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WJEC CG1 1.2

Topic 2: The Operating System

Time slices Time slice is a finite amount of processor time. Each task has chance of a time slice. All users are asked polled to determine if a time slice is required. Priority can be altered using time slices. Scheduler is responsible for allocating time slices
Partition Memory is divided up into fixed or variable size chunks. Necessary to accommodate more than one task/ensures tasks do not corrupt each other. Each task has a chunk / partition to reside in. When task is finished memory is released
Interrupt An interrupt is a signal generated by a device or software, which may cause a break in the execution of the current routine. Or an interrupt is a signal generated by a device or software, requesting attention
When an interrupt is generated O/S halts current task; O/S runs interrupt routine / checks priority; O/S returns to original task or another interrupt routine and continues.
Batch processing involves the collection of programs/data for (non-interactive) processing together. Suitable example e.g. payroll. Suitable description e.g. all time sheets, etc. are collected until the end of the month then the payroll system is run.
Interrupt examples Hardware or Software fault Input/output device requesting attention e.g. printer out of paper / key press / mouse click; User interrupt e.g. <ctrl> <break>; Operating system generated interrupt e.g. end of time slice; Run time error e.g. division by zero
Interrupt order Another interrupt arises [in this order]; O/S suspends current interrupt routine; runs the new higher priority interrupt routine; Finally, the O/S returns to original interrupt routine and continues.
Multi-programming (MP) computer system A MP computer system is one where more than one job is held in the computer's main memory at the same time and can be processed in the computer's CPU at apparently the same time. MP is used to ensure efficient use of the CPU and prevent from being idle.
Buffer A buffer is an area of computer memory where data is held while transferring it to or from a (slower) peripheral.
Ready the process is waiting to be executed by the processor (because another process is currently being run)
Blocked the process that is waiting for some event (such as input / output operation / error)
Scheduling is the method by which the processor time is allocated [1] in a multi-programming / multi-tasking / multi-access operating system.
Multi-tasking occurs when more than one task or application is available to the user at the same time / can run at the same time. Example: Could be using e.g. a spreadsheet and an internet browser at the same time on a single computer.
Benefit of standalone Security may be improved – reduced chance of hacking / unauthorised access via a network / virus spreading / cheaper and easier to buy and set up; not susceptible to network or server failure.
Double Buffer While one buffer is being emptied, another can be filled. This avoids waiting for the data transfer. Printer -one buffer can be filled while another one is being emptied to the printer whereas a single buffer is adequate for a keyboard.
Created by: mcounsell