Transcript LECT03

Operating Systems I
An Introduction
to
Operating System
Concepts
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OS Introduction
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Computer applications today require a
single machine to perform many
operations and the applications may
compete for the resources of the machine.
This demands a high degree of
coordination
This coordination is handled by system
software known as the operating system
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Evolution of Operating System
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OS for batch jobs
o Program execution required significant
preparation of equipment
o Program execution (job)
o OS was a system to simplify program setup
and simplify transition between jobs
o Physical separation of users and
equipment led to computer operators
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OS Evolution (cont’d)
o Users left jobs with the operator and came
back the next day (batch jobs)
o Users had no interaction with computer
during program execution. Maybe okay for
some applications, but not for all.
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OS Evolution
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OS for Interactive Processing
o Allowed programs to carry on dialogue with
user via remote terminals (workstations)
o Real-time processing
o Users demand timely response
o Machines too expensive to serve only one
user
o Common for several users to want
interactive services at the same time
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OS Evolution (cont’d)
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OS for time-sharing
o To accommodate multiple real-time users,
the OS rotates its various jobs in and out of
execution via time-sharing
o Each job gets a predetermined “time slice”
o At end of time slice current job is set aside
and a new one starts
o By rapidly shuffling jobs, illusion of several
jobs executing simultaneously is created
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OS Evolution (cont’d)
o Without time slicing, a computer spends
most of its time waiting for peripheral
devices or users
o A collection of tasks can be completed in
less time with time-sharing than when
completed sequentially
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Different Operating Systems on the
Same Machine ?
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It is possible to have more than one
operating system available to be used
on a machine.
Only one operating system is run at a
time, though.
Examples:
o VAX -- VMS or Ultrix
o PCs -- DOS, Windows, or Linux
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Types of software
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Applications software
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Performs tasks specific to the machine’s
utilization.
System Software
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Performs tasks common to computer
systems in general
Operating systems vary based on the
hardware they’re used on
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Types of software (cont’d)
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Utility software
o Provides fundamental activities, yet not
included with OS
o “Extends” the OS
o Distinction between applications and
utilities is often vague
o Distinction between OS and utilities is also
vague
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The OS Shell
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Defines interface between OS and
users
o Windows GUI
o UNIX command line
o UNIX users can choose among a variety of
shells
•csh is the “C shell”
•tcsh is an enhanced “C shell”
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OS Shell interface
Users
Users
O/S
Users
shell
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The OS Kernel
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The internal part of the OS is often
called the kernel
Kernel Components
o File Manager
o Device Drivers
o Memory Manager
o Scheduler
o Dispatcher
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OS File Manager
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Maintains information about the files
that are available on the system
Where files are located in mass storage,
their size and type and their protections,
what part of mass storage is available
Files usually allowed to be grouped in
directories or folders. Allows
hierarchical organization.
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OS Device Drivers
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Software to communicate with
peripheral devices or controllers
Each driver is unique
Translates general requests into specific
steps for that device
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OS Memory Manager
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Responsible for coordinating the use of
the machine’s main memory
Decides what area of memory is to be
allocated for a program and its data
Allocates and deallocates memory for
different programs and always knows
what areas are free
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OS Scheduler
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Maintains a record of processes that are
present, adds new processes, removes
completed processes
o memory area(s) assigned
o priority
o state of readiness to execute (ready/wait)
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OS Dispatcher
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Ensures that processes that are ready
to run are actually executed
Time is divided into small (50 ms)
segments called a time slice
When the time slice is over, the
dispatcher allows scheduler to update
process state for each process, then
selects the next process to run
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OS Summary
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Shell -- interface to user
File Manager -- manages mass memory
Device Drivers -- communicate with
peripherals
Memory Manager -- manages main
memory
Scheduler & Dispatcher -- manage
processes
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Utilities
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Operating systems usually come with
some associated utility programs
UNIX usually has the text editors emacs
and vi (and sometimes pico)
UNIX has its own sort utility
UNIX has its own mail utility
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