Systems Software

Question 1

What is an operating system?

Answer 1

It is a program or set of programs that manages the operations of the computer for the user. It provides an interface for the user, application software and hardware since the user cannot directly communicate with hardware.

Question 2

How does an operating system work?

Answer 2

1. OS is held in permanent storage eg. hard disk
2. Loader (small program) is held in ROM
3. Computer switched on - loader in ROM sends instructions to load OS by copying it from storage into RAM.

Question 3

what are the functions of an operating system?

Answer 3

• memory management
• interrupt service routines
• processor scheduling
• backing store management
• management of all input and output

Question 4

what is memory management?

Answer 4

it is when the OS must manage allocation of RAM to different programs fairly since there may not be sufficient RAM for everything to be in it at once.

Question 5

what are 3 methods of memory management?

Answer 5

• paging
• segmentation
• virtual memory

Question 6

What is paging?

Answer 6

• memory (RAM mainly) is divided into fixed sections called pages (4Kb)
• process in memory will be allocated enough pages to run - but these may not be contiguous physically
• page table uses mapping to store a link between physical memory address + logical address space of each process

Question 7

What does contiguous mean?

Answer 7

next to each other/adjacent physically

Question 8

What is segmentation?

Answer 8

• similar to paging except memory is divided into segments which can be different sizes.
• Things stored in the segment may relate to parts of a program - eg. a particular function/subroutine may occupy a segment.
• segment size relates to the program structure

Question 9

what is the difference between paging and segmentation?

Answer 9

Paging uses chunks of memory that are the same size while segmentation uses chunks of memory which are different sizes.

Question 10

What is virtual memory?

Answer 10

• an area of secondary storage that can be used as an extension of memory when RAM is filled up and another instruction needs processor time
• pages of the current process will be stored in virtual memory until they are needed - then they will be swapped with an unused page into RAM
• advantage: it’s extra strage and can optimise computer performance when used correctly
• disadvantage: slower than using RAM

Question 11

What is disk thrashing?

Answer 11

It is when the OS is excessively swapping pages into and out of RAM which slows down performance significantly because so much time is spent swapping pages.

Question 12

What is an interrupt?

Answer 12

• its a signal from a software program, hardware component or internal clock to the CPU
• software interrupt = when an application program terminates or requests certain services from the OS
• hardware interrupt = when in I/O operation is complete or an error occurs eg. printer paper out

Question 13

why are interrupts needed?

Answer 13

• I/O process is completed/sends an interrupt signal
• an error occurs eg. printer paper out
• power failure
• scheduled interrupt from the internal clock - to enable multitasking to take place

Question 14

when are interrupts detected and serviced?

Answer 14

The CPU checks for interrupts at the end of a clock cycle for any interrupts that need to be processed