2.1 System Sofware

Question 1

What are operating systems?

Answer 1

OS manages hardware and provides an interface for the users and the application software

Question 2

What functions does the OS provide?

Answer 2

• User interface
• Memory management
• Interrupt handling
• Processor scheduling

Question 3

User interface

Answer 3

Allows the operating system to hide the complexity of hardware

Question 4

Memory management

Answer 4

• Programs and data need to be loaded into RAM
• Operating systems must manage allocation of RAM to different programs
• There may not be sufficient RAM for all desired processes to be completely loaded into RAM at once

Question 5

Pages

Answer 5

• Available memory is divided into fixed size chunks called pages
• Each page has an address

Question 6

What process does paging fix?

Answer 6

• A process loaded into RAM is allocated sufficient pages, but those pages may not be contiguous in physical terms

Question 7

How do page tables fix problems?

Answer 7

A page table maps between the logical locations and the physical memory locations

Question 8

Segments

Answer 8

Spaces of memory that can relate to parts of a program

Question 9

Segmentation

Answer 9

Dividing memory into segments

Question 10

Virtual memory

Answer 10

Some of the pages of a current process are stored in virtual memory until they are swapped into RAM

Question 11

Disk Thrashing

Answer 11

Where the computer has to constantly swap pages between virtual and primary memory and therefore slows down the computer

Question 12

Interrupts

Answer 12

CPU instructions can be interrupted when necessary

Question 13

How are interrupts processed?

Answer 13

CPU checks at the end of each clock cycle whether there are any interrupts to be processed

Question 14

What happens when an interrupt is detected?

Answer 14

The processor stops fetching instructions and instead pushes the current contents of its registers onto a stack

Question 15

Interrupt priority

Answer 15

Interrupts can themselves be interrupted if the new interrupt is of a higher priority

Question 16

Processor Scheduling

Answer 16

OS schedules when each application can use the CPU

Question 17

Round Robin

Answer 17

• Each job is allocated by FIFO a time during which it can use the CPU’s resources
• If the hob has not been completed by the end of its slice, the next job is allocated a time slice

Question 18

First come first serve

Answer 18

The first job to arrive is executed until it completes

Question 19

Shortest remaining time

Answer 19

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

Question 20

Multi level feedback queues

Answer 20

• 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

Question 21

Starvation

Answer 21

When high priority processes keep executing and low priority processes get blocked for indefinite time