1.2.1 Systems software OCR A-Level computer science

What are the functions of an operating system

UIs (User Interfaces), user management, memory management, interrupt handling, file management, processor scheduling, device drivers

What is paging

A memory management method where memory is divided into fixed-size chunks (pages) and a certain number of pages are allocated to each process trying to run. Pages may not be physically next to each other in memory, so a page table is needed to keep track of which programs are using which pages.

What is segmentation

A memory management method where memory is divided into chunks of varying sizes (segments) that do not have to be contiguous (next to each other) in physical memory.

What is Virtual Memory

An area of secondary storage where processes actively running but not currently being used can be stored until they need to be used, where they are loaded back into RAM. This frees up space in main memory for other programs that are currently being used to be stored.

What are interrupts

They are signals that cause the CPU to stop executing its current processes so it can handle a more urgent event.

What is the Interrupt Service Routine

It is a program that is executed automatically when an interrupt is sent, moving current processes onto a stack and analysing the nature of the interrupt so the CPU knows how to handle it.

How does the First Come First Serve scheduling algorithm work

The instruction/process that requests access to the CPU first is allocated time with the CPU first. All others are put into a queue and have to wait until the process using the CPU is done.

What are the advantages and disadvantages of First Come First Serve

It is the simplest scheduling algorithm, but it has the longest average wait time for shorter processes.

How does the Round Robin scheduling algorithm work

Each process requesting the CPU is assigned a particular amount of time. Once it gets to the CPU, if it does not process and execute in the amount of time it was given, it goes to the back of the queue and has to wait again.

What are the advantages and disadvantages of Round Robin

It is more complicated than FCFS, but still simple and easy to implement. Shorter processes are favoured due to there being a time limit.

How does the Shortest Job First scheduling algorithm work

It gives the waiting process with the shortest execution time to the CPU first.

What are the advantages and disadvantages of Shortest Job First

Average wait time is reduced since shorter processes are executed first. However, it is more complicated to predict the execution time of each process, and longer processes may be continually ignored.

How does the Shortest Time Remaining scheduling algorithm work

It gives the process with the shortest remaining processing/execution time to the CPU. Processes run until either they complete or another shorter process comes along.

What are the advantages and disadvantages of Shortest Remaining Time

Average waiting time is reduced since short processes are executed quickly. However, longer programs will be neglected if shorter processes keep coming along, and it is complicated to predict execution time.

How do Multi-level Feedback Queues work

Processes are assigned to a queue based on their priority level. Higher priority queues are executed first. Instructions can be dynamically moved between queues and priority level can be changed based on certain factors, like how long it has been waiting for.

What are the advantages and disadvantages of Multi-level Feedback Queues

It is flexible and prevents starvation of processes by adjusting priority based on wait time and execution time. However, it is very complicated and takes a lot of CPU overhead to manage.

What does a distributed OS do

It can coordinate the processing of a single program across multiple devices, utilising the resources and processing power of many computers to complete a single task.

What is an open-source OS

An OS whose source code and design is publicly available for users to view, modify and distribute.

What is an embedded OS

A specialized OS designed to manage the hardware of embedded systems in specific devices (like cars, appliances, etc.). Normally, they are able to work on very little RAM and secondary storage.

What is a real-time OS

An OS that is designed to operate in time-critical environments, responding to inputs instantaneously and managing many inputs at once.

What is the BIOS (Basic Input Output System)

It is a program stored in ROM that initialises and tests hardware and loads the OS into RAM.

What are device drivers

They are programs that are responsible for communication between the OS and another (often external) hardware device. Device drivers translate command signals and outputs between the OS and the external hardware.

What is a Virtual Machine

Software that is used to emulate the software of another device on hardware that it was not written on or for.