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Summary Functions of an operating system
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A cheat sheet for chapter 7 of the PG Online textbook chapter 7 that summarises the functions of the operating system.
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Unit 1.2.1 to 1.2.2 Computing OCR A-Level Notes
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Unit 1.1 Computing A-Level OCR Notes
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Terminology for Computer Science, partially covered.
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SCHEDULING Types of scheduling algorithms:PAGING Ensuring that all sections of
● First come, first served:
processed in chronologica
programs being run receive fair which they enter the queu
Memory is organised into fixed amounts of processing time. ● Shortest job first: Jobs a
sized units called pages. Scheduling algorithms can be completed in order from s
Individual pages can be stored either: largest.
into any available space in ● Round robin: Each job is
section of processing time
memory and the addresses of ● Pre-emptive: Jobs can be which it’s allowed to exec
pages are stored in a page table. made to start and stop by the each job has used its first
operating system. slice, this processed is re
Disadvantages: ● Non pre-emptive: A job is
until it a job has been com
● Multi-level feedback que
left alone to be completed Multiple queues are order
● Procedures may be split once started on a different priority.
across multiple pages so this ● Shortest remaining time
is an inefficient method that have the least amoun
● Code is not divided logically VIRTUAL MEMORY remaining for completion
● A better solution would be in the front of the queue.
segmentation When secondary storage is full, a
section of the hard drive can be
used to act as RAM.
Disadvantage: Disk thrashing.
SEGMENTATION This is where the computer BIOS
freezes due to pages being
Allocating segments of memory to
swapped too frequently between Basic Input Output Syste
processes. The memory
main memory and the hard disk. first program that runs w
segments vary in size and contain
complete sections of code. computer is switched on.
Segmentation uses logical tests performed:
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