Chapter 3.1 The Functions of Operating Systems3.1 (a) The Main Features of Operating SystemsThe operating system (OS) must provide and manage hardware resources as well asprovide an interface between the user and the machine and between applicationssoftware and the machine. The OS must also provide other services such as datasecurity.Originally, if a program needed input, the program would have to contain the code toTo make full use of the processor, more than one program should be stored inmemory and the processor should give time to each of the programs. Suppose twoprograms are stored in memory and, if one is using an input or output device (bothvery slow compared to the processor), it makes sense for the other program to use theprocessor. In fact this can be extended to more than two programs as shown in Fig.3.1.a.1.The OS must now manage the memory so that all three programs shown in Fig.3.1.a.1 are kept separate as well as any data that they use. It must also schedule thejobs into a sequence that makes best use of the processor. Using I/O UsingProgram A processor required processor Using I/O UsingProgram B processor required processor Using I/OProgram C processor requiredProcessor Processor in use Processor in use Processor idle Fig. 3.1.a.1The I/O phase should not hold up the processor too much which can easily happen ifthe I/O devices are very slow, like a keyboard or printer. This can be overcome byusing Simultaneous Peripheral Operations On-Line (spooling). The idea is to store all
input and output on a high-speed device such as a disk. Fig. 3.1.a.2 shows how thismay be achieved, Input Output device Input Output device spool spool Application Read Write program process process Fig. 3.1.a.2Another problem is that programs may not be loaded into the same memory locationseach time they are loaded. For example, suppose that three programs are loaded inthe order A, B, C on one occasion and in the order C, A, B on another occasion. Theresults are shown in Fig. 3.1.a.3. OS OS Program A Program C Program B Program A Program C Program B Free Free Fig. 3.1.a.3A further problem occurs if two or more users wish to use the same program at thesame time. For example, suppose user X and user Y both wish to use a compiler forC++ at the same time. Clearly it is a waste of memory if two copies of the compilerhave to be loaded into main memory at the same time. It would make much moresense if user Xs program and user Ys program are stored in main memory togetherwith a single copy of the compiler as shown in Fig. 3.1.a.4.
OS User Xs program and data User Ys Program and data Compiler Free Fig. 3.1.a.4Now the two users can use the compiler in turns and will want to use different parts ofthe compiler. Also note that there are two different sets of data for the compiler, userXs program and user Ys program. These two sets of data and the outputs from thecompiler for the two programs must be kept separate. Programs such as this compiler,working in the way described, are called re-entrant.3.1 (b) InterruptsThe simplest way of obeying instructions is shown in Fig. 3.1.b.1. Start Fetch instruction Execute instruction Any more instructions? Yes No End Fig. 3.1.b.1
This is satisfactory so long as nothing goes wrong. Unfortunately things do go wrongand sometimes the normal order of operation needs to be changed. For example, auser has used up all the time allocated to his use of the processor. This change inorder is instigated by messages to the processor, called interrupts. There are a numberof different types of interrupt. The nature of each of these types of interrupt is I/O interrupt o Generated by an I/O device to signal that a job is complete or an error has occurred. E.g. printer is out of paper or is not connected. Timer interrupt o Generated by an internal clock indicating that the processor must attend to time critical activities (see scheduling later). Hardware error o For example, power failure which indicates that the OS must close down as safely as possible. Program interrupt o Generated due to an error in a program such as violation of memory use (trying to use part of the memory reserved by the OS for other use) or an attempt to execute an invalid instruction (such as division by zero).If the OS is to manage interrupts, the sequence in Fig. 3.1.b.1 needs to be modified asshown in Fig. 3.1.b.2. Start Fetch instruction Execute instruction Is there an interrupt? No Yes Service the interrupt Any more instructions? Yes No
End Fig. 3.1.b.2 This diagram shows that, after the execution of an instruction, the OS must see if an interrupt has occurred. If one has occurred, the OS must service the interrupt if it is more important than the task already being carried out (see priorities later). This involves obeying a new set of instructions. The real problem is how can the OS arrange for the interrupted program to resume from exactly where it left off? In order to do this the contents of all the registers in the processor must be saved so that the OS can use them to service the interrupt. Chapter 3.3 explains registers that have to have their contents stored as well as explaining the processing cycle in more detail. Another problem the OS has to deal with happens if an interrupt occurs while another interrupt is being serviced. There are several ways of dealing with this but the simplest is to place the interrupts in a queue and only allow return to the originally interrupted program when the queue is empty. Alternative systems are explained in Section 3.1.c. Taking the simplest case, the order of processing is shown in Fig.3.1.b.3. Start Fetch instruction Yes Execute instructionAny more instructions? Is there an interrupt No in the interrupt queue? No Yes Service the next interrupt End in the interrupt queue
Fig. 3.1.b.3The queue of interrupts is the normal first in first out (FIFO) queue and holdsindicators to the next interrupt that needs to be serviced.3.1 (c) SchedulingOne of the tasks of the OS is to arrange the jobs that need to be done into anappropriate order. The order may be chosen to ensure that maximum use is made ofthe processor; another order may make one job more important than another. In thelatter case the OS makes use of priorities.Suppose the processor is required by program A, which is printing wage slips for theemployees of a large company, and by program B, which is analysing the annual,world-wide sales of the company which has a turnover of many millions of pounds.Program A makes little use of the processor and is said to be I/O bound. Program Bmakes a great deal of use of the processor and is said to be processor bound.If program B has priority over program A for use of the processor, it could be a longtime before program A can print any wage slips.The objectives of scheduling are to maximise the use of the whole of the computer system; be fair to all users; provide a reasonable response time to all users, whether they are on-line users or a batch processing user; prevent the system failing if it is becoming overloaded; make sure that the system is consistent by always giving similar response times to similar activities from day to day.To achieve these objectives some criteria are needed in order to determine the order inwhich jobs are executed. The following is a list of criteria which may be used todetermine a schedule which will achieve the above objectives. Priority. Give some jobs a greater priority than others when deciding which job should be given access to the processor. I/O or processor bound. If a processor bound job is given the main access to the processor it could prevent the I/O devices being serviced efficiently. Type of job. Batch processing, on-line and real-time jobs all require different response times. Resource requirements. The amount of time needed to complete the job, the memory required, I/O and processor time. Resources used so far. The amount of processor time used so far, how much I/O used so far. Waiting time. The time the job has been waiting to use the system.
In order to understand how scheduling is accomplished it is important to realise thatany job may be in one, and only one, of three states. A job may be ready to start,running on the system or blocked because it is waiting for a peripheral, for example.Fig. 3.1.c.3 shows how jobs may be moved from one state to another. Note that a jobcan only enter the running state from the ready state. The ready and blocked statesare queues that may hold several jobs. On a standard single processor computer onlyone job can be in the running state. Also, all jobs entering the system normally entervia the ready state and (normally) only leave the system from the running state.When entering the system a job is placed in the ready queue by a part of the OS calledthe High Level Scheduler (HLS). The HLS makes sure that the system is not overloaded.Sometimes it is necessary to swap jobs between the main memory and backing store(see Memory Management in Section 3.1.d. This is done by the Medium LevelScheduler (MLS).Moving jobs in and out of the ready state is done by the Low Level Scheduler (LLS).The LLS decides the order in which jobs are to be placed in the running state. Thereare many policies that may be used to do scheduling, but they can all be placed in oneof two classes. These are pre-emptive and non-pre-emptive policies.A pre-emptive scheme allows the LLS to remove a job from the running state so thatanother job can be placed in the running state. In a non-pre-emptive scheme each jobruns until it no longer requires the processor. This may be because it has finished orbecause it needs an I/O device.Some common scheduling policies are First Come First Served (FCFS) Shortest Job First (SJF) Round Robin (RR) Shortest Remaining Time (SRT) Multi-level Feedback Queues (MFQ)and there are many more. FCFS o simply means that the first job to enter the ready queue is the first to enter the running state. This favours long jobs. SJF o simply means sort jobs in the ready queue in ascending order of time expected to be needed by each job. New jobs are added to the queue in such a way as to preserve this order. RR o this gives each job a maximum length of processor time (called a time slice) after which the job is put at the back of the ready queue and the
job at the front of the queue is given use of the processor. If a job is completed before the maximum time is up it leaves the system. SRT o the ready queue is sorted on the amount of expected time still required by a job. This scheme favours short jobs even more than SJF. Also there is a danger of long jobs being prevented from running. MFQ o involves several queues of different priorities with jobs migrating downwards.There are other ways of allocating priorities. Safety critical jobs will be given veryhigh priority, on-line and real time applications will also have to have high priorities.For example, a computer monitoring the temperature and pressure in a chemicalprocess whilst analysing results of readings taken over a period of time must give thehigh priority to the control program. If the temperature or pressure goes out of a pre-defined range, the control program must take over immediately. Similarly, if a bankscomputer is printing bank statements over night and someone wishes to use a cashpoint, the cash point job must take priority. This scheme is shown in Fig. 3.1.c.4; thisshows that queues are needed for jobs with the same priority. Priority 1 (high) Queue P r o Priority 2 Queue c e s Priority 3 Queue s o r Priority 4 (low) Queue Fig. 3.1.c.4In this scheme, any job can only be given use of the processor if all the jobs at higherlevels have been completed. Also, if a job enters a queue that has a higher prioritythan the queue from which the running program has come, the running program isplaced back in the queue from which it came and the job that has entered the higherpriority queue is placed in the running state.Multi-level feedback queues work in a similar way except that each job is given amaximum length of processor time. When this time is up, and the job is notcompletely finished, the job is placed in the queue which has the next lower prioritylevel. At the lowest level, instead of a first in first out queue a round robin system isused.
3.1 (d) Memory ManagementThis section can become very complex. In an examination the questions will belimited to basic definitions and explanations. Calculations of the addresses and otherdetail will not be required, they are included here for completeness of the topic forthose students who wish to understand in more detail.In order for a job to be able to use the processor the job must be stored in thecomputers main memory. If there are several jobs to be stored, they, and their data,must be protected from the actions of other jobs.Suppose jobs A, B, C and D require 50k, 20k, 10k and 30k of memory respectivelyand the computer has a total of 130k available for jobs. (Remember the OS willrequire some memory.) Fig. 3.1.d.1 shows one possible arrangement of the jobs. Free 20k Job D 30k Job C 10k 130k available Job B 20k for jobs Job A 50k OS Fig 3.1.d.1Now suppose job C terminates and job E, requiring 25k of memory, is next in theready queue. Clearly job E cannot be loaded into the space that job C hasrelinquished. However, there is 20k + 10 k = 30k of memory free in total. So the OSmust find some way of using it. One solution to the problem would be to move job Dup to job B. This would make heavy use of the processor as not only must all theinstructions be moved but all addresses used in the instructions would have to berecalculated because all the addresses will have changed.
When jobs are loaded into memory, they may not always occupy the same locations.Supposing, instead of jobs A, B, C and D being needed and loaded in that order, it isrequired to load jobs A, B, D and E in that order. Now job D occupies differentlocations in memory to those shown above. So again there is a problem of usingdifferent addresses.The OS has the task of both loading the jobs and adjusting the addresses. The part ofthe OS which carries out these tasks is a program called the loader. The calculation ofaddresses can be done by recalculating each address used in the instructions once theaddress of the first instruction is known. Alternatively, relative addressing can beused. That is, addresses are specified relative to the first instruction.Another problem to be solved is when to move jobs. Possible solutions are whenever a job terminates; when a new job is too large for any existing space; at regular intervals; when the user decides.This system is known as variable partitioning with compaction. Imagine that each jobneeds a space to fit into, this space is the partition. Each of the jobs requires adifferent size of space, hence “variable partitions”. These variable partitions arenormally called segments and the method of dividing memory up is calledsegmentation. We also saw that sometimes it is necessary to move jobs around so thatthey fill the „holes‟ left by jobs that leave, this is called “compaction”.An alternative method is to divide both the memory and the jobs into fixed size unitscalled “pages”. As an example, suppose jobs A, B, C, D and E consist of 6, 4, 1, 3and 2 pages respectively. Also suppose that the available memory for jobs consists of12 pages and jobs A, B and C have been loaded into memory as shown in Fig. 3.1.d.2. Job A Job B Job C Memory Page 6 Page 4 Page 1 Free Page 5 Page 3 C1 Page 4 Page 2 B4 Page 3 Page 1 B3 Page 2 B2 Page 1 B1 A6 A5 A4 A3 A2 A1 Fig. 3.1.d.2
Now suppose job B terminates, releasing four pages, and jobs D and E are ready to beloaded. Clearly we have a similar problem to that caused by segmentation. The holeconsists of four pages into which job D (three pages) will fit, leaving one page plusthe original one page of free memory. E consists of two pages, so there is enoughmemory for E but the pages are not contiguous, in other words they are not joinedtogether and we have the situation shown in Fig. 3.1.d.3. Job E Memory Page 2 Free Page 1 C1 Free D3 D2 D1 A6 A5 A4 A3 A2 A1 Fig. 3.1.d.3The big difference between partitioning and paging is that jobs do not have to occupycontiguous pages. Thus the solution is shown in Fig. 3.1.d.4. Memory E2 C1 E split E1 D3 D2 D1 A6 A5 A4 A3 A2 A1 Fig. 3.1.d.4The problem with paging is again address allocation. This can be overcome bykeeping a table that shows which memory pages are used for the job pages. Then, ifeach address used in a job consists of a page number and the distance the requiredlocation is from the start of the page, a suitable conversion is possible.
Suppose, in job A, an instruction refers to a location that is on page 5 and is 46locations from the start of page 5. This may be represented by 5 46Now suppose we have the following table Job Page Memory Page A1 4 A2 5 A3 6 A4 7 A5 8 A6 9We see that page A5 is stored in page 8 of memory, thus 5 46 Becomes 8 46Paging uses fixed length blocks of memory. An alternative is to use variable lengthblocks. This method is called segmentation. In segmentation, programmers dividejobs into segments, possibly of different sizes. Usually, the segments would consistof data, or sub-routines or groups of related sub-routines.Since segments may be of different lengths, address calculation has to be carefullychecked. The segment table must not only contain the start position of each segmentbut also the size of each segment. This is needed to ensure that an address does notgo out of range. Fig. 3.1.d.5 shows how two jobs may be stored in memory. In thiscase the programmer split Job A into 4 segments and Job B into 3 segments. Thesetwo jobs, when loaded into memory, took up the positions shown in the Figure.
Now suppose that an instruction specifies an address as segment 3, displacement(from start of segment) 132. The OS will look up, in the process segment table, thebasic address (in memory) of segment 3. The OS checks that the displacement is notgreater than the segment size. If it is, an error is reported. Otherwise thedisplacement is added to the base address to produce the actual address in memory tobe used. The algorithm for this process is 1. Get segment number. 2. Get displacement. 3. Use segment number to find length of segment from segment table. 4. If displacement is greater than segment size, 4.1 produce error message 4.2 stop. 5. Use segment number to find base address of segment from segment table. 6. Add displacement to base address to produce physical address.3.1 (e) SpoolingSpooling was mentioned in Section 3.1.a and is used to place input and output on afast access device, such as disk, so that slow peripheral devices do not hold up theprocessor. In a multi-programming, multi-access or network system, several jobs maywish to use the peripheral devices at the same time. It is essential that the input andoutput for different jobs do not become mixed up. This can be achieved by usingSimultaneous Peripheral Operations On-Line (spooling).Suppose two jobs, in a network system, are producing output that is to go to a singleprinter. The output is being produced in sections and must be kept separate for eachjob. Opening two files on a disk, one for each job, can do this. Suppose we call thesefiles File1 and File2. As the files are on disk, job 1 can write to File1 whenever itwishes and job 2 can write to File2. When the output from a job is finished, the name(and other details) of the file can be placed in a queue. This means that the OS nowcan send the output to the printer in the order in which the file details enter the queue.As the name of a file does not enter the queue until all output from the job to thecorresponding file is complete, the output from different jobs is kept separate.Spooling can be used for any number of jobs. It is important to realise that the outputitself is not placed in the queue. The queue simply contains the details of the files thatneed to be printed so that the OS sends the contents of the files to the printer onlywhen the file is complete. The part of the OS that handles this task is called thespooler or print spooler.
3.1.(f) Desktop PC Operating SystemsThere are basically two types of OS used on PCs. These are command driven andthose that use a graphical user interface (GUI). Probably the best known of these areMS-DOS (command driven) and Windows (GUI). These differ in the way the useruses them and in the tasks that can be carried out.All OSs for PCs allow the user to copy, delete and move files as well as letting theuser create an hierarchical structure for storing files. They also allow the user tocheck the disk and tidy up the files on the disk.However, Windows allows the user to use much more memory than MS-DOS and itallows multi-tasking. This is when the user opens more than one program at a timeand can move from one to another. Try opening a word processor and the clipboardin Windows at the same time. Adjust the sizes of the windows so that you can seeboth at the same time. Now mark a piece of text and copy it to the clipboard. Youwill see the text appear in the clipboard window although it is not the active window.This is because the OS can handle both tasks apparently at the same time. In fact theOS is swapping between the tasks so fast that the user is not aware of the swapping.Another good example of multi-tasking is to run the clock program while usinganother program. You will see that the clock is keeping time although you are usinganother piece of software. Try playing a CD while writing a report!The OS not only offers the user certain facilities, it also provides application softwarewith I/O facilities. In this Section you will see how an OS is loaded and how itcontrols the PC.This section, printed with a shaded background, is not required by the CIE ComputingSpecification, but may be interesting and useful for understanding how the systemworks.When a PC is switched on, it contains only a very few instructions. The first step thecomputer does is to run the power-on-self-test (POST) routine that resides inpermanent memory. The POST routine clears the registers in the CPU and loads theaddress of the first instruction in the boot program into the program counter. Thisboot program is stored in read-only memory (ROM) and contains the basicinput/output system (BIOS).Control is now passed to the boot program which first checks itself and the POSTprogram. The CPU then sends signals to check that all the hardware is workingproperly. This includes checking the buses, systems clock, RAM, disk drives andkeyboard. If any of these devices, such as the hard disk, contain their own BIOS, thisis incorporated with the systems BIOS. Often the BIOS is copied from a slow CMOSBIOS chip to the faster RAM chips.The PC is now ready to load the OS. The boot program first checks drive A to see if adisk is present. If one is present, it looks for an OS on the disk. If no OS is found, anerror message is produced. If there is no disk in drive A, the boot program looks foran OS on disk C. Once found, the boot program looks, in the case of Windows
systems, for the files IO.SYS and MSDOS.SYS. Once the files are found, the bootprogram loads the boot record, about 512 bytes, which then loads IO.SYS. IO.SYSholds extensions to the ROM BIOS and contains a routine called SYSINIT. SYSINITcontrols the rest of the boot procedure. SYSINIT now takes control and loadsMSDOS.SYS which works with the BIOS to manage files and execute programs.The OS searches the root directory for a boot file such as CONFIG.SYS which tellsthe OS how many files may be opened at the same time. It may also containinstructions to load various device drivers. The OS tells MSDOS.SYS to load a filecalled COMMAND.COM. This OS file is in three parts. The first part is a furtherextension to the I/O functions and it joins the BIOS to become part of the OS. Thesecond part contains resident OS commands, such as DIR and COPY.The files CONFIG.SYS and AUTOEXEC.BAT are created by the user so that the PCstarts up in the same configuration each time it is switched on.The OS supplies the user, and applications programs, with facilities to handle inputand output, copy and move files, handle memory allocation and any other basic tasks.In the case of Windows, the operating system loads into different parts of memory.The OS then guarantees the use of a block of memory to an application program andprotects this memory from being accessed by another application program. If anapplication program needs to use a particular piece of hardware, Windows will loadthe appropriate device driver. Windows also uses virtual memory if an applicationhas not been allocated sufficient main memory.As mentioned above, Windows allows multi-tasking; that is, the running of severalapplications at the same time. To do this, Windows uses the memory managementtechniques described in Section 3.1.d. In order to multi-task, Windows gives eachapplication a very short period of time, called a time-slice. When a time-slice is up,an interrupt occurs and Windows passes control to the next application. In order to dothis, the OS has to save the contents of the CPU registers at the end of a time-slice andload the registers with the values needed by the next application. Control is thenpassed to the next application. This is continued so that all the applications have useof the processor in turn. If an application needs to use a hardware device, Windowschecks to see if that device is available. If it is, the application is given the use of thatdevice. If not, the request is placed in a queue. In the case of a slow peripheral suchas a printer, Windows saves the output to the hard disk first and then does the printingin the background so that the user can continue to use the application. If furtherprinting is needed before other printing is completed, then spooling is used asdescribed in Section 3.1.e.Any OS has to be able to find files on a disk and to be able to store users files. To dothis, the OS uses the File Allocation Table (FAT). This table uses a linked list topoint to the blocks on the disk that contain files. In order to do this the OS has aroutine that will format a disk. This simply means dividing the disk radially intosectors and into concentric circles called tracks. Two or more sectors on a singletrack make up a cluster. This is shown in Fig. 3.1.f.1.
Cluster Sectors using 3 sectors Tracks Fig 3.1.f.1A typical FAT table is shown in Fig 3.1.f.2. The first column gives the clusternumber and the second column is a pointer to the next cluster used to store a file. Thelast cluster used has a null pointer (usually FFFFH) to indicate the end of the linking.The directory entry for a file has a pointer to the first cluster in the FAT table. Thediagram shows details of two files stored on a disk. Cluster Pointer Pointer from 0 FFFD directory entry for 1 FFFF File 1 2 3 3 5 Pointer from 4 6 directory entry for 5 8 File 2 6 7 7 10 8 9 9 FFFF End of File 1 is in 10 11 cluster 9 11 FFFF End of File 2 is in cluster 11 Fig. 3.1.f.2In order to find a file, the OS looks in the directory for the filename and, if it finds it,the OS gets the cluster number for the start of the file. The OS can then follow thepointers in the FAT to find the rest of the file.In this table any unused clusters have a zero entry. Thus, when a file is deleted, theclusters that were used to save the file can be set to zero. In order to store a new file,all the OS has to do is to find the first cluster with a zero entry and to enter the clusternumber in the directory. Now the OS only has to linearly search for clusters with zeroentries to set up the linked list.
It may appear that using linear searches will take a long time. However, the FATtable is normally loaded into RAM so that continual disk accesses can be avoided.This will speed up the search of the FAT.Note that Windows 95/98 uses virtual FAT (VFAT) which allows files to be saved 32bits at a time (FAT uses 16 bits). It also allows file names of up to 255 characters.Windows 98 uses FAT 32 which allows hard drives greater than 2 Gbytes to beformatted.
3.1.(g) Network Operating SystemsThis Section should be read in conjunction with Chapters 1.6 from the AS text and3.10 from this A2 text.The facilities provided by a NOS depend on the size and type of network. Forexample, in a peer-to-peer network all the stations on the network have equal status.In this system one station may act as a file server and another as a print server. At thesame time, all the stations are clients. A client is a computer that can be used by usersof the network. A peer-to-peer network has little security so the NOS only has tohandle communications, file sharing, printing.If a network contains one or more servers, the NOS has to manage file sharing, file security, accounting, software sharing, hardware sharing (including print spooling), communications, the user interface.File sharing allows many users to use the same file at the same time. In order to avoidcorruption and inconsistency, the NOS must only allow one user write access to thefile, other users must only be allowed read access. Also, the NOS must only allowusers with access rights permission to use files; that is, it must prevent unauthorisedaccess to data. It is important that users do not change system files (files that areneeded by the NOS). It is common practice for the NOS to not only make these filesread only, but to hide them from the users. If a user looks at the disk to see what filesare present, these hidden files will not appear. To prevent users changing read onlyfiles to read write files, and to prevent users showing hidden files, the NOS does notallow ordinary users to change these attributes.To ensure the security of data, the network manager gives users access rights. Whenusers log onto a network they must enter their user identity and password. The NOSthen looks up, in a table, the users access rights and only allows them access to thosefiles for which access is permitted. The NOS also keeps a note of how the users wanttheir desktops to look. This means that when users log on they are always presentedwith the same screen. Users are allowed to change how their desktops look and theseare stored by the NOS for future reference.As many users may use the network and its resources, it may be necessary for theNOS to keep details of who has used the network, when and for how long and forwhat purpose. It may also record which files the user has accessed. This is so that theuser can be charged for such things as printing, the amount of time that the network
has been used and storage of files. This part of the NOS may also restrict the usersamount of storage available, the amount of paper used for printing and so on. Fromtime to time the network manager can print out details of usage so that charges maybe made.The NOS must share the use of applications such as word processors, spreadsheetsand so on. Thus when a user requests an application, the NOS must send a copy ofthat application to the users station.Several users may well wish to use the same hardware at the same time. This isparticularly true of printers. When a user sends a file for printing, the file is split intopackets. As many users may wish to use a printer, the packets from different userswill arrive at the print server and will have to be sorted so that the data from differentusers are kept separate. The NOS receives these packets and stores the data indifferent files for different users. When a particular file is complete, it can be addedto the print queue as described in Section 3.1.e.The NOS must also ensure that users files are saved on the server and that theycannot be accessed by other users. To do this the network manager will allocate eachuser a fixed amount of disk space and the NOS will prevent a user exceeding theamount of storage allocated. If a user tries to save work when there is insufficientspace left, the NOS will ask the user to delete some files before the user can save anymore. In order to do this, the servers hard drive may be partitioned into many logicaldrives. This means that, although there may be only one hard drive, different parts ofit can be treated as though they are different drives. For example, one part may becalled the H drive which is where users are allowed to save their work. This drivewill be divided up into folders, each of which is allocated to a different user. Usersonly have access to their own folders but can create sub-folders in their own folders.The NOS must provide this service as well as preventing users accessing other usersfolders. Another part of the drive may be called (say) the U drive where some userscan store files for other users who will be allowed to retrieve, but not alter, themunless they are saved in the users own area. The NOS will also only allow access tocertain logical drives by a restricted set of users.For all the above to work, the NOS will have to handle communications betweenstations and servers. Thus, the NOS is in two parts. One part is in each station andthe other is in the server(s). These two parts must be able to communicate with oneanother so that messages and data can be sent around the network. The need for rulesto ensure that communication is successful was explained in Chapter 1.6 in the AStext.Finally, the NOS must provide a user interface between the hardware, software anduser. This has been discussed in Section 3.1.f, but a NOS has to offer different usersdifferent interfaces. When a user logs onto a network, the NOS looks up the needs ofthe user and displays the appropriate icons and menus for that user, no matter whichstation the user uses. The NOS must also allow users to define their own interfaceswithin the restrictions laid down by the network manager.It must be remembered that users must not need an understanding of all the tasksundertaken by the NOS. As far as users are concerned they are using a PC as if it
were solely for their own use. The whole system is said to be transparent to the user.This simply means that users are unaware of the hardware and software actions. Agood user interface has a high level of transparency and this should be true of alloperating systems.