1.2.1
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What is an operating system?
A collection o programs that work together to provide an interface between the user and computer
Name some functions of an operating system:
-memory management -resource management -user interface -utility software
What is resource management/multitasking?
-Multitasking – when you have more than one program open and running at the same time. -The processor allocates a small amount of time to each process and cycles between them. -appears to be simultaneous
What is file management?
-Data is stored in files -An extension to the filename tells the OS which application to load the file into -The OS may present a logical structure of files in folders and allow the user to rename, delete, copy and move files
What is user management/security?
-User management – allows multiple users to log into the same computer -OS will retainsettings for each user -may have differentaccess rights
What is a user interface?
-WIMP – windows, icons, menus and pointers -Interactive, Intuitive -Optimised for mouse and touch gesture input
What is paging?
-pages are fixed size -physical divisions programs are split up to fit into a given number of pages
What is segmentation?
-fixed sizes -logical divisions -complete sections of code
Similarities between paging and segmentation:
-both allow programs to run despite insufficient memory -stored on disk -transferred into memory when needed
What is virtual memory?
-when RAM is full, VM is used -sections of programs not currently being used are temporarily moved into VM through paging -frees up memory in RAM
What is an interrupt?
A signal generated by software/hardware to indicate that the processor requires attention
What is the interrupt service routine (ISR)?
a program with a set of instructions that need to be fetched, decoded and executed to carry out the operations of the interrupt
What happens when the CPU receives an interrupt:
-complete the current FDE cycle -check priority of interrupt -if its higher… -values held in registers are pushed onto the stack -interrupt is executed -pop the frame off the top of the stack -load values back into registers -carry on with FDE cycle
What is scheduling?
-In a multi-user environment, a number of users will need servicing, seemingly at the same time so will need a scheduler
First come first serve (FCFS):
-Processes are executed in the order they arrive -not pre-emptive
Shortest job first (SJF):
-Picks the processes that take the shortest amount of time and runs them until they finish -not pre-emptive
Round Robin (RR):
-Each process is allocated a fixed amount of time, known as time slice or quantum -If the process is not complete by the end of its time slice, it returns to the back of the ready queue -pre-emptive
Shortest time remaining (STR):
-the ready queue is sorted by the estimated time to complete the process -pre-emptive
Multi-level feedback queues (MLFQ):
-several ready queues are used, each with a different scheduling algorithm -jobs are able to move between queues as their priorities change pre-emptive
What does pre-emptive mean?
-Jobs are actively made to start and stop by the operating system
What does non pre-emptive mean?
-Once a job is started, it is left alone until it is completed
What are the different types of operating systems?
-distributed -multitasking -multi-user -embedded -real time