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What does the OS do?
The OS manages the hardware and provides an interface for the user and the application software
Memory management- paging
-A page is a fixed-sized division of physical memory.
-Memory manager gives each page a number and records the location in a ‘page table’.
-A process currently in use may be held in non-contiguous pages.
-A page table uses mapping to store a link between the physical memory address and the logical address space of each process.
Memory management- segmentation
-Segments are differently-sized complete logical sections of programs
-They are logical divisions
-E.g., if there are two free spaces but a program is too big to fit all of it in either space then the program will be split up LOGICALLY so a subroutine could be stored in one free space and the main program is stored in the other space
-WHEREAS, if it were paging, the program would fill up the first free space and then the rest of the program would be stored in the remaining free space.
Memory management- virtual memory
-When the RAM runs out of space, programs that aren’t being used currently are moved to the virtual memory on the hard disk.
-When the programs are needed again, they can be swapped out for another program in the main memory.
-However, the disadvantage of using virtual memory is that it is slower since fetching programs from virtual memory takes longer than fetching from RAM.
What is an interrupt?
An interrupt is a way for other devices and applications to signal to the processor that they require attention during a FDE cycle- they always have higher priority than normal programs.
Interrupt Service Routine
-When there is an interrupt, the processor needs to stop executing its current program to run code for the interrupt so there is an interrupt service routine (a program with a set of instructions that need to be fetched, decoded, and executed to carry out operations of the interrupt).
-When an interrupt is received, the values held in the registers (PC, MAR, MDR, CIR, ACC) are pushed onto a stack in a stack frame, saving them for later retrieval.
-The interrupt is executed.
-Once the interrupt is complete, the frame is popped off the top of the stack and the processor carries on executing the original program.
-IF an interrupt is interrupted, the interrupt service routine is suspended, its registers’ contents are pushed onto a stack and the second interrupt service routine is executed.
-Interrupt examples: screen recording application, power supply failure, etc.
Peripheral Management
-The OS has software called device drivers which enables it to control and interact with peripheral devices e.g., printers, graphics cards, network cards, etc.
-Many external devices have embedded system software, so a driver bridges the gap between a major operating system and a tiny hardware OS.
-Device drivers allow the OS to perform specific operations on the hardware e.g. a printer driver enables the OS to send print commands and manage print jobs
-Most hardware manufacturers write their own device driver software, meaning a single operating system may have several printer drivers installed
-When data is sent to a printer, the OS will communicate with the printer to ensure it’s switched on, check that it is a printer, etc. and then the data to be printed will be transferred to an area of memory called a buffer, so the CPU can carry on with another task- the buffer compensates for the difference in speed between the CPU and the printer/output device.
Backing Store Management
-When files/applications are loaded, they are transferred from backing storage into main memory.
-The OS keeps a directory of where files are stored so they can be accessed quickly.
-The OS also needs to know which areas of storage are empty so new files/applications can be stored there.
-The file management system that comes with a desktop OS enables us to move/delete files and folders and protect others from unauthorised access.
What is preemptive scheduling?
Allocates the CPU for a specific time quantum to a process
Allows interruption of processes currently being handled
It can result in low-priority processes being neglected if high-priority processes arrive frequently
Example algorithms include Round Robin and Shortest Remaining Time First
What is non-preemptive scheduling?
Once the CPU is allocated to a process, the process holds it until it completes its burst time or switches to a 'waiting' state
A process cannot be interrupted unless it completes or its burst time is reached
If a process with a long burst time is running, shorter processes will be neglected
Example algorithms include First Come First Served and Shortest Job First
Processor scheduling- round robin
-Pre-emptive program
-Each job is allocated a time slice during which it can use the CPU’s resources.
-If the job has not completed by the end of its time slice, the next job is given a time slice.
-The allocation of time slices continues until the jobs are finished.
Processor scheduling- first come, first served
-Non-pre-emptive algorithm
-The first job to arrive is executed to completion (similar to a supermarket queue)
Processor scheduling- shortest job first
-Non-pre-emptive algorithm
-All programs are continuously sorted by burst time from shortest to longest
-When new processes arrive on the queue, they are prioritised based on their burst time in the next cycle
-Shorter jobs are placed at the front of the priority queue
-Longer jobs have lower priority, so they are placed at the back
Processor scheduling- shortest remaining time
-Pre-emptive algorithm
-The job with the shortest estimated time to completion is run until a new job with a shorter estimated completion time arrives
-This can cause “job starvation”, which is said to occur when long jobs are held off indefinitely as new, shorter jobs continually join the queue.
Processor scheduling- multi-level feedback queues
-Pre-emptive algorithm
-Multiple queues with different priority levels are created
-If a job uses too much CPU time it is moved to a lower priority queue
-Processes can also be moved to a higher priority queue if they have waited a long time
Note 
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