1.2.1 - Systems Software

Question 1

What are the key features of an operating system?

Answer 1

• Provides a user interface
• Manages hardware (peripherals)
• Manages CPU usage / Interrupts
• Provides utilities
• Manages Memory
• Provide system security (user permissions)
• Provides a platform for software to run

Question 2

What are the key facts about memory management (RAM)?

Answer 2

• Provides security as it does not let programs access memory reserved for other programs.
• Ensures that RAM is used efficiently and not wasted.
• Removes data not needed anymore (garbage collection) and allocates memory to applications.
• Provides multitasking as it allows multiple programs to run at once

Question 3

What are the key facts about paging?

Answer 3

• Physical addressing with fixed-size divisions
• Data can be held non-contiguously and makes the best use of storage.
• Can cause internal fragmentation (space wasted inside page due to fixed size)

Question 4

What are the key facts about segmentation?

Answer 4

• Logical addressing with variable size divisions.
• Can cause external fragmentation - Can leave gaps in RAM due to differing sizes of segments

Question 5

What are the key facts about virtual memory?

Answer 5

• Uses secondary storage (HDD)
• Pages not currently needed get moved to virtual memory to create room
• Pages are swapped between virtual memory and RAM as nothing can be running from virtual memory.

Question 6

What is an interrupt?

Answer 6

A signal to the processor indicating a device/process needs attention

Question 7

What happens when an interrupt is received?

Answer 7

• If it is of a higher priority, the CPU finishes its current FDE cycle.
• The contents of the CPU’s registers are copied to a stack in memory.
• The location of the appropriate interrupt service routine is loaded into the program counter.
• When complete, the previous contents are popped from the stack and loaded back into the registers.
• If the interrupt is of a lower/equal priority to the current process, then the current process continues.

Question 8

What are virtual machines?

Answer 8

A theoretical computer that runs like software

Shares the resources with the actual OS that it is running on

Question 9

What are the benefits of virtual machines?

Answer 9

• Allows you to run the software on several different systems without needing multiple physical machines.
• Protects the physical machine from a virus as the VM can quickly be reset to its original state.
• Some VM are used to execute intermediate code (Example Java VM)

Question 10

What is the purpose of scheduling (CPU Management)

Answer 10

• Used to process as many jobs as possible in the least possible time
• Ensures all jobs are processed fairly
• Enables the most efficient use of the processor

Question 11

What are the five main types of scheduling algorithms?

Answer 11

• Round Robin (RR)
• First come first served (FCFS)
• Shortest job first (SJF)
• Shortest remaining time (SRT)
• Multi-level feedback queues (MLFQ)

Question 12

Describe round-robin scheduling

Answer 12

• Each process is given a fixed equal time slice
• If the process is not completed, then it joins the back of the queue and the next one starts.

Question 13

Describe first come first served (FCFS) scheduling.

Answer 13

• Jobs are processed to completion in the order in which they arrive (First in First Out)
• The job will be completed before the next one starts
• Each job has an equal priority.

Question 14

Describe the shortest job first (SJF) scheduler.

Answer 14

• Picks the job that will take the shortest time and runs it until it finishes
• Needs to know the time each job will take in advance

Question 15

Describe the shortest remaining time (SRT) scheduler.

Answer 15

The quickest job is completed first but stops and re-evaluates when a new job arrives.

Question 16

Describe multi-level feedback queues (MLFQ)

Answer 16

• Puts processes into different queues
• Each queue has a different priority
• The algorithm can move jobs between these queues based on their priority
• Designed to minimise Input/Output bottlenecks

Question 17

What are the 5 main types of operating systems?

Answer 17

Distributed

Multi-User

Real-time

Multi-Tasking

Embedded

Question 18

Describe a distributed OS + give example of possible use.

Answer 18

• Shares processing between the processors on a network
• Allow multiple computers to be treated as one
• Multiple computers work on the same problem, completing it quicker
• Example: Video Rendering / SETI

Question 19

Describe a multi-user OS + give example of possible use.

Answer 19

• Allows multiple users to use the same computer at the same time
• Each user is given a time slice of the processor
• Example: Mainframe

Question 20

Describe a real-time OS + give example of possible use.

Answer 20

• Offers a guaranteed response time
• Will make decisions and act immediately

Examples: Self-Driving Cars & Airplane Auto Pilot

Question 21

Describe an embedded OS + give example of possible use.

Answer 21

• Has limited function and is read-only – stored in ROM.
• System software used to manage the device
• Built into the device itself
• Specific to the hardware/purpose

Examples: Washing machines & Microwaves

Question 22

Describe a multi-tasking OS + give example of possible use.

Answer 22

• Allows the user to run more than one program at the same time.
• Example uses include running word processing software whilst checking emails.

Examples: Typical home/personal computer OS

Question 23

Describe and give examples of a device driver

Answer 23

• A program that allows the operating-system to communicate with external hardware
• Translates operating-system instructions into a form that devices can use
• Allows peripherals from different manufacturers to work with different OS
• Abstracts hardware complexity

Examples: Printer drivers| Graphics Card drivers| Sound Card drivers

Question 24

Define BIOS and state its role

Answer 24

Basic Input Output System (BIOS)

• Contains the computer start up instructions
• Loads settings
• Performs the Power On Self-Test (Tests hardware is working) and reports errors
• Determines the drive on which the OS is stored
• Stores and loads the Bootstrap loader (used to boot up the OS)
• Can change settings such as which storage device the computer boots from