1.2.1: Systems Software

Question 1

Operating systems

Answer 1

• collection of programs working together to provide interface between user + computer
• allow communication (user –> computer) –> perform certain low-level tasks –> management of memory + resources
• windows, macOS –> iOS + Android

Question 2

Features of OS

Answer 2

• memory management (paging, segmentation, virtual memory)
• resource management (scheduling)
• file management (moving, editing, deleting files + folders)
• I/O management (device drivers)
• Interrupt management
• Utility software (disk defragmentation, backup, formatting)
• Security (firewall)
• user interface

Question 3

memory management

Answer 3

• memory must be shared fairly between multiple programs + apps
• when main memory isn’t large enough:
• OS, determines + allocates length of processor time
• paging, segmentation, virtual memory

Question 4

Paging

Answer 4

• memory is split into equal sized sections (pages)
• programs made up of certain number of pages can be swapped between between main memory and hard disk

Question 5

Segmentation

Answer 5

• memory is split into logical sized divisions (segments)
• vary in size
• represent structure + logic flow of program
• segments allocated to code –> conditional statements/loops

Question 6

Virtual memory

Answer 6

• uses sections of hard drive to act as RAM when space in main memory is enough to store programs used
• sections of programs not used –> temporarily moved to virtual memory –> paging

Question 7

Disk Thrashing

Answer 7

• computer ‘freezes’ –> result of pages being swapped too frequently
• progressively worse as virtual memory fills up

Question 8

Interrupts

Answer 8

• signals generated by software/hardware to show the processor a process needs attention
• different interrupts, different priorities –> how urgent
• stored in order of priority –> priority queue in interrupt register
• OS makes sure interrupts are serviced fairly by ISR
• printer signalling, peripherals signalling power failure

Question 9

Interrupt service routine (ISR)

Answer 9

• processor checks content of interrupt register at end of FDE cycle
• if interrupt exits of higher priority, current content in special purpose registers in CPU –> transferred into a stack
• processor responds with appropriate ISR loading into RAM
• ISR has begun –> flag sent
• interrupt queue checked for higher priority
• repeated until all priority interrupts are serviced
• contents of stack transferred back into registers in memory
• FDE cycle resumes

Question 10

Scheduling

Answer 10

• ensures all sections of programs begin run are receiving a fair amount of processing time
• this is achieved with scheduling algorithms

Question 11

Pre-emptive

Answer 11

• jobs made to start/ stop by OS
• multilevel feedback queues, shortest time remaining, Round Robin

Question 12

Non pre-emptive

Answer 12

• once a jobs is started, it is left alone until completed
• first come first serve, shortest job first

Question 13

Round Robin

Answer 13

• each job given a section of processor time (slice) –> allowed to execute
• after job in queue has used 1st time slice, OS again grants each job an equal slice of processor time
• continues until a job has finished –> then removed from queue
• longer jobs take longer –> inefficiently split up into multiple cycles
• doesn’t take into account job priority

Question 14

first come, first served

Answer 14

• job processed in chronological order
• easy implementation
• no priority of jobs

Question 15

Multilevel feedback queues

Answer 15

• uses multiple queues –> ordered in different priority
• difficult to implement –> deciding which jobs to prioritise based on a combination of priorities

Question 16

Shortest job first

Answer 16

• stored in time required to complete
• batch systems
• requires processor to calculate process time
• risk if shortest jobs keep getting added (starvation)

Question 17

Shortest remaining time

Answer 17

• ordered in time left for completion
• least time serviced first
• risk of starvation

Question 18

Distributed OS

Answer 18

• runs across multiple drivers –> load to be spread across multiple computer processors –> when task runs

Question 19

Embedded OS

Answer 19

• perform small range of specific tasks.
• limited to functionality, hard to update + consume less power than other OS

Question 20

Multi-tasking OS

Answer 20

• allows user to carry out tasks seemingly simultaneously
• done by time slicing between programs and applications

Question 21

Multi-user OS

Answer 21

• uses 1 computer in a multiple-user system
• Scheduling algorithm needed –> processing time shared fairly

Question 22

Real time OS

Answer 22

• time critical computer systems –> perform task in time frame guaranteed

Question 23

BIOS

Answer 23

• Basic Input Output System
• 1st program run when computer is switched on
• PC points to location of BIOS

Question 24

BIOS responsibilities

Answer 24

• POST (power-on self test) all hardware connected correctly
• Checking CPU clock, memory, processor
• Testing for external memory

Question 25

Device drivers

Answer 25

• computer programs allowing OS to interact with hardware
• different drivers used for different device types + specific to OS installed

Question 26

Virtual memory

Answer 26

• theoretical computers –> software implementation of computer system
• provides environment + translator for intermediate code to run
• protection from malware –> affected before main memory
• running incompatible software