Chapter 7 - Functions of an operating system
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What are some functions of the operating system?
» Hardware » User interface » Memory management » File management » Multitasking » Scheduling
What is a helpful acronym to remember the functions of the operating system?
» MUSH FM
What is multitasking? 2 marks
» When you have more than one program open and running at the same time » Processor allocates a small amount of time to each process and cycles between them
What is user management?
» Allows multiple users to log into the same computer » OS will retain settings for each use
What is WIMP?
» Windows » Icon » Menus » Pointers
What is an operating system?
» A collection of programs that work together to provide an interface between the user and the computer » Manages the computer’s hardware
What is the user interface?
» The means by which the computer and user interact » Can be a commmand line interface » Can be a GUI interface
Where is the OS held?
» On the hard disk
How does is the OS loaded into RAM?
» Small program called loader which is in ROM sends instructions to load operating system by copying it from storage into RAM
Why is memory management needed? 2 marks
» Protects programs from each other, such as being overwritten » To enable memory to be shared » To rellocate memory when necessary
How does the operating system manage memory management?
» Memory is used efficently - program that are being used need to be stored in memory without space being wasted
What are the 2 ways in which memory can be split?
» Paging » Segmentation
What is paging? 3 marks
» Memory is split into equal-sized sections known as pages » Can be swapped between main memory and the hard disk as needed » Pages are physical divison that are made to fit sections of memory
What is segmentation? 3 marks
» Splitting up of memory into logical sized divisions known as segments » Segments are different sizes » Segments are complete sections of programs
When is virtual memory needed?
» When there is insufficient memory space in RAM to store prgorams
Which is more expensive, RAM or secondary storage?
» RAM
How does virtual memory work?
» Uses a section of the hard drive to act as RAM » Sections of program that are not currently in use are temporarily moved into virtual memory through paging. » Pages are swapped to main memory when required
What is disk thrashing?
» When the computer slows down » Due to pages being swapped too frequently between the hard disk and main memory » As a result more time is spent transferring these pages, then actually running the program
What is an interrupt?
» Signals generated by software or hardware to indicate to the processor that a process needs attention
What is the interrupt Service routine?
» Program code that is called when an interrupt is called » Checks to see the priority of the interrupt
What is a priority queue?
» Stores the interrupts in order of their priority
When does the processor check the contents of the interrupt?
» At the end of each FDE cycle
What are the steps of the ISR when the priority is higher priority then the process being executed?
» Contents of the program counter and other special purpose registers are copied transferred to a stack » Relevant ISR can be loaded by changing the program counter to the locaiton of the ISR in memory » When ISR is complete, previous values of the program counter and other registers can be restored by popping the stack
What happens when an interrupt is being services a new higher priority interrupt is raised?
» The interrupt currently being services is also added to a stack in memory and the new interrupy is serviced