1. Operating Systems c. define and explain the purpose of scheduling, job queues, priorities and how they are used to manage job throughput; d. explain how memory is managed in a typical modern computer system (virtual memory, paging and segmentation should be described along with some of the problems which could occur, such as disk threshing); e. describe spooling, explaining why it is used; f. 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.

2. Scheduling The operating system allocates priorities to jobs. Deciding which job to run next is the job of the scheduler. The scheduler must: ● Maximise throughput, try to process as many jobs as possible in as little time as possible. ● Balance resource use, ie if a printer is doing nothing a higher priority could be assigned to a program that needs a printer. ● Avoid pushing low priority jobs to the back of the queue forever.

3. Round robin scheduling In Round robin scheduling each task is dealt with in a first-in- first-out basis. Each task is given a limited amount of CPU time. If the task is not completed in the time allocated the processor is given to the next task.

4. Memory management Computers hold several programs in memory at once. The Operating System needs to allocate memory to each application as well as itself.

5. Virtual memory is a technique for making the computer appear to have more memory than it actually does. Processes are held on a specially designated area of the hard disk with only the part of the process that is currently being used loaded in to RAM. Parts of the process (called pages) are swapped in and out of RAM as necessary. The OS loads the different processes in and out of RAM when necessary. Virtual memory PROCESS

7. Paging In Paging each process is divided up in to fixed length blocks called pages. When a process is to be executed only the pages that are immediately required are loaded in to memory. Each process has a page managment table which states whether or not a particular page of the process is loaded or not.

8. Disk Thrashing Thrashing occurs when a computer's virtual memory is in a constant state of paging, rapidly exchanging data in memory for data on disk, to the exclusion of most application-level processing.This causes the performance of the computer to degrade or collapse. Basically the OS concentrates too much on memory management and not enough on executing processes.

9. Segmentation Segmentation just means the division of the computer's main memory into smaller units or segments. Each segment has a size (ie 1 byte) and a set of permissions that decide which process can write to it.

10. Spooling SPOOL stands for Simultaneous Peripheral Operations OnLine. It just means storing jobs in a queue so the process can do other things. It is most often used today for printing. Imagine if you couldn't use Word until your 200 page document had printed.

11. FAT (File Allocation Table) Space on the harddrive is split into fixed size allocation units. A file will take up a number of allocation units. The file system maintains a table called a File Allocation Table which details the content of every unit on the disk. H^*88^VYTNB54$£$%GART)(90NVHTYV&& test.doc rage.mp3

12. Boot file The OS is held on disk and must be loaded when the computer is turned on. This done using a boot program. The boot program is a very small program that searches the available storage locations for an OS and loads the first one that it finds. You can configure the boot program from the BIOS.