1.2.1 Operating Systems Flashcards
What is an OS?
Operating System. A program or set of programs that manage the computers hardware and interactions with the user
What are the functions of the operating system?
Providing a user interface
Managing the CPU, scheduling, and interrupts
Managing memory and storage allocation
Managing hardware and peripherals
Managing user permissions and account information
Providing utilites and libraries
What is paging?
Splitting memory into equal fixed size chunks/units/blocks
What is segmentation?
Splitting memory into variable sized chunks/units/blocks
What is a logical address?
The position of a page/segment within a program
What is a physical address?
The position of a page/segment in memory
Why is memory management important?
Security - Memory is reserved for each program and cannot be overwritten by other programs
Multitasking - Allows multiple programs to run at once
Explain how virtual memory works?
Memory contents are divided into pages or segments.
If RAM is full,
then pages or segments not currently in use will be moved to an area in storage called virtual memory to make room.
When the pages or segments are needed, they are moved back to memory again.
What is an advantage of virtual memory?
Makes room in RAM
What is a disadvantage of virtual memory?
It’s slower to acces pages/segments in virtual memory than pages in RAM.
Extra processing time involved in moving pages/segments.
What is an interrupt?
A signal to the processor indicating that a device/process needs attention.
What is an ISR?
Interrupt Service Routine. A program that is run when an interrupt happens.
What are the three reasons for an interrupt?
Power Failure
Clock interrupt (scheduled)
Input/Output device signal
How are interrupts handled in the FDE cycle?
When the computer completes an FDE cycle
it will check the priority of the interrupt.
If the interrupt is a higher priority then the current tasks, then the current contents of all registers will be stored in the stack in memory.
The relevant inerrupt service routine is then loaded by placing the address of the first instruction in the Program Counter.
When the ISR is complete, the previous state is popped from the stack and loaded back into the registers.
If a higher priority interrupt arrives while completing the ISR that will be added to the stack and the new interrupt will be dealt with.
What are the five main scheduling algorithms?
Round robin
First come first served
Shortest job first
Shortest remaining time
Multi-level feedback queues
How does the round robin scheduling algorithm work?
Processes are allocated a fixed slice of CPU time and are cycled through in the order that they arrived, switching processes at the end of each time slice or when a higher priority interrupt arrives.
What is a disadvantage of the round robin scheduling algorithm?
Extra time is spent switching jobs
What are the advantages of the round robin scheduling algorithm?
No risk of starvation.
Easy to adapt to prioritise tasks by allowing high priority tasks to have multiple time slices per cycle.
How does the first come first served scheduling algorithm work?
Processes are executed in the order that they arrive with no system of priorities
What are the advantages of the first come first served scheduling algorithm?
No extra processing is used
No risk of starvation
What is the disadvantage of the first come first served scheduling algorithm?
Important jobs may be left waiting for too long
How does the shortest job first scheduling algorithm work?
The process with the smallest estimated running time is run first.
What is the advantage of the shortest job first scheduling algorithm?
Reduces the number of jobs waiting
What are the disadvantages of the shortest job first algorithm?
Requires extra processing for time estimation.
Can cause starvation
How does the shortest remaining time scheduling algorithm work?
The process with the smallest estimated remaining time is run first.
What is the advantage of the shortest remaining time scheuling algorithm?
Reduces the amount of waiting jobs
What are the disadvantages of the shortest remaining time scheduling algorithm?
Requires extra processing for time estimation
Can cause starvation
How do multi-level feedback queue scheduling algorithms work?
Multiple queues are set up based on process length, input/output device use, and processor use.
Each queue schedules based on its own priority syste.
Processes might move between queues
What is the advantage of multi-level feedback queues?
Most effient use of devices which are often a bottleneck, since input/output opperations are slower than processor ones.
What is the disadvantage of multi-level feedback queues?
Lots of extra processing involved
Why is scheduling important?
Efficiency - maximise the thoughput of the processor and harware devices
Fairness - Provide an acceptable responce time for all users on a multi-user system
What are the five main types of operating systems?
Distributed, emedded, multi-tasking, multi-user, real time
What are the characteristics of the distributed operating system?
Multiple computers can work together on a single task.
What are the characteristics of an embedded operating system?
It is specialised to one task and for the hardware it is running on.
It is normally stored in ROM instead of the BIOS.
What are the characteristics of a multi-tasking operating system?
Can run multiple programs at the same time.
What are the characteristics of a multi-user operating system?
Allows multiple users at the same time
What are the characteristics of a real time operating system?
Guarenteed fast response time
for multiple simultanious inputs.
Inforportates failsafes to detect failures and switch to redundant systems.
What does BIOS stand for?
Basic Input Output System
What does the BIOS do?
Load/boot the OS
Perform initial checks on the computer hardware to ensure it works when the computer is turned on
Set up the initial state of the computer
What is a virtual machine?
Software that simulates one operating system while running on a different operating system.
What are the advantages of using a virtual machine?
Code can be tested on multiple platforms without needing the physical devices.
Malware can be run on the virtual machine without putting the physical machine at risk.
What are the disadvantages of using a virtual machine?
Slower, because less memory, storage and CPU power are availible.
Performance can be unstable if hardware requirements are not met.
What is utility software?
Programs that help the OS with the basic functioning of the computer
Give three examples of utility software.
Any from:
Encryption, Defragmentation, Compression, Backup, Driver
What do encryption utilities do?
Scrambles the data in files using a key to obscure their meaning from unauthorised parties.
What do defragmentation utilities do?
Organises files conigiously in secondary storage to decrease loading times.
What do backup utilities do?
Make regular coppies of files to prevent them from being lost.
What do device driver utilities do?
Enable communication between the OS and a hardware device.
