Computer Systems 1 Fundamentals of Computing

1. Computer Systems 1Fundamentals of Computing Operating Systems

2. CS1 - Week 22 • Introduction to the OS • OS Fundamentals • Processes • Memory • Files • I/O Computer Systems 1 (2004-2005)

3. The Operating System (OS) • Manages the hardware and software resources of the system • Resources • Processor • Memory • Disk space, etc. • Allows applications to deal with the hardware without having to know all the details of the hardware Computer Systems 1 (2004-2005)

4. Operating Systems • The OS acts a a ‘shell’ around the hardware. • The applications software need an OS to work, and users need the applications software to interact with the hardware via the OS. Application Operating System User Hardware Application Application User User Computer Systems 1 (2004-2005)

5. Types of OS • Real-Time Operating System (RTOS) • Fast Response (‘Real Time’ / Instant) • Used to control machinery and industrial systems • Usually Poor HCI • Single-User, Single Task • One user can do one thing at a time • Useful for low spec systems • Single-User, Multi-Tasking • One user can perform many tasks at a time • Common for current PC systems • Multi-User • Many users can use the computer's resources simultaneously • OS balances resources allocation against users and tasks • N.B- A machine on a network is not necessarily multi-user!!! Computer Systems 1 (2004-2005)

6. OS Generic Structure • Kernel • Memory & Process Management • Input / Output • File System • User Interface Computer Systems 1 (2004-2005)

7. The Kernel • Heart of the OS • Provides secure access to the computer hardware • Low-level software / hardware transfer • Manages resource availability / usage • A kernel includes • Interrupt handler • Handles all requests for kernel service • Scheduler that • Decides how and which programs share the kernel processing time • Supervisor • Gives use of the computer to each process when required Computer Systems 1 (2004-2005)

8. Types of kernels • Monolithic Kernel • Runs most of its services in the kernel workspace • Error prone due to the amount of tasks in the kernel itself • Used in most Linux systems • Microkernel • Runs most services - like networking, filesystem, etc. - in user space • More stable, but more complex designs • Hybrid (Modified Microkernel) • Microkernels that have some "non-essential" code in kernel space in order for that code to run more quickly. • Windows NT/2000 and Macintosh OS X use hybrid kernels Computer Systems 1 (2004-2005)

9. Process Management • Process • A single sequence of operation(s) • The basic unit of software the operating system deals with in scheduling the work done by the processor • Not the same as an application • An application can contain multiple processes • Process has some key attributes • Process ID (PID) • Priority Level • Pointers to upper and lower bounds of memory allocated • Status of all I/O devices needed Computer Systems 1 (2004-2005)

10. Process Management • CPU can only do one thing at a time • If a process wants to use the CPU it must tell the CPU • Interrupts or Polling • To give the appearance multi-tasking, the quickly OS switches between different processes • This function is performed by a process scheduler • Schedulers decide what process should run, when, and for how long • Scheduling decision factors • Priority • Resources needed • Resources used to date • Waiting time to date Computer Systems 1 (2004-2005)

11. Memory Management • Provide memory space to enable multiple processes to be executed at the same time • Give satisfactory level of performance for users • Protect each process • Don’t get data mixed-up between processes • Enable sharing of memory between processes • More dynamic & efficient use of the memory resources • Make addressing of memory transparent to programmers • Provides Virtual Memory • Extra memory, larger than the physical allocation • Done by storing portions of data to backing storage Computer Systems 1 (2004-2005)

12. Input / Output • Devices use an I/O sub-system of a computer use to communicate • Communication carried out via registers • I/O devices use interrupts to signal to the CPU that a task has been completed • IRQ – Interrupt ReQuests. • Each device has a unique IRQ • On ISA buses, data can be sent from the I/O controller to memory, bypassing the the CPU • This is DMA (Direct Memory Access) Need more input…. Computer Systems 1 (2004-2005)

13. Application Program Input-output Control System (IOCS) I/O Device Device Driver Device Controller } System Calls Operating System I/O Bus Input / Output • Input / Output Control System (IOCS) • The part of the O/S that deals with I/O activity • Deals with requests and routes to and from devices • Device Driver • A software module which manages the communication with, and control of, a specific I/O device • Interprets requests from the application to specific commands to the I/O device Computer Systems 1 (2004-2005)

14. File System • File can be stored data, although programs and folders are also considered files at a low level • OS knows nothing about file structure • Applications can interpret file data • File System tasks • Creating & deleting files • Storing and organising files on storage medium • A new file is allocated one cluster • As the file grows the cluster becomes filled and more clusters are assigned to the file Computer Systems 1 (2004-2005)

15. A A B A B C C B B Free cluster End of file B End of file A File System • Ideally, successive file clusters should be in sequence on the disk • Faster access as less disk movement in required • Usually a disk contains many files • Clusters get mixed out of sequence • This is known as fragmentation • Fragmentation can reach a critical level where system performance becomes greatly reduced Computer Systems 1 (2004-2005)

16. File System • Windows file systems are FAT • File Allocation Table • FAT16 • Uses 16-bit value to record file properties • FAT32 • Very similar to FAT16, except it uses 32-bit values • Linux / UNIX Use iNodes • Similar procedure to Windows • Usually 32-bit values • Every file has an iNode • OS tracks iNodes and file names Computer Systems 1 (2004-2005)

17. User Interface • The user interface (UI) is a layer above the operating system • What users “see” or interact with • UI deals with getting user commands and providing feedback / representation • Two common types of user interfaces: • Command-Line Interface (CLI) • Graphical User Interface (GUI) Computer Systems 1 (2004-2005)

18. Command-Line Interface • Textual commands • Either from the keyboard or from a script • User types a command after the computer displays a prompt character • For example, MS-DOS, UNIX, etc. • Advantages • Faster for skilled users • All options and commands available in a standard format • Can perform operations in a batch processing mode without user interaction • Disadvantages • Perplexes new users / Not user-friendly • Multiple operations made complex • Have to remember commands / not intuitive Computer Systems 1 (2004-2005)

19. Graphical User Interface • Graphical representations & metaphors • GUIs have a number of common features: • Windows, Icons, Menus, Pointers (WIMP) • As well as buttons, toolbars, etc. • Advantages • Consistent look and feel, even across platforms • Commands can be executed using either mouse or keyboard • High levels of operation and dialog • Disadvantages • Often restrictive • Use more system resources / more intense Computer Systems 1 (2004-2005)

20. UNIX • Designed by Ken Thomson of AT&T Bell Laboratories improved by Dennis Ritchie • Multi-user • More than one person suing the OS at a time • Multitasking • Each user can run more than one process • Platform independent • Runs on almost any hardware platform • Complex • Large distribution • User control and administration • Used as multiple types of server Computer Systems 1 (2004-2005)

21. LINUX • Created by Linus Torvalds • Particularly aimed at PC users • Two parts: • Kernel • The Linux operations section • The UNIX part of things • Distribution • Particular flavour of Linux • Open source • Free! (more or less) • Many now have a windows style interface • Easy to get to grips with • Some expertise is required to become a master • Low hardware requirements • 100 MHz CPU, 12Mb RAM, 500Mb HDD Computer Systems 1 (2004-2005)

22. Microsoft Windows • Founded by Bill Gates • Developed from MS-DOS • Microsoft Disk Operating System • ‘Borrowed’ from XEROX Q-DOS • Windows 1.0 began life as an interface running on MS-DOS • GUI improved and networking capabilities added in future versions • Is now independent of MS-DOS • Many varieties available • Majority are multi-user and multi-task • Windows XP • Windows Server • Windows 2000 Computer Systems 1 (2004-2005)

23. Macintosh OS X • Rooted in a UNIX system • Providing all the good bits of a powerful UNIX OS • Super-nice GUI as front-end • Ease of use and interaction • Macintosh OS have always been pioneers in user-friendly interfaces and functionality • High level of accessibility • Scalable from first-time users to advanced • Seamless integration of OS and system applications Computer Systems 1 (2004-2005)

24. Week 22 • Operating System Structure & Components • Kernel • Memory management • Process management • Input / output • File system • User interface Computer Systems 1 (2004-2005)