Transcript Virtual memory

Chapter 4: operating
systems
What is an operating system?

A program or collection of programs
that coordinate computer usage
among users and handle common
tasks.
OS - functions
1.
2.
3.
4.
5.
6.
Hide details of input and output
Provide an environment for multiple
users/programs to work concurrently –
multitasking and multi-slacking
Allocate memory to different
users/programs
Share resources (files, printer) among
users
Provide means for users/programs to
communicate with each other
Provide protection and security for
programs
OS functions - examples
1.
2.
3.
4.
5.
You can access disks as file cabinets without the
knowledge of tracks, sector, etc – hide I/O details
You can work with multiple programs at the same
time, e.g., using IE to download files and use
Word to write a report - multitasking
In STT475 and 375, we share the same printers –
share resources
You can interact with machines via the Internet –
communicate among users/machines
When you use password on your machine, no one
can use your computer without the password protection
Why are there difference OS’s?

Different types of cars for different purposes
– Vans for family
– Compact cars for gas mileage
– SUV for rough terrains

Different OS’s for different environments
and applications
– Windows 2000/XP for personal computers
– Windows 2003 Servers for web servers and big
corporations
– Unix/Linux for personal computers and servers
What is “multitasking”?

The speed of CPUs is much faster than any
I/O device
– If a computer only execute one program (e.g.,
Word) from beginning to end, the CPU would
stay idle most the time since no human can type
fast enough.

Multitasking allows multiple programs to
reside in the memory/virtual memory at the
same time and switch the CPU between
those programs
Why OS’s need to coordinate
programs?

Printing example
– When you print a Word document and a
webpage at the same time on your home
printer, did you ever find that one is printed in
mid of the other?


The OS determines which program uses the printer
first and then next program
Memory example
– When you use Word and IE at the same time,
the OS must allocate memory space for both
programs
What can’t Windows read Mac
or Linux disks



Different OS’s define tracks and sectors
differently on the disk – formatting
Windows machines do not know how to
interpret Mac’s format
Today, Mac machines can read Windows
disks is because that Mac OS tries to read
disks in Windows format after it fails to read
it in Mac’s format.
Portable storage devices

Floppy disks
– Limited capacity, very slow, not reliable
– Most of today’s PC’s do not even install
floppy disk drives

Jumpdrive, pin drive, USB memory
stick, flash memory, etc
– High capacity, fast, and reliable
– Use USB connection
What is “booting”?


Start up the computer and allow the
OS to set up itself
Then the OS starts behind-the-scenes
programs (services) for special tasks
– Monitoring email, coordinating printing
requests, monitoring data on the
network.
What is “reboot”?





Stop all current processing and
Restart the computer
If everything were perfect in a computer,
reboot would not be needed
Reboot sometime is the most efficient way
to restore the computer back to normal
Reboot is the only way to restore the
system when the computer does not
respond to any keystroke and mouse
Virtual memory




A technique for today’s computers to run
many applications beyond the limit of the
physical memory (RAM)
Virtual memory is part of the hard disk
Often it uses the free space of the hard disk
All programs that are being executed and
data are being processed are all located in
virtual memory.
Virtual memory
Hard disk
Used space
Data files
And
Applications
Running
programs and
their data
Virtual Memory
Free space
RAM and virtual memory
You
Workbench
.limited space
.hand reachable
Workshop
. Large
. Has everything
Your computer
CPU
RAM
Virtual memory
CPU, RAM, Hard Disks - Program
You want to use MS WORD to write a letter.

–
–
–
–
–
–
MS Word and its application data are installed onto the
hard disk
When you double-click the Word icon, Word is copied
from the hard disk to virtual memory along with its
application data
When you start using Word, part of Word is copied from
virtual memory to RAM
The text you enter from keyboard is saved in RAM
When you save the letter, the text along with format info
(bold, italic, etc) is saved into a file on the hard disk.
When Word exits, it is removed from RAM and virtual
memory along all data related to it.
CPU, RAM, Hard Disks - Program

–
–
–
–
–
–
–
–
What happens when no free space in RAM?
As you open more and more applications (multitasking) and/or
enter more data, RAM may become all occupied
This is like your workbench is all covered by tools and materials.
The operating system moves some part of some applications to
virtual memory to free up some RAM space
This is like you move some tools and materials from workbench
to the walls of the workshop.
Now you can continue entering more data or opening more
applications.
This is like you continue your work on workbench using newly
freed space.
When you need a part of a program or a program that was
moved to virtual memory, it may be swapped back in RAM.
This is like you need a tool you moved to the walls earlier, you
need to clear some space on workbench so you can move it
back.
Add More Memory






Virtual memory allows us to run applications that
together need more than the available physical RAM
Virtual memory (hard disk) is much slower than main
memory (RAM).
The more swapping between RAM and virtual memory,
the slower of the system
The more RAM (bigger workbench), the less likely
swapping occurs
The more RAM, the faster the system
Stop applications that no longer needed, this frees the
RAM space occupied by those applications  remove
no longer needed tools from your workbench.
File Management



Programs and user data are stored as
files on the hard disk
Each file always takes multiple sectors
Folders can be used to organize files
like file cabinets
Before DELETE
After DELETE
After ERASE
Deleting a file vs. Erasing it

Deletion of a file
1.
2.

Removes the record of the file from the corresponding
directory
Marks that the various sectors of the file are now free
Erasure of a file
1.
2.
3.
Writes over all information in the various sectors containing
file data
Removes the record of the file from the corresponding
directory
Marks that the various sectors of the file are now free
Delete a file === throw it into a garbage can
someone may steal it and read it
Erase a file === shred it
no one can read it anymore
I/O Devices - Drivers


OS needs a special program, called device driver,
for each I/O device
The driver is designed for the device and the OS.
– A HP printer driver designed for Windows XP normally
would not work Vista



Many OS’s have device drivers of commonly used
devices included in their installation
Device drivers may be updated by the manufacture
for various reasons
My advice: download drivers only from the
manufacture’s website
Which OS is best?

Three most common OS’s
– Windows by Microsoft



Aggressively marketed
Take a large fraction of individual stations
As a result, users can easily share programs and data
– Macintosh OS X by Apple

Offers a simple, clean, and powerful graphical user
interface
– Linux by Linus Torvalds and others

Offer higher reliability and ability to link complex tasks
Summary – chapter 4







Operating systems handle common tasks
Multitasking allows a computer to split its CPU time and
resources among multiple programs and users
Booting restarts the computer and the OS sets up the
environment
Rebooting involves shutting down the computer and starting it
up again
Different OS’s are designed for different purposes
Different OS’s define different formats on disks
Virtual memory
– Allow more applications beyond the limit of RAM
– Located on the hard disk



Deleting vs. erasing files
Disk fragmentation and defragmentation
Viruses and anti-virus programs
Terminology






Operating systems
Booting a computer
Rebooting a
computer
Single-tasking
Multitasking
Virtual memory







Fragmentation
Defragmentation
Deleting files
Erasing files
MS Windows
Mac OS X
Linux