Topic 6 Operating Systems

1. Topic 6Operating Systems

2. Applications software allow the user to communicate with the system software. The system software communicates with the hardware. The operating system provides a set of system calls that can be used by the programmer of an application to work the computer system. The operating system can also provide a library of subroutines, often called an Application Programming Interface (API)

3. The operating system is part of the system software. It can be regarded as an interface in three different ways. First, it is an interface between the parts of a computer system. These work at different speeds and in different ways, and the operating system enables them to communicate. Second, the operating system is an interface between an application package and hardware. Software houses, that create application packages, write them to run under a particular operating system. Third, the operating system is an interface between the hardware and the user. There are two kinds of user interface: command-line interfaces (CLIs) and graphical user interfaces (GUIs). • MS-DOS had a command line interface. • Apple Macs from the start offered a GUI.

4. Operating systems are expected to have similar features. These are: • memory management; • processing modes; • input/output; • filing system; • resource sharing; • user interface; • applications support; • security.

5. Additional criteria required when looking at an operating system is:- • cost; • ease of installation; • ease of management • reliability • Performance • range of platforms • range of software • customisation

6. What a process is The term process is basic to the understanding of the workings of an operating system. A programmer writes programs. The source code of these programs will be in assembly language or in some high level language like Java. When a program is actually running, it contains run-time data and a stack; other information necessary to keep the program running include a program counter and a stack pointer. All of these things together make up a process. A process can be defined as a program in execution. Another name for process is task.

7. When you run a program, the operating system has to convert it into a process. It is the process, rather than the simple program, that the processor executes. When the process terminates, the program itself remains (on backing store), fit to be converted into a process again if needed. 6.3

8. Pupil task Now log onto scholar and work through web animation on layers.

9. An operating system needs room in memory for itself and for the processes it creates, and user programs need memory for themselves and the data they use. Part of the operating system, known as the memory manager, allocates memory to these processes and keeps track of the use of memory. Some operating systems, such as early versions of Windows (95, 98, NT) provides only a single-user environment. Multi-user memory management is provided by most modern operating systems. Most operating systems support multi-tasking. In multi-tasking, several applications or tasks are (apparently) available at the same time, and the user can switch easily between one application and another.

10. Most operating systems also support multi-threading. Threads are sometimes called lightweight processes. One process can have several threads. The threads act independently of one another, but share the same memory space. For example, in a database application, one thread might run the user interface and another might handle the actual database. Process loading and swapping If a program is to be run (as a process), it needs to be copied into memory from backing store. This action - by no means instantaneous - is known as process loading. Some systems, as we shall see, also take processes out of memory and store them on disc, to make room for another process. This is known as process swapping.

11. Monoprogramming The simplest method of memory management is to have just one process in memory at a time, and to allow it to use all the memory there is. One obvious problem with this approach is that a program needs to be entirely self-reliant and contain, Multiprogramming Monoprogramming would not do on a large computer that has lots of users. Having several programs in memory at once is called multiprogramming. In multiprogramming, memory is divided into parts, of one kind or another. Names for these parts include partitions, segments, and pages. Multitasking Animation Log onto scholar 6.3 using web animation..

12. Fixed partitions Using fixed partitions is a relatively straightforward way of effecting multiprogramming. The partitions are fixed in size but don’t have to be all the same size. 1000k Partition 4 400k Process D 700k Partition 3 300k Process C 400k Partition 2 200k Process B 200k Partition 1 100k Process A 100k Operating system

13. Drawbacks to fixed partitions The main problem with fixed partitions is that, processes don’t usually fit partitions exactly. As a rule, there will be wasted space in each partition. A map of memory, as in the diagram above, looks fragmented. This waste of space is known as internal fragmentation (internal because the space wasted is inside the partitions). Another drawback is that a process might come along that is too big for any of the partitions.

14. Variable partitions One solution, is to allocate to each process exactly the amount of memory it needs. When a process turns up, it is loaded into the next available area. This can go on until there is no room left. 1000k Available Space 750k Process D 550k Process C 325k Process B 175k Process A 100k Operating system

15. The arrival of process E Process C terminates Process F can be executed once process B terminates

16. Pupil task looked at how the processor is switched between tasks resident in memory (page 156 Scholar multitasking animation)

17. Once everything is up and running, new processes need to be fitted into the HOLES available. (spaces in memory can be known as holes) There are three policies to choose from:- first fit best fit worst fit

19. Information about holes is often kept in a linked list. Coalescing holes is straightforward. First fit will take the first hole large enough; the other two methods will need to scan the entire list before a decision can be made. Compaction Fragmentation can be kept down to one hole (of whatever size) by shifting processes around whenever one leaves memory. All remaining processes are packed down (compacted) so as to leave a single hole at the top. In the previous series of memory maps, process F arrived and process C terminated. Process F did not have enough room. However, with compaction, processes D and E could be moved down in memory, leaving enough room at the top for F.

20. 1000k Available Space 975k Process F 675k Process E 525k Process D 325k Process B 175k Process A 100k Operating system Process C Terminated Memory after Compaction 1000k Available Space 975k Process E 675k Process D 525k Available Space 325k Process B 175k Process A 100k Operating system

21. All such gains in sophistication have a cost in terms of complexity of implementation. The more intricate the algorithm, the more intricate the code needed to make it happen. Furthermore, an action such as compaction is not instantaneous but takes time. The overheads that compaction calls for tend, in fact, to be so great and make things soslow that the scheme is rarely used in practice. Paging if we assume that processes are relatively small and can fit into memory. From the very early days, there has been the problem that some programs are too large to fit into memory at all, never mind into a fraction of it such as a partition. In a paged system, processes are chopped up into a number of chunks all of the same size. These chunks are called pages.

22. Complete questions 5 – 12 on page’s 161/162.

23. Process management is to do with choosing which process to run, and how long to run it for. This is managed by part of the operating system called the scheduler. A scheduler works according to a particular scheduling algorithm. The algorithm is subject to certain constraints: • it should be fair, so that each process gets its fair share of the processor; • it should be efficient, to maximise use of the processor; • it should achieve high throughput; • it should have a good response time. Many processes are, obviously, started directly by the user. Others are started by the operating system, however processes can themselves start other processes. All these processes have a program counter, registers, and other information. The processor performs what’s called a context switch when it changes from one process to another

24. a process, once it had gained control of the processor, would run to completion. Such an approach is still tenable on PCs that expect to have only one user at a time (DOS works in this way), but won’t do at all on a multi-user or multi-tasking system. One answer is cooperative scheduling where each process would, in principle, run for a little and get a bit of work done. The problem with this is that processes tend to act selfishly and keep the processor to themselves for as long as they can. This is due to the operating system giving up too much control. Cooperative scheduling would work even worse in a multi-user system. Somebody The scheduler needs, then, to be able to suspend one process, at any point, and give another process a turn. The operating system keeps control. Such scheduling is known as pre-emptive scheduling.

25. Process states • A process enters the system and starts running. When it’s finished, it terminates. • While it’s in the system, the process can be in one of three states: • running; • blocked; • ready. • The process runs until obliged to wait for I/O or pre-empted by the scheduler (to give • another process a turn). • If it is pre-empted, it is still ready to use the processor. • If it is waiting for I/O, it is described as blocked, and stays in that condition until the I/O is over • look at Diagram 6.9 on page 163 There are 2 different scheduling algorithms. Round robin Priority scheduling

26. Round robin Processes are put in a sort of circular list. Each process in turn is assigned a time-slice. If the process is still running at the end of the time slice, it is pre-empted and has to wait its turn again Switching processes takes time and thus wastes processor time. Too long a time slice might mean irritatingly long waits. Round robin is suitable to interactive systems ( not real time). The algorithm is also fair, in that all jobs are treated as though they are equally important Priority scheduling In priority scheduling, each process is given a priority, and the job with the highest priority is processed. In many systems, there is a queue of jobs for each level or priority, and the jobs within a certain level are treated to round-robin scheduling. Because low-priority jobs might, on a busy system, never get a turn, things are sometimes organised so that jobs that have been waiting a long time have their priority raised.

27. Three algorithms used for priority scheduling are first come first served, shortest job first and multi-level feedback queues. First come, first served (FCFS) This algorithm is straightforward, and works well enough for long jobs. Job Processingtime Turnaroundtime A 20 20 B 10 30 C 4 34 D 2 36 This gives an average turnaround time of 30 minutes, which compares badly with an average run-time of 9 minutes. Shortest job first In batch systems, and other systems where the running time of a process can be predicted, we can use the principle that the shortest jobs should be run first. This gives an average turnaround time of 15 minutes: the longest job does worse but the three other jobs do a lot better. The shortest-job-first approach gives optimal turnaround times. On the other hand, long jobs on busy systems might have to wait indefinitely because shorter jobs keep getting in the way.

28. Multi-level Feedback Queues Each of these methods has advantages and also disadvantages. The multi-level feedback queue algorithm tries to combine the best of each. The top level queue has highest priority, but each process in this queue is only allocated one very short time slice. for a longer time independently of the processor (perhaps using DMA). If it is a longer process, it is given a short time slice, then moved down to the second level queue, so that the top level queue is not held up. The second level queue gets processor time if the top level queue is empty. If a process is till not completed within the longer time slice, it is pushed down to the next queue. The final queue is a round robin queue, which continues to allocate time slices to processes (when the processor is not being used by the higher queues) until they are completed.

29. Pupil task Now complete questions 13 -18 on page 166

30. Application program Input and output (I/O) Peripheral devices are, in general, different from one another: they work in different ways and at different speeds. In the I/O system, the software can be thought of as organised in layers: the upper one presents an interface to the user or application. The device driver communicates with the device controller, which is part of the hardware, plugged into the computer’s I/O bus. I/O Control System _________ Device Driver I/O Bus Device Controller ______________ Device the lower one deals directly with the hardware

31. Device independence The I/O system hides the physical details of a device from the users and presents then with straightforward means of access and control Input/Output in Windows A DLL is a piece of executable code (a device driver, for example) that can be shared by different processes. The routines in a DLL are only linked into an application at run time. In Windows, all the device drivers are dynamic link libraries. Plug and play Adding a new device to a system has in the past been rather a fiddly business. Often a new control card has to be plugged into the computer. Very often, switches on the device would have to be set by hand. Plug and play can be regarded as a sort of convention or standard agreed between computer manufacturers, chip manufacturers, device manufacturers, and so on. The user has to supply very little information during installation and nothing at all from then on.

32. Direct memory access A device controller will have its own memory on board. This enables it to use Direct Memory Access (DMA) to get data to and from the computer’s memory. In DMA, the processor only sets up the transfer. Thereafter, some clock cycles are used by the I/O system for the data transfer. Virtual devices A virtual device seems, to the user, like an actual device but is, in fact, a simulation maintained by the I/O system. A print spooler is a virtual device, that takes data sent to the printer, stores them temporarily on disc, and prints them when the system is ready to do so. Buffering Buffering is used in sending blocks of data. Data is transferred into the buffer until it is full. Then the entire block is sent at once. This is more efficient than having the data trickle through the system. Spooling Spooling involves the input or output of data to a tape or a disk. The print spooler stores the data in files and sends it to the printer when it is ready, using a print queue.

33. Pupil task Question 19,20 & 21 on page 170

34. File Management • The most familiar aspect of the Operating System to most users is whenever they carry out any of the following operations: • loading an application • saving a document • creating a new folder • renaming a file • deleting an unwanted application A major function of the file management section of the operating system is to map the logical view (as seen by the user) on to the physical view (as seen by the computer). Logical View of files • A file is a meaningful collection of data. The file will have • a name, • a file type, • a location (within a folder on a disc), • a size. • other attributes such as its creation date, • date last modified, • read-only.

35. Folders and Directories. • Applications install themselves into folders. • Users arrange their work into folders. • The arrangement of folders on the root containing folders that contain folders and so on, is often described as hierarchical. • Paths • In figure 6.11 (page 172), There is no confusion because the folders are on different paths: their full names, so to speak, are different. Physical View Most storage devices are divided up into tracks and sectors, forming blocks of a fixed size, perhaps 512 bytes. See diagram page 173 The term cluster (or blocks) is also used to describe the smallest section of a storage device that the operating system will recognise.

36. Contiguous Allocation the following sequence of user commands were required using a disk with 30 clusters, each of size 32K Command 1: save file 1 (size 64K) Command 2: save file 2 (size 97K) Command 3: save file 3 (size 3K) Command 4: save file 4 (size 220K)

37. Command 5: delete file 2 Command 6: save file 5 (220K) Command 7: copy file 1 (64K) Command 8: save file 6 (70K)

38. Command 9: save file 7 (190K) This will need 6 clusters. These must be contiguous, so clusters 6 and 7 or clusters 26 to 30 cannot be used. Although there are more than 6 free clusters, the file cannot be copied as the clusters are not contiguous. Command 10: add data to file 1 Perhaps file 1 is a word processed document, and the user loads it up, adds some extra paragraphs, then tries to save it. The system will be unable to save the file as the next cluster (cluster 4) is already in use.

39. Linked Allocation page 179 Indexed Allocation page 180 File Allocation Tables page 181***(FAT) The file management system in operation page 182 understand Diagram

40. Pupil task REVISION QUESTIONS ON PAGE 185