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Chapter 3
Operating Systems
Chapter 3 Operating Systems
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3.1 The Evolution of Operating Systems
3.2 Operating System Architecture
3.3 Coordinating the Machine’s
Activities
3.4 Handling Competition Among
Processes
3.5 Security
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Some functions of an
operating system
Control and Oversee operation of
computer
 Store and retrieve files
 Schedule programs for execution
 Execute programs
Ex:Microsoft
Linux(By Linus Torvalds)Open sourcce
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Evolution of shared computing
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Single-Processor Systems:
Program execution of earlier computers requires significant preparation
of equipments.
Mounting, placing the punched cards, setting the switches, and so on.
Each program execution is referred to as a job, an isolated activity.
The next user only gains the full control of the computer after the
completion of the current user. [sequentially]
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Batching processing executes a batch of collected jobs
pending in the job queue(first-in, first-out) (FIFO) without further
interaction with users.
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Evolution of shared computing
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Single-Processor Systems:
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Interactive processing carries on a dialogue with the user.
--It involves real-time processing for the machine to interact with its
environment.
--Reservation systems, word processing, computer games.
To serve more than one user or job at a time, time-sharing technique
divides time into time intervals or time slices, each designated to one
task.
--Time-sharing used in single-user systems is usually called
multitasking.
--Tradeoffs:
The overhead to rapidly shuffle the jobs to gain the illusion of
concurrent execution of several jobs.
The time wasted to await the completion of peripheral devices or
user’s inputs.
Multiprocessor Systems: With networks like the Internet, many small5
machines can flexibly collaborate for computing, in contrast to an
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Evolution of shared computing
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Batch processing
Job queue
Time-sharing
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Multi-tasking: multiple tasks for a single
user
Interactive processing
Real-time processing
Scheduling multiprocessor machines
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Figure 3.1 Batch processing
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Figure 3.2 Interactive processing
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Types of software
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Application software
 perform specific tasks for users spreadsheets,
database systems, word processing systems,
program development software, games, and so
forth.
 System software:
systems
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Operating system
software
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Utility software
the OS for simplicity.
perform tasks needed by all computer
abstracts the underlying hardware for the rest of
performs fundamental activities, yet not included in
Disk formatting, file copying, dial-up, compression,
etc.
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Figure 3.3 Software classification
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Components of an
operating system
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Shell: portion that communicates with users
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Graphical user interface (GUI)
pictorially represents the
objects on the screen, and accept the commands like mouse-clicking for
execution.
Textual shells(Borne shell, Korn shell, C shell, TC
shell, MS-DOS) is light weighted only accepting
the commands issued by a keyboard.
Window manager is the component to govern the
screen layout of GUI
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Components of an
operating system
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Kernel: contains components
performing basic required functions
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File manager--coordinates the use of mass storage
facilities.It keeps records of all the files: location (directory, folder,
sub-directory, path), access permission and history (file descriptor),
free available space, etc.
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Device drivers--are installed into the kernel to drive the
corresponding peripheral devices using encapsulated activation
steps specified for each device.
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Memory manager
Scheduler and dispatcher
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Components of an
operating system
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Scheduler--According to some priorities or concerns, it
determines which activities should be considered for execution.
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Dispatcher--It administrates the allocation of time slices to
the activitiesbeing executed.
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Figure 3.4 The shell as an interface
between users and
the operating system
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File Manager
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Directory or folder: user-created group
or bundle of files
Path: position of a file in directory
hierarchy
File descriptor: information needed to
access an open file
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Memory manager
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Coordinate the machine’s use of main
memory
Total main memory required exceeds
available(I.e., 512MB is requires but only
256MB available), it creates the illusion of
additional memory space by rotating back
and forth of program and data between main
memory and mass storage
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Memory manager
Page: divide the required space into units
called page (a few kilobytes) and store
the content of the page in mass storage
 Virtual memory: illusionary memory
space
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Created by shuffling units of data, called
pages, between actual main memory space
and mass storage
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Getting it started (bootstrapping)
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Bootstrap: program in read only
memory (ROM)
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Run by the CPU when power is turned on
Transfers operating system from mass
storage to main memory
Executes jump to operating system
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Figure 3.5 The booting
process
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BIOS(Basic Input/output System)
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In addition to bootstrap, read only
memory (ROM) contains a set of
routines for fundamental input/output
activities
Receive information from keyboard,
display message on screen
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Processes
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Program = a static set of directions
--A program can be run multiple times, each
instance/activity called a process.
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Process = the activity of executing a program
--Each process has its own process state,
recording the current status of the activity.
Process state = current status of the activity
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Snapshot of relevant parts of the machine state
Program counter, other registers, associated main
memory
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Process administration
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Scheduler
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Keeps state of all processes in an process
table(in main memory)
It contains—assigned memory area,
priority and ready/waiting
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Ready if its progress can continue, or waiting if
it is awaiting some external events to occur,
and so on
Priority
Non-scheduling information: memory pages,
etc.
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Process administration (continued)
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Dispatcher
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Gives one time slice or quantum to a process that is ready
Executes a process switch (or context switch) when the running
process’s time slice is over
A timer circuit counts down the quantum and generates an
interrupt at the end.
Such process switch involves updating the snapshot of the
current process, restoring the snapshot of the next process, and
then starting its execution.
A program, called interrupt handler, is run for scheduler to update
the process table, select the next process, …Early termination of
quantum is also possible.
 Interrupt indicates that time slice is over
 Interrupt handler: part of dispatcher
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Figure 3.6 Time-sharing between
process A and process B
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Handling competition for a
resource
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System resources are shared among
processes.
--Some non-shareable resources like printers should not be
simultaneously written by more than one process.
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Using a flag with the status of set(allocated) and
clear(available), there seems to be only one process allowed to
write to the very resource.
OS checks the flag and to decide set or clear(note:check and
set may require several machine cycle to finish)
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Due to process switch, two processes may just be
aware of the clear flag right before being preempted.
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During the following quantum, they will proceed to 25
Handling competition for a
resource
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Remedies: testing and setting the flag must
be completed w/o interruption. (atomic)
Semaphore = a control flag telling if resource
is in use
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Critical region = sequence of instructions that
should be executed by only one process at a
time
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Test and set must be done together for proper
function
Usually protected by a semaphore
Mutual exclusion = requirement for proper
implementation of a critical region
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Deadlock
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Two processes block each other from
continuing
Conditions that lead to deadlock
1. Competition for non-sharable resources
2. At least two resources are needed by both
processes
The resources are requested on a partial basis: having some, while
requesting more.
3. An allocated resource can not be forcibly retrieved
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Deadlock
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Deadlock ⇔cyclic wait-for graphs in the
process table.
The super-users may kill some processes for
the rest to be finished.
Deadlock avoidance schemes try to alter the
first two conditions.
To avoid the 2nd condition, each process is
required to request all its resources at a time.
To prevent the 1st condition, non-shareable resources
are converted into sharable ones.
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Figure 3.7 A deadlock resulting from
competition for nonshareable railroad
intersections
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Spooling
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Postpone requested operation until a later
time
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Makes a non-shareable resource appear shareable
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Technique of deadlock avoidance
Printer spooling may be used for processes to write to
their own designated disk space, instead of directly to the
printer.Then, OS sends these printouts to the printer in
series.
Multiple read accesses to a file finely coexist, but conflict
with any other write access.
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Security from outside attacks
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Most common protection: require user
name and password
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Problem: password stealing
Problem: automated password guessers
Counter measures:
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Always tell user when he/she last logged in
Report repeated bad guesses
Log the guesser into a captive account to spy
on the guesser
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Security from attacks from within
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Operating system prevents illegal
access to resources
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Different memory for different processes
Privileged instructions only allowed in
kernel
All file access passes through the kernel
Other devices can only be accessed
through the kernel
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