Download Operating Systems

Operation Systems Overview
Objective
Provide a grand tour of the major operating
system components and responsibilities
Operating System Concepts
1.2
Silberschatz, Galvin and Gagne ©2005
Operating Systems
 OS: A program that acts as an intermediary between a user of
a computer and the computer hardware.
 No universally accepted definition
 “Everything a vendor ships when you order an operating
system” is good approximation

But varies wildly
 “The one program running at all times on the computer” is the
kernel. Everything else is either a system program (ships with
the operating system) or an application program
 Computer system can be divided into four components

Hardware – provides basic computing resources

Operating system

Application programs

Users
Operating System Concepts
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Silberschatz, Galvin and Gagne ©2005
Four Components of a Computer System
Operating System Concepts
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Operating System Definition
 OS is a resource allocator

Manages all resources

Decides between conflicting requests for efficient and fair
resource use
 OS is a control program

Controls execution of programs to prevent errors and improper
use of the computer
Operating System Concepts
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Computer System Organization
 Computer-system operation

One or more CPUs, device controllers connect through
common bus providing access to shared memory

Concurrent execution of CPUs and devices competing for
memory cycles
Operating System Concepts
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Computer-System Operation
Concepts and Terms
 Each device controller is in charge of a particular device type.
 Each device controller has a local buffer.
 CPU moves data from/to main memory to/from local buffers
 I/O is from the device to local buffer of controller.
 Device controller informs CPU that it has finished its operation by
causing an interrupt.
 A trap is a software-generated interrupt caused either by an error or
a user request.
 An operating system is interrupt driven.
Operating System Concepts
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Storage Structure
 Main memory – only large storage media that the CPU can access
directly.
 Secondary storage – extension of main memory that provides large
nonvolatile storage capacity.
 Magnetic disks – rigid metal or glass platters covered with
magnetic recording material
Operating System Concepts
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Storage Hierarchy
 Storage systems organized in hierarchy.

Speed

Cost

Volatility
 Caching – copying information into faster storage system; main
memory can be viewed as a last cache for secondary storage.
 Multitasking environments must be careful to use most recent
value, no matter where it is stored in the storage hierarchy
 Multiprocessor environment must provide cache coherency in
hardware such that all CPUs have the most recent value in their
cache
Operating System Concepts
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Operating System Structure

Multiprogramming needed for efficiency

Timesharing (multitasking) is logical extension in which CPU switches jobs
so frequently that users can interact with each job while it is running,
creating interactive computing

Each user has at least one program executing in memory process

If several jobs ready to run at the same time  CPU scheduling

If processes don’t fit in memory, swapping moves them in and out to
run

Virtual memory allows execution of processes not completely in
memory
Operating System Concepts
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Operating-System Operations
 Interrupt driven by hardware
 Software error or request creates exception or trap

Division by zero, request for operating system service
 Other process problems include infinite loop, processes modifying
each other or the operating system
 Dual-mode operation allows OS to protect itself and other system
components
 User mode and kernel mode
 Mode bit provided by hardware
 Provides ability to distinguish when system is running user
code or kernel code
Some instructions designated as privileged, only
executable in kernel mode
 System call changes mode to kernel, return from call resets
it to user

Operating System Concepts
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Transition from User to Kernel Mode
 Timer to prevent infinite loop / process hogging resources

Set interrupt after specific period

Operating system decrements counter

When counter zero generate an interrupt

Set up before scheduling process to regain control or terminate
program that exceeds allotted time
Operating System Concepts
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System Calls
 Programming interface to the services provided by the OS
 Typically written in a high-level language (C or C++)
 Mostly accessed by programs via a high-level Application
Program Interface (API) rather than direct system call use
 Three most common APIs are Win32 API for Windows, POSIX API
for POSIX-based systems (including virtually all versions of UNIX,
Linux, and Mac OS X), and Java API for the Java virtual machine
(JVM)
 Why use APIs rather than system calls?
Operating System Concepts
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Example of System Calls
 System call sequence to copy the contents of one file to another
file
Operating System Concepts
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Standard C Library Example
 C program invoking printf() library call, which calls write() system call
Operating System Concepts
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3 Management Actions

Process Mgt

Memory Mgt

Storage Mgt
Operating System Concepts
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Process Management

A process is a program in execution. It is a unit of work within the system.
Program is a passive entity, process is an active entity.

Process termination requires reclaim of any reusable resources
Operating System Concepts
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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
Operating System Concepts
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CPU Scheduling
Basic Concepts
Every time a process has to wait, another process
can take over use of the CPU
Scheduling of the CPU is a fundamental OS function
 Maximum CPU utilization obtained with multiprogramming
 CPU–I/O Burst Cycle – Process execution consists of a cycle of
CPU execution and I/O wait
Operating System Concepts
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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 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,
not output (for time-sharing environment)
 McDonalds queue example
Operating System Concepts
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Scheduling Algorithms
 First Come First Serve
 Shortest Job First
 Priority Based (SJF is priority based where shortest job
gets highest priority)
 Round Robin (time quantum…if ‘q’ really large->FCFS…if
‘q’ really small too much context switching…swapping with
no real progress)
 Multiple combinations of the above and others
 Book Store Example
Operating System Concepts
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Memory Management
 All data in memory before and after processing
 All instructions in memory in order to execute
 Memory management activities

Keeping track of which parts of memory are currently being
used and by whom

Deciding which processes (or parts thereof) and data to move
into and out of memory

Allocating and deallocating memory space as needed
Operating System Concepts
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Memory Layout for Multiprogrammed System
Operating System Concepts
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Allocating Physical Memory
Contiguous Allocation
 Multiple-partition allocation

Hole – block of available memory; holes of various size are
scattered throughout memory

When a process arrives, it is allocated memory from a hole
large enough to accommodate it

Operating system maintains information about:
a) allocated partitions b) free partitions (hole)
OS
OS
OS
OS
process 5
process 5
process 5
process 5
process 9
process 9
process 8
process 2
Operating System Concepts
process 10
process 2
process 2
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process 2
Silberschatz, Galvin and Gagne ©2005
Allocating Physical Memory
Paging Model of Logical and Physical Memory
Operating System Concepts
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Segmentation
 Memory-management scheme that supports user view of memory
 A program is a collection of segments. A segment is a logical unit
such as:
main program,
procedure,
function,
method,
object,
local variables, global variables,
common block,
stack,
symbol table, arrays
Operating System Concepts
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Silberschatz, Galvin and Gagne ©2005
Example of Segmentation
Operating System Concepts
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Virtual Memory
 Virtual memory – separation of user logical memory from physical
memory.

Only part of the program needs to be in memory for execution

Logical address space can therefore be much larger than
physical address space (figure 9.1)

Allows address spaces to be shared by several processes

Allows for more efficient process creation
 Virtual memory can be implemented via demand paging
Operating System Concepts
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Virtual Memory That is Larger Than Physical Memory

Operating System Concepts
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Page Table When Some Pages Are Not in Main Memory
Operating System Concepts
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Virtual-address Space
 Virtual address space of process refers
ot the logical (virtual) view of how a
process is stored

View starts at zero

Exists in contiguous memory

Allows heap to grow upward as it is
used for dynamic memory allocation

Allows stack to grow downward
through successive function calls
Heap: memory dynamically allocated during
process run time
Stack: contains temporary data such as function
parameters, return addresses, local variables
Operating System Concepts
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Silberschatz, Galvin and Gagne ©2005
Storage Management
 OS provides uniform, logical view of information storage

Abstracts physical properties to logical storage unit - file
 File-System management
Operating System Concepts
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File Attributes
 Name – only information kept in human-readable form
 Identifier – unique tag (number) identifies file within file system
 Type – needed for systems that support different types
 Location – pointer to file location on device
 Size – current file size
 Protection – controls who can do reading, writing, executing
 Time, date, and user identification – data for protection, security,
and usage monitoring
 Information about files are kept in the directory structure, which is
maintained on the disk
Operating System Concepts
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Sequential-access File
Operating System Concepts
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Two-Level Directory
 Separate directory for each user
 Path name
 Can have the same file name for different user
 Efficient searching
 No grouping capability
Operating System Concepts
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Silberschatz, Galvin and Gagne ©2005
Tree-Structured Directories
Operating System Concepts
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Silberschatz, Galvin and Gagne ©2005
File System Mounting
 A file system must be mounted before it can be accessed
 A unmounted file system (i.e. Fig. 11-11(b)) is mounted at a
mount point
 A discover process is done by OS

What (name of device)

Where (mount point)

Verify valid file system on device

Then….Notes in dir structure that a file system mounted at the
mount point
 Enables an OS to traverse its directory structure, switching among
files systems as appropriate
Operating System Concepts
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Silberschatz, Galvin and Gagne ©2005
(a) Existing. (b) Unmounted Partition
Operating System Concepts
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Access Lists and Groups

Mode of access: read, write, execute

Three classes of users (used to condense ACLs)
a) owner access
7

b) group access
6

c) public access
1

RWX
111
RWX
110
RWX
001

Ask manager to create a group (unique name), say G, and add
some users to the group.

For a particular file (say game) or subdirectory, define an
appropriate access.
owner
chmod
group
761
public
game
Most common
approach to protection
is make access dependent
on identify of user
Attach a group to a file
chgrp
Operating System Concepts
G
game
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Silberschatz, Galvin and Gagne ©2005
Allocation Methods
Secondary Storage
 An allocation method refers to how disk blocks are allocated for
files:
 Contiguous allocation (similar to allocating physical memory)
 Linked allocation (via a trail)
 Indexed allocation (via a block dedicated to be an index)
Operating System Concepts
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Silberschatz, Galvin and Gagne ©2005
Free-Space Management
 Need to reuse space from deleted files, if possible

Maintain free-space list to keep track of free space…records
all free disk blocks
 Frequently implemented as a bit vector (n blocks in example)
0 1
2
n-1
bit[i] =
Operating System Concepts

…
0  block[i] occupied/allocated
1  block[i] free
1.41
Silberschatz, Galvin and Gagne ©2005
End of Lesson