Information Technology Operating Systems

Information Technology I Outline: Operating Systems � What is an OS � OS Functions � � � � � Multitasking Virtual Memory File Systems Window systems PC Operating System Wars: Windows vs. Linux Operating System provides � a way to boot (start) the computer � control of the computer hardware: keyboard, display, mouse, printer � a file system, a way to name and organize files for storage on disk, hence Disk Operating System (DOS) Information Technology I Operating System provides � a way to load and run user (application) programs � a way for application programs to use the hardware devices and file system Operating System provides � a “User Environment” � a consistent way for application programs to interact with the user � Much of the UI is provided by Windows, not by individual programs � Windows have a Graphical User Interface (GUI Interface) Information Technology I Operating System provides � multiple users - several people may use the computer at one time � security - to prevent individual programs and users from interfering with each other � Examples: UNIX MetaFrame Linux Windows NT with Operating System provides � Ease of use: presents three useful illusions � � � Many separate computers, one for each process Large memory Disks and other secondary storage are organized as collections of files Information Technology I Illusion #1: Multitasking � Reality: � � � Illusion: � � One CPU One instruction at a time Several application programs executing concurrently Implementation: � Operating system divides CPU time among application programs (time sharing) - each program “thinks” it is the only one running Task Switching Illustrated Application Memory Area � Process A (e.g. MS Word) Initially, the CPU executes MS Word � � IP points to instruction in MS Word code CPU registers contain data related to MS Word CPU Process B (e.g. Netscape) IP Registers Operating System Memory Area Process A register save area Process B register save area Information Technology I Task Switching Illustrated Application Memory Area � Process A (e.g. MS Word) After a short period of time (a few milliseconds) � � CPU is interrupted by operating system IP and other CPU registers related to MS Word are saved to a special memory area reserved by the OS Process B (e.g. Netscape) CPU IP Registers Operating System Memory Area Process A register save area Process B register save area Task Switching Illustrated Application Memory Area � Process A (e.g. MS Word) After a short period of time (a few milliseconds) � � � CPU is interrupted by operating system IP and other CPU registers related to MS Word are saved to a special memory area reserved by the OS IP and other CPU registers related to Netscape are loaded from special memory area CPU Process B (e.g. Netscape) IP Registers Operating System Memory Area Process A register save area Process B register save area Information Technology I Task Switching Illustrated Application Memory Area � Process A (e.g. MS Word) This makes the CPU start executing instructions from the Netscape code Process B (e.g. Netscape) CPU IP Registers Operating System Memory Area Process A register save area Process B register save area Cooperative Multitasking � Used in early versions of Windows � Application explicitly passes control back to OS � A badly behaved application may never pass control back. What happens? Information Technology I Preemptive Multitasking � OS/2, UNIX, NT, DOS applications running under Windows � OS preempts when it wants to take control � When application asks for I/O � Any I/O request requires a branch to the OS The I/O request won't be handled right away anyway (I/O devices are much slower) So OS initiates I/O request, then saves application state, and gives a time slice to another application Or when application has run longer than a preset limit - A special timer interrupt signal causes CPU to branch to an interrupt handler program, which is part of the OS Illusion #2: Memory Management � Assign memory to each process � Some OS provide memory protection � Don't let a process access memory not assigned to it P2 (150K) P1 (100K) OS Memory area Information Technology I Non-contiguous Memory � Example: Processes 1, 2, and 3 � � � � � Process 1 starts with 100K of memory Process 2 starts with 150K of memory Process 1 asks for 50K more of memory Process 2 exits Process 3 starts with 200K of memory P1 P1 P2 P3 P1 P3 P1 P1 OS Memory area OS Memory area P1 OS Memory area Issues � OS loads applications in dynamically determined, non-contiguous pieces of memory � Program code “thinks” it is stored in a fixed, contiguous memory area � � for example, branch instructions refer to fixed memory addresses How can the system reconcile the two views? Information Technology I How to Do it: Paging � Divide all of main memory into frames � Divide all program code and data into pages � � � page size = frame size Place each program page into next available memory frame � � � � 110 100 Process 1 gets frames 51-70 Process 2 gets frames 71-100 Process 1 gets frames 101-110 Process 2 frees 71-100 P1 110 100 P2 70 50 Each frame holds 1 page (say 1000 bytes) 70 P1 OS Memory area 50 P1 P3 P1 110 100 P3 70 P1 P1 50 OS Memory area OS Memory area Address Translation � “Virtual Address” consists of page number and location within the page � Page may be located anywhere in real memory � Lookup in page table Translate page number to correct frame number � Location within the page is unchanged Page # 1 2 30 31 32 Frame # 71 72 100 111 112 � Page table for P3 110 100 70 50 P3 P1 P3 P1 OS Memory area Information Technology I Address Translation Example � Use decimal numbers for simplicity � Assume page size = frame size = 1000 bytes � CPU wants to access memory address 30,127 � � Address is intercepted by paging hardware � � page number = 30, location within page = 127 page table indicates that page 30 is stored in frame 100 Actual address sent to memory hardware is 100,127 CPU Paging Hardware Page # Frame # 1 71 2 72 Memory 30 100 31 111 32 112 Virtual Memory � Some pages are not used for a long time � Why keep all of them in memory? � Copy page to hard disk and use frame for some other page � Copy pages back from hard disk to main memory frames as they're needed � Total number of memory pages allocated to processes can exceed total number of memory frames � Process (and its programmer) not aware that main memory is too small (the big memory illusion) � � � It asks for a main memory location (Page #, offset on page) OS has to get that page into main memory if not already there Called Virtual Memory Information Technology I VM and Caches � Both techniques exploit a small, fast memory and a big, slow memory � Caches � � � Ask for address of location in slow memory Check if that location is also in fast memory (cache) Virtual Memory � � Ask for address of location in fast memory Not enough fast memory for all the addresses - Some fast memory addresses are actually in slow memory - Copy needed page from slow memory to fast memory Illusion #3: File Systems � Reality: � � � � Disks are sets of tracks Tracks are sets of sectors Sectors can store fixed-sized byte blocks Illusion: � � � � Disks are sets of directories Directories contain other directories or files Files are variable-size byte sequences Directories and files have names Information Technology I The File System � A DIRECTORY kept by the operating system � � � Keeps track of each file’s name Contains information about the file’s physical location May keep additional information such as date created, etc. The File System � Special files (subdirectories) are themselves directories � Directories may contain subdirectories, nesting to multiple levels Information Technology I Hierarchical directories Drive C: \ [root] My Documents Program Files W indows Hierarchical directories Drive C: \ [root] My Documents Program Files Windows Desktop Start Menu System Information Technology I Windows file naming � Four part names � � � � � Drive (or device) Path File name File type (extended name) C:\Windows\System\WinTrust.hlp C:\Windows\System\WinTrust.hlp � C: = Drive (hard disk named C:) � \Windows\System\ = Path � � � begin in “\” (root) go to “Windows” subdirectory go to “System” subdirectory of Windows � WinTrust = File Name � hlp = File Type (help file) Information Technology I Windows file types � Identify the kind of file � For data files, identify the application that usually opens the file � EXE - program (executable file) � COM - command � DOC - MS Word document � PPT - PowerPoint presentation � XLS - Excel spreadsheet � HLP - Help file File System Abstract Functions � File operations � � � � � � Directory operations � � � Set access privileges on file Create or delete a file Open or close Seek to particular position in file Read or write at current position Create or delete a directory Move a file or directory inside another directory General operations � Search for a file with given name Information Technology I File Representation � Combine disk sectors into clusters � � 1 cluster ~ 10000 bytes Implement file as collection of clusters � Clusters need not be contiguous (near each other on the disk) � � � Why is this an advantage? Why is this a disadvantage? How big should the clusters be? Keep a map of the clusters used in each file Which is better: contiguous or fragmented? Keep a map of the free clusters DOS Hard Disk Organization Hard Disk Cluster 0 Boot Sector Cluster 1 File Allocation Table Cluster 2 Root Directory File Data Cluster 3 ... File Data Information Technology I Example: DOS Directories and FAT Root Directory File Allocation Table 0 Filename file1.txt file2.doc Cluster 2 6 1 0 5 2 0 6 12 10 7 7 11 11 15 8 0 5 9 9 13 16 17 0 4 4 8 12 16 0 3 3 10 14 14 18 0 15 19 0 0 Legend: FAT Block Number 10 11 Block 10 of the File Allocation Table contains the value 11 Contents of FAT Block OS Wars: Windows vs. Linux � Linux is a type of Unix � Free operating System � Original version developed by Linus Torvalds in 1991 � Greatly enhanced and extended by the global “Open Source” software development community Lecture notes for 15.564: Information Technology I Comparison of Windows NT and Linux � Appearance � Cost � Applications � Scalability � Software Support � Technical Support � Compatibility � Configurations Appearance � NT � � GUI Linux � Command Line Information Technology I Cost � NT � � � 5 user version $809 10 user version $1129 25 user version $3,999 � Other costs � Office Suite, Compiler / Development Tools � Netware, Virus Protection, Backup packages, Database operations � Linux � Free Applications � Many more programs for NT than for Linux. � Choice of vendors is much larger for NT. � Many of Linux applications are not available in GUI Information Technology I Scalability � NT � � � � Supports 4GB of RAM NTFS provides 64 bit file system Integrated file cache Linux � � Supports 2 GB of RAM Maximum file size of 2GB Software Support � NT Wide range of commercial software � Single user system � � Linux � Windowing protocol Information Technology I Technical Support � NT � � � � Dedicated support network Support through partners and OEM’s 350, 000 Microsoft trained professionals Linux � � Free Online Support “Peer to Peer” support Compatibility � NT � � � � Difficult to access UNIX file systems No UNIX programs run under NT NT Backups only run on NT Linux � Can access DOS/Windows/NT file systems � Compatible with any version of UNIX � Many window programs run under Linux � Backups are compatible between distributions of Linux, and UNIX but not with NT Information Technology I Configurations � NT � � � Admin user is not authorized to make all changes Configuration Linux � � � Easily changed to meet needs Interfaces Wide range of free tools to configure system Security � NT � � � � Access Permissions Bound to machine and Domain Have to check by hand for any file changes Linux � � � Security is tied to the file Can easily tell if a file has been changed Firewall functionality is built in to the server