Operating System

Question 1

What is an operating system?

Answer 1

• a program or set of programs that manages the operations of the computer for the user.

Question 2

What functions does the operating system provide?

Answer 2

• Memory management
• Interrupt service routines
• Processor scheduling
• Backing store management
• management of all input and output

Question 3

How does the operating system handle memory management?

Answer 3

• Programs and their data need to be loaded into RAM
• Operating system must manage the allocation of RAM to the different programs
• There may not be sufficient RAM for all desired processes to be completely loaded into RAM at once, so the allocation and management of space is controlled by the operating operating system.

Question 4

What are pages?

Answer 4

• Memory is divided into fixed size chunks called pages.

* Each page has an address.

Question 5

What happens to a process loaded into RAM?

Answer 5

It is allocated sufficient pages.

Question 6

What does a page table do?

Answer 6

Maps between the logical memory locations and the physical memory locations.

Question 7

What is segmentation?

Answer 7

• Logical division of address space into varying length segments which depend on the program structure.
• It is possible to load only part of a program into memory initially.

Question 8

What happens to the hard disk for virtual memory to become available?

Answer 8

An area of the hard disk that is designated as virtual memory.

Question 9

What is disk thrashing?

Answer 9

• if many processes are running and the computer has insufficient RAM, lots of time is spent swapping pages in and out of virtual memory.
• Repeatedly swapping pages can noticeably slow down the computer, this is disk thrashing.

Question 10

What is an interrupt?

Answer 10

A signal from a software program, hardware device or internal clock to the CPU.

Question 11

What happens when an interrupt is detected?

Answer 11

• The processor stops fetching instructions and instead pushes the current contents of its registers onto a stack.
• Suspends execution of the running programs, or process and disables all interrupts of a lower priority.
• Then it puts the values of the program counter and of each register onto the system stack.

Question 12

What is the purpose of an interrupt?

Answer 12

• Interrupts are triggered regularly by a timer.
• This is to indicate that it is the turn of the next process to have processor time.
• It is because the processor is able to be interrupted, that multi tasking can take place.

Question 13

what processes the interrupt?

Answer 13

Interrupt service routine.

Question 14

What is interrupt priority?

Answer 14

• Interrupts have different priorities, and can be processed in order of priority.
• Interrupts can be interrupted if the new interrupt is of a higher priority.

Question 15

What happens if a higher priority interrupt occurs while an interrupt is being processed?

Answer 15

The original interrupt’s registers will also be pushed into the stack

Question 16

What is the structure of the Stack?

Answer 16

LIFO

Last in first out

Question 17

What is the processor scheduling?

Answer 17

• A single CPU can only process instructions for one application at a time.
• The operating system must schedule when each application can use the CPU.
• This provides the illusion of multi tasking.

Question 18

What are the the objectives of scheduling?

Answer 18

• Provide an acceptable response time to all users
• Maximise the time the CPU is usefully engaged
• To ensure fairness on a multi user system .

Question 19

What is round robin scheduling?

Answer 19

Processes are dispatched on a first in first out basis.
• Each job is allocated a time slice during which it can use the CPU’s resources.
• If the job has not completed by the end of its time slice, the next job is allocated a time slice.

Question 20

How does the operating system achieve round robin?

Answer 20

• Sets an interrupting clock to generate interrupts at specific times.

Question 21

What are some advantages of round robin?

Answer 21

Guarantees a reasonable response time to all users of the system.

Question 22

What is first come first served?

Answer 22

First job to arrive is executed till completion

Question 23

What is shortest remaining time?

Answer 23

• The time to completion is estimated as each new job arrives
• Job with the shortest remaining time to completion is executed, meaning that a shorter new job can take over fro the current process.

Question 24

What is shortest job first?

Answer 24

• Total execution time is estimated by the user.

* Job with the smallest total execution time is executed when the current job completes.

Question 25

What are the purposes of multi level feedback queues?

Answer 25

• Give preference to short jobs.
• Give preference to I/O bound processes
• Separate processes into categories based on their need for the processor.

Question 26

How do multi level feedback queues work?

Answer 26

Implements several job queues and jobs can move between queues depending on how much processor time they have used.
• Multiple queues are created with different priority levels.
• If a job uses too much CPU time it is moved to a lower priority queue.
• Processes can also be moved to a higher priority queue if they have waited a long time.