Why Operating Systems? Topic 3. Operating Systems. Why Operating Systems? Why Operating Systems?

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1 Topic 3 Why Operating Systems? Operating Systems Abstracting away from the Nuts and Bolts Early computers had no operating system. Programmers were responsible for: telling the computer when to load and execute programs specifying absolute memory addresses their program would use connecting I/O devices at appropriate times, etc! slow and complex Why Operating Systems? Why Operating Systems? Modern computers are far more complicated! multiple users many programs or processes running at once greater interconnection with I/O devices networking platform compatibility Operating system provides a layer of abstraction between the programmer and the computer hardware (or microprogram level)! buffers the programmer from very complex details

2 The Operating System The Operating System Application Software Operating System Hardware A user of a computer interacts with (or writes) application software This application software interacts with the operating system rather than with the hardware directly The operating system is designed specifically to interface with the hardware. Traditionally often written by the hardware designers, tho less so as fewer hardware platforms have dominated The primary program that runs the machine Can be very large! And getting larger every year... Examples of well-known operating systems: VMS, Unix, CP/M, MS-DOS, OS/2, MacOS, Linux, Windows, MacOS-X Some operating systems are open source - you can modify them yourself and even contribute to their development! eg Linux, Darwin (core of MacOS X) The Operating System Time Sharing Responsibilities include: job scheduling processor allocation memory management I/O transfers and communication file system structure and integrity GUI management (may be an extra layer - window manager) networking and load balancing security A number of processes are active at any one time User processes - eg applications, user programs System processes - eg window manager, scheduling, etc CPU time must be shared between the processes - time slicing Cannot allow one process to dominate for too long - eg user experience deteriorates Each process only gets a fraction of a second at once Appears as if processes are running in parallel Multi-processor and multi-core systems allow some true parallel processing 7

3 Time Sharing Processes may have higher or lower priorities eg. The window a user is working may be given high priority File system indexing may be given low priority (background task) In Unix systems you can tell the operating system what priority to assign to your job - be nice to your fellow users! > man nice Time Sharing Memory Management Virtual Memory Problems: Although the cost of memory is decreasing rapidly, it is still more expensive than disk space Memory requirements increase as cost decreases Single programs may require more memory for program and data than is available Many processes competing for limited memory for programs and data Separate concepts of address space and memory locations virtual address space - addresses a program can refer to physical address space - actual hardwired memory addresses Virtual address space can be many times larger than physical address space Programs written as if they have access to full virtual memory 11

4 Paging When a program refers to an address outside of main (physical) memory contents of main memory saved to secondary memory (disk) appropriate block of secondary memory found and loaded (or swapped) into main memory virtual addresses mapped into physical memory addresses Usually divide memory and disk into fixed size chunks or pages Demand Paging vs Working Set Demand paging bring in each page only as needed Working set paging most programs don t use their address space uniformly some pages used more than others keep a track of these bring all pages into memory when program is started (predictive) Each strategy has merits! paging Page Replacement File Systems Bringing in new pages for a process means swapping out pages from that or another process Strategies - eg. first-in first-out (FIFO) least recently used (LRU) If working set is too large, can get thrashing! large virtual memory use for program is OK providing working set is small An important task of the operating system is managing the file system Providing a structure for applications to read and write data on a hard disc. Basic unit of storage is the file (Windows-speak document ) Files are organised in a hierarchy of directories (folders). Directories hold files and other (sub)directories. A directory is just a special kind of file.

5 File System Example File Systems Letters Documents CES1005Assignments extension-request assn1 assn2 / (root directory) Applications MATLAB Firefox Excel Any file in this hierarchy is uniquely specified by its path. The operating system provides a means of navigating the directory tree to access a particular file. Many operating systems and versions are in use e.g. DOS, MacOS, Windows 95/98, Windows NT/2000/XP/Vista, Unix, Linux, MacOS X. Different operating systems handle the details of file handling differently, but the fundamentals remain the same. The path to file assn2 might be specified as: \Documents\CES1005\Assignments\assn2 (DOS, Windows) /Documents/CES1005/Assignments/assn2 (Unix/Linux/MacOS) File Systems File Systems All operating systems support: file/directory creation file/directory deletion file/directory renaming file/directory moving file/directory display file editing Serious weakness of the Windows operating system is that the names of files are *not* case sensitive. The file name of a file might appear as MyFile, but specifying MYFILE or myfile will still match the name of the file. This causes problems in languages such as MATLAB or Java where the names of files are central to the language! MS-DOS based filesystems, including many versions of Windows, also required exactly 3 character filename extension - hence we have mypage.htm instead of mypage.html! Unix everything is a character stream philosophy

6 Text Files and Binary Files There are many types of files. Two most common ones: ASCII (American Standard code for Information Interchange) plain text (more recently extended with internationalisation to Unicode) Binary ASCII is in fact binary each successive group of 8 bits in a file corresponds to the ASCII code of a letter. ASCII uses 8 bit code to represent all usual characters plus some control characters e.g. The character A is represented as 0x41) (65 decimal, B (66, 0x42) a (97, 0x61) 22 Text Files and Binary Files Text Files and Binary Files An ASCII file containing the characters cat dog would contain the following bit pattern c a t space d o g A binary file may have bits corresponding to the binary representations of numbers, data structures or program instructions. also used for compression Some of the standard editors for creating & modifying text files Notepad (Windows) Emacs, vi, pico (Unix, Mac) When using a word processing package, such as Word, and you save a document as text, you re just saving the characters in your document as ASCII. When you save in say Word format, you re saving the characters along with additional control characters or codes in binary format. ASCII files are (mostly) portable across different computers and operating systems. Binary files are usually not portable, but provide efficient storage.

7 Text Files and Binary Files A file may be for data storage or may be an executable An executable (sometimes referred to as a binary ) is a program to be run on the computer binary instructions for execution on the CPU (via the operating system). Different kinds of machines have different CPUs and operating systems executables are not portable, e.g. a Mac OSX executable will not run on Windows Programs are typically written as ASCII text files then converted into binary machine instructions by a compiler, or interpreted by a separate program directly can be compiled for different CPUs The Concept of The User and Security Another important role of (modern) OSs is that of security Early OSs didn t have the concept of individual users. Everyone using the computer has the same privilege (complete access to all files). Problem when many users access the same computer(s) People can interfere with each others files, change configurations, etc. Many home PCs are still operated in this manner! Microsoft has been slow in implementing the concept of individual users in their operating systems This concept existed in other OSs for over 25 years. The Concept of The User and Security Networked File Systems Modern operating systems implement the concept of a user. To use the machine, you have to log in specifying your username and enter a password. Once logged in, you will only have access to your own area - typically a directory in the file system where the user can create, delete, modify files. A user may have read-only access to directories outside of his/ her area but not writing privileges. Concept of the user implies security of some kind Allow you to keep your files safe from tampering Allow you to create your own customised environment without interfering with others. Following from the concept of a user, one can have networked file systems Typically one or more computers, known as servers, store all the users files. Any number of workstation computers can be connected to the servers. These workstations will not have any users files, all they will have is copies of applications such as Matlab and Emacs. Access to users files is handled automatically by the OS through the network

8 Networked File Systems Networked File Systems Server for lab ug01 Users / home ug02 Workstation pc01 pc02 pc03 From any workstation, the path to your home area will be (eg) /home/ug01/username... Thus, the OS makes a separate machine on the network simply look like another directory You can use any computer within a network to access your files Even access your files via the internet from overseas This means that administration tasks are centralised e.g. backups only need to be made of the server s files The job of a system administrator is simplified System Integrity Another role of the OS is to protect itself and other programs from malfunctioning programs or malicious programs such as viruses. Main mechanism is *memory protection* When a program is loaded: It allocates an area of memory it can use. The OS then monitors all memory accesses If a program attempts to access a location outside its own area, then it is shut down Microsoft has been slow in implementing memory protection in the OSs the concept existed for 25 years!