Computing Higher - Unit 1 - Computer Systems: Topic 7: Computer Software 1 Systems Software System software controls the actual operation of the computer

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Computing Higher - Unit 1 - Computer Systems: Topic 7: Computer Software

Systems SoftwareSystem software controls the actual operation of the computer system. The operating system is part of the system software. The purpose of an operating system is to provide the user with a means of operating the computer. An operating system can be described as carrying out these main tasks:1. Interpreting user commands;2. File management;3. Memory management;4. Input and output; 5. Managing Processes;6. Resource Allocation.

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Starting Up

When a computer is powered up it carries out a series of tasks to check that the basic hardware is operating correctly. It then tries to load the operating system program into the main memory (RAM). A small program called a bootstrap loader that is held in ROM carries out checking hardware, locating and loading the operating system. Once the operating system is loaded into main memory, it runs until the computer is switched off.

Interpreting the user's commands

This part of the operating system accepts the commands that the user enters into the system. It checks that the commands are valid and then translates them into instructions that the computer system can follow, then passes them on to the correct part of the OS to be carried out. User commands are either typed at the keyboard or are entered using a mouse. Systems that allow the user to use a mouse to issue commands use a WIMP environment.

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File Management

The file system manages how files are stored.

In order to determine where on a disk to place a file for later retrieval the file system must be able to distinguish which sectors are free, and which sectors are already in use.

The simplest way of finding free space would be to search the disk until a suitable space can be found. This would be too slow for practical purposes.

A better approach is to use an index file to keep track of the files and space on a disk.

The use of an index means that some of the space on the disk is used to keep the index file, reducing the amount of space available for user files.

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Hierarchical Filing Systems

A hierarchical filing system is shown opposite. The folder icons represent directories, while the rectangle icons represent files.

This representation allows users to group related data into directories.

It is possible with this structure to have many files of the same name existing in different directories.

The file management system holds pathnames, providing unique identifiers for every file and directory that is created.

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Memory ManagementThe memory manager is the part of the operating system that regulates the way that memory is used. Look at the memory map below.The lowest area of memory is used by the operating system (OS);The second block of memory (area A) is currently unused;Area B holds the program and data of Process 1;Area C holds the program and data of Process 2;Area D is currently unused;Area E holds the program and data of Process 3;Area F is currently unused.

In an unregulated system it is possible that Process 1 (currently residing in area B) could require more memory, and expand upwards into area C. Unfortunately area C is currently occupied by Process 2, and the expansion of Process 1 would result in data loss/ corruption in Process 2. The role of a memory manager is to prevent this sort of occurrence, by managing the memory resource.

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Input/OutputThe I/O system handles the interactions between the processor and all the devices that are part of the computer system as a whole. Among devices that are regarded as part of the computer could be listed the keyboard, the mouse, disk drives, modems and screen (often called a monitor).These devices are, in general, different from one another: they work in different ways and at different speeds. All these speeds, however, are slower than the computer’s processor and memory. One of the I/O system’s objectives is to make the attached devices to work at their highest useful rates.A device driver is a piece of software that handles one device or type of device. For example, every printer on a computer system will need a device driver, but that device driver might work with more than one model of printer. The device driver converts commands originating from the user into instructions for the device to carry out.

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Managing Processes: A process is a program that is being executed as well as all the resources which are associated with that program, (buffers, memory locations, input and output devices, data files etc). In a single-program operating system it only has to choose between the operating system itself and the package being run. The managing processes part of the OS allocates resources to the process such as memory, files and buffers eg keyboard buffers.It schedules processor time, planning out the processors workload.Maintains the integrity of a process making sure it does not become corruptTerminates the process and restores all system facilities so that they can be accessed by other processes.

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Resource Allocation: This is at the heart of managing processes. This is allocating memory and processor time to programs. Allocating Memory: This is a matter of admitting processes to memory if there is enough space, or delaying until there is space available.In a single-program OS the only processes being run are the OS and the user’s one program that is running.

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Utility ProgramsUtility programs are those programs that are used to enhance the operating system, or in some other way improve the usefulness of the system.Most operating systems will ship with a collection of utility programs. Examples of these include:Disk formatting programs - used to prepare the surface of writeable media for use.Disk defragmentation tools - reorganises the way the files are laid out on the disk so that the components of a file are near to each other. This makes files must faster to load and improves systems performance.

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Utility ProgramsDe-FragmentationMost modern filing systems save files where they can on a disk. Files are stored in areas on the disk called blocks. They will try to save a file in consecutive blocks. Below Program 1 takes 5 blocks and Program 2 takes only two blocks.

However, as time goes on, and files are saved and/or deleted, this isn’t always possible. For example, in the above situation, Program 1 is edited until it is only 3 blocks large. Program 3 is developed and saved taking up 3 blocks.

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Utility ProgramsDe-FragmentationProgram 1 is further developed and it becomes only 2 blocks large.

You can see above that soon spaces appear and programs appear disjointed. This will obviously slow down the loading of the file as the drive head will have to keep moving to different parts of the disc to load the file.A ‘defragger’ is a utility program that moves files (and parts of files) around the disc until each file is stored in consecutive sectors with no fragmentation.

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Utility ProgramsFile compression - File compression utilities are used to reduce the amount of space that a file takes up on disk.

Virus checking tools - used to prevent the spread of small, usually malignant programs which need to be executed before they can be effective, so viruses often attach themselves to program files.

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Graphic File FormatsJPEGA JPEG file is created by choosing one of the set of compression algorithms available. For example, similar colours and minor changes in colour, need not be coded separately.JPEG processing makes image files small by removing detail. This is called a lossy compression. This will reduce the number of colours used to store the image and avoid unnecessary repetition of bit patterns.The highest quality results in the largest file size so a trade off has to be made between image quality and file size.

ExampleA graphic is scanned creating a bit-mapped image. To save on file size the image is saved as a JPG file. However, software allows the file to be saved in Low, Medium, High and Maximum quality settings. There is a trade off between image quality and file size. The user of the system must decide on what they want:

better quality image (closer to original) : smaller file size

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GIFGIF images are based on a compression algorithm that creates a codebook or dictionary of particular bit patterns. These in turn, are then substituted resulting in a smaller file. When decoding, the algorithm uncompresses the file to generate the original image.Significantly, GIFs preserve all the precise shapes in an image i.e. result in ‘lossless’ compression.

It uses 8 bit colour (256 colours)It is normally used for internet graphics because files compress well using lossless compression.Compression saves on storage space and transmission time (bandwidth)Can be used for internet charts, cartoons, tech drawings or screen dumpsNOT suitable for photographic data images which lose quality if saved in 8-bit format.

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TIFFTIFF (Tag Image File Format) is a common format for encoding bitmap images e.g. scanner images, and can be any resolution.

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Computing Higher - Unit 1 - Computer Systems: Topic 7: Computer Software 1 Systems Software System software controls the actual operation of the computer