systems
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Dr. E.C. Kulasekere
IEE Sri Lanka
What to look for in the previous slides
The resource requirements increase.
The resource requirements will be higher for
a multiple user system.
The resource constraints are not a problem in
UNIX multi-user systems.
Increase in efficiency of operation as the
operating system improves.
The graphical requirements as the operating
system improves hog a lot of resources.
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IEE Sri Lanka
More advance topics
in Operating
Systems
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IEE Sri Lanka
Chapter 6 Expectations
explain what a process is and how an
operating system manages each
process
identify the relationships between the
three process states
list four items associated with a process
control block
define the terms thread, process,
critical section and semaphore
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Dr. E.C. Kulasekere
IEE Sri Lanka
Chapter 6 Expectations (Cont …)
illustrate how access to a shared
resource can be controlled using a
semaphore
discuss the problems associated with
critical sections
explain the difference between a
foreground and a background task
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IEE Sri Lanka
What is a process?
A process or task is a portion of a program in
some stage of execution.
A process can be periodically communicating
with each other as well.
Each process has a unique PCB.
The process priority is used to determine how
often the process receives processor time.
How does this scheduling occur? Or priority
increase?
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IEE Sri Lanka
What about process states?
Some possible states are:
ready - if it can run when the processor
becomes free.
running - it currently has the processor.
blocked - it cannot run when the processor
becomes free
Running processes are interrupted to
change state. The PCB is saved to recall
it back.
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Dr. E.C. Kulasekere
IEE Sri Lanka
What is a PCB?
The data structure for a PCB contains
the location in memory of where the
process is.
the priority of the process
a unique process identification number
(called PID)
the current process state (ready, running,
blocked)
associated data for the process
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IEE Sri Lanka
Process synchronization
Problems associated with
synchronization.
Semaphore method.
Critical section method.
Use of synchronization in single user
systems and multi user systems.
Deadlock.
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