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. A program or collection of programs that coordinate computer usage among users and handle common tasks.

OS - functions 1. Hide details of input and output 2. Provide an environment for multiple users/programs to work concurrently – multitasking and multi-slacking 3. Allocate memory to different users/programs 4. Share resources (files, printer) among users 5. Provide means for users/programs to communicate with each other 6. Provide protection and security for programs

OS functions - examples 1. You can access disks as file cabinets without the knowledge of tracks, sector, etc – hide I/O details 2. 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 3. In STT475 and 375, we share the same printers – share resources 4. You can interact with machines via the Internet – communicate among users/machines 5. 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 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 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 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 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 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 The OS determines which program uses the printer first and then next program Memory example 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 Different OS’s define tracks and sectors differently on the disk – formatting Windows machines do not know how to interpret Mac’s format 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. 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 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 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 Start up the computer and allow the OS to set up itself Then the OS starts behind-the-scenes programs (services) for special tasks Then the OS starts behind-the-scenes programs (services) for special tasks –Monitoring , coordinating printing requests, monitoring data on the network.

What is “reboot”? Stop all current processing and Stop all current processing and Restart the computer Restart the computer If everything were perfect in a computer, reboot would not be needed 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 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 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) 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 Virtual memory is part of the hard disk Often it uses the free space 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. All programs that are being executed and data are being processed are all located in virtual memory.

Virtual memory Hard disk Data files And Applications Used space Free space Running programs and their data Virtual Memory

RAM and virtual memory Virtual memory You Workbench.limited space.hand reachable Workshop. Large. Has everything RAMCPU Your computer

CPU, RAM, Hard Disks - Program You want to use MS WORD to write a letter. 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? 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 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). 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 swapping between RAM and virtual memory, the slower of the system The more RAM (bigger workbench), the less likely swapping occurs The more RAM (bigger workbench), the less likely swapping occurs The more RAM, the faster the system 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. 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 Programs and user data are stored as files on the hard disk Each file always takes multiple sectors Each file always takes multiple sectors Folders can be used to organize files like file cabinets 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 Deletion of a file 1.Removes the record of the file from the corresponding directory 2.Marks that the various sectors of the file are now free Erasure of a file Erasure of a file 1.Writes over all information in the various sectors containing file data 2.Removes the record of the file from the corresponding directory 3.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 someone may steal it and read it Erase a file === shred it no one can read it anymore no one can read it anymore

I/O Devices - Drivers OS needs a special program, called device driver, for each I/O device OS needs a special program, called device driver, for each I/O device The driver is designed for the device and the OS. 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 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 Device drivers may be updated by the manufacture for various reasons My advice: download drivers only from the manufacture’s website My advice: download drivers only from the manufacture’s website

Which OS is best? Three most common OS’s Three most common OS’s –Windows by Microsoft Aggressively marketed Aggressively marketed Take a large fraction of individual stations Take a large fraction of individual stations As a result, users can easily share programs and data As a result, users can easily share programs and data –Macintosh OS X by Apple Offers a simple, clean, and powerful graphical user interface Offers a simple, clean, and powerful graphical user interface –Linux by Linus Torvalds and others Offer higher reliability and ability to link complex tasks Offer higher reliability and ability to link complex tasks

Summary – chapter 4 Operating systems handle common tasks Operating systems handle common tasks Multitasking allows a computer to split its CPU time and resources among multiple programs and users 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 Booting restarts the computer and the OS sets up the environment Rebooting involves shutting down the computer and starting it up again Rebooting involves shutting down the computer and starting it up again Different OS’s are designed for different purposes Different OS’s are designed for different purposes Different OS’s define different formats on disks Different OS’s define different formats on disks Virtual memory Virtual memory –Allow more applications beyond the limit of RAM –Located on the hard disk Deleting vs. erasing files Deleting vs. erasing files Disk fragmentation and defragmentation Disk fragmentation and defragmentation Viruses and anti-virus programs Viruses and anti-virus programs

Terminology Operating systems Operating systems Booting a computer Booting a computer Rebooting a computer Rebooting a computer Single-tasking Single-tasking Multitasking Multitasking Virtual memory Virtual memory Fragmentation Fragmentation Defragmentation Defragmentation Deleting files Deleting files Erasing files Erasing files MS Windows MS Windows Mac OS X Mac OS X Linux Linux