3.3.1.6 Modern PC operating systems Describe the main components of a typical desktop PC operating system, including the file allocation table (FAT) and how it is used, and the purpose of the boot file.

Modern PC operating systems There are basically two types of OS used on PCs. GUI (Windows) Allows the user to use much more memory than MS-DOS Allows multi-tasking. Command Driven (MS-DOS) Relies on user typed commands. Only one command (per prompt) can be executed at one time. All OS's for PC's allow the user to copy, delete and move files as well as letting the user create an hierarchical structure for storing files. They also allow the user to check the disk and tidy up the files on the disk. The OS not only offers the user certain facilities, it also provides application software with I/O facilities.

Disk management - FAT The file allocation table (FAT) is a map of a disk’s storage area, which contains details of the clusters that are used by files and directories, those that are un-used and those that cannot be used because they contain bad sectors. A hard disk is divided into identically sized clusters each of which contains several sectors. Each file will occupy one or more clusters. For each file, the FAT will store: the ID of the first cluster in a chain; a pointer to the next cluster in the chain; and a special end of chain (EOC) entry that indicates the end of a chain.

Disk management - FAT most disks contain two FATs – the second copy protects against loss of data if the first FAT is damaged; the file allocation table is created (or replaced) when a disk is formatted; early versions of FAT have been replaced with NTFS in modern versions of Windows™; the FAT and disk directory, both take up storage space on a disk – this is why a disk has less capacity after formatting than its theoretical maximum.

Disk management – Disk directory This stores information about the files on a disk. It contains: file names; file sizes; date and time of last modification; details on access rights. Both the FAT and the disk directory are used by the operating system whenever files are accessed and it is updated whenever they are saved or deleted.

Saving and loading files When a command is issued to save/ load a file to/ from disk, the file management sub-system does the following: Saving checks that there is enough available space on the disk; uses the FAT to find the first available space on the disk; instructs the I/O sub-system to actually write the file to the specified location; updates the FAT and directory to include the details of the new file. Loading searches the directory to find a match with the given filename; uses the FAT to obtain the address of the file’s first block; obtains the size of the file; starts to load the file into memory checks that the file is not corrupted; check the file’s status – not already opened.

Booting Booting is the process of starting the computer. The main sequence of start-up events are to check the basic hardware, load low-level drivers and then initialise the loading of the operating system into memory. This whole process is started by the boot file which is located in the BIOS. Part of the booting process is to determine the boot device (the device that contains the operating system). The user can decide the order in which potential boot devices should be tried and save these settings in the BIOS. During start-up, the computer goes through this pre-configured list until it finds a device that is bootable – a common setting is to try the CD/DVD drive first followed by the primary hard disk When a bootable device is found, the boot sector of the device is loaded otherwise an error is displayed.

Boot file The boot file is used to start a computer. The boot file: is executed automatically whenever the computer is switched on or restarted; runs a Power-On Self Test (POST) – to check and initialise the required hardware such as memory and hard disks; obtains the settings that have been saved by the user – e.g. which devices are disabled; the boot order; finds the boot loader in ROM; initialises the operating system; passes control to the operating system.