1.2 Software and Software Development

Operating system

Software that controls the computer's hardware and software resources

Function of OS

Allows apps to interact directly with hardware on computer

Tasks performed by OS

Processor management, inputs, outputs, interrupts, memory management (inc. backing up), security

Purpose of OS

To prevent apps each having to include code to interact with hardware

Memory management requirement

Memory must be shared fairly between multiple programs and apps being used

OS role: memory management

OS determines and allocates the length of processor time each task receives

Memory management techniques

Paging, segmentation, virtual memory

Paging description

Memory split up into equal-sized pages, physical divisions without any logical decisions

Paging function

Pages swapped between main memory and hard disk as needed

Segmentation description

Memory split up into logical, differently sized divisions called segments, complete sections of programs, logical divisions

Segmentation advantage

Increases efficiency as code kept together where necessary

Virtual memory description

Section of hard drive acts as RAM when space in main memory is insufficient to store programs being used

Virtual memory function

Sections of programs not in use temporarily moved to into virtual memory then moved back to RAM when needed

Disadvantage of memory management

Disk thrashing

Disk thrashing description

Pages/segments swapped too frequently between hard disk and main memory so more time spent transferring data than running programs

Disk thrashing factors

Worsens as virtual memory fills up

Interrupts definition

Signals generated by hardware/software to indicate to processor that a process needs attention

Interrupt priorities

Different interrupts have different priorities, urgency (should be) taken into account by OS

Interrupt register

Stores interrupts in order of priority within priority queue

Define priority queue

Abstract structure holding interrupts in order of priority

Use of interrupt service routine (ISR)

Used by OS to make sure interrupts are serviced fairly

ISR description

Processor checks contents of interrupt register at end of each FDE cycle

Stack

Holds current contents of special purpose register if processor finds interrupt of higher priority than current process

Processor finds interrupt of higher priority than current process...

Loads appropriate ISR into RAM, flag set to signal ISR has begun

Interrupt serviced...

Flag is reset

After ISR complete

Interrupt queue checked again for further interrupts of higher priority than original process

No additional (priority) interrupts found

Contents of stack transferred back into registers in memory

Additional priority interrupts found

Process repeated until all priority interrupts have been serviced

Job definition

Section of program being run

Scheduling description

OS ensures all jobs receive fair amount of processing time

Scheduling function

Scheduling algorithm implemented, appropriate to task

Pre-emptive scheduling algorithm description

Involves jobs being actively made to start and stop

Pre-emptive scheduling algorithms

Multilevel feedback queues, shortest remaining time, round robin

Multilevel feedback queues description

Multiple queues used, each order based on a different priority

Multilevel feedback queues disadvantage

Requires decision on which job to prioritise based on combination of properties

Shortest remaining time description

Orders queue storing jobs to be processed according to time left for completion, shortest time left first

Shortest remaining time disadvantage

Risk of processor starvation for longer jobs if short jobs continuously added to queue

Processor starvation

Occurs when a particular process does not receive enough process time to execute and be completed

Round robin

Each job given an equal time slice in which it can execute, OS grants each another time slice after 1st round

Time slice

Section of processor time

Round robin disadvantage (multiple cycles)

Longer jobs take much longer time for completion due to execution being inefficiently split into into multiple cycles

Round robin disadvantage (jobs)

Does not take into account job priority

Non pre-emptive scheduling algorithm description

Leave jobs alone once started

Non pre-emptive scheduling algorithms

First come first served, shortest job first

First come first served

Jobs processed in chronological order according to when they entered the queue

First come first served advantage

Straightforward to implement

First come first served disadvantage

Does not take into account job priority

Shortest job first

Orders queue storing jobs to be processed according to time required for completion, longest last

Shortest job first suitability

Suited to batch systems where shorter jobs are given preference to minimise waiting time

Shortest job first disadvantage

Processor must know/calculate time for each job, risk of processor starvation

Types of OS (not mutually exclusive)

Distributed, embedded, multi-tasking, multi-user, real time

Distributed OS

Run across multiple devices, spreading load across multiple processor when a task is run

Embedded OS

Built to perform small range of tasks and cater to specific device, limited functionality

Embedded OS disadvantage

Hard to update

Embedded OS advantage

Consume less power, efficient due to being highly specialised

Multi-tasking OS

Enable user to carry out tasks seemingly simultaneously

Multi-tasking OS function

Uses time slicing to switch quickly between programs and apps in memory

Multi-user OS

Allows multiple users to use one computer

Multi-user OS typical use

Supercomputers

Multi-user OS requirement

Scheduling algorithm must be used to prevent risk of processor starvation and share memory appropriately

Real time OS

Designed to perform task within guaranteed time frame

Real time OS uses

Time-critical computer games, any situation where response within certain time period is crucial to safety

Basic Input Output System (BIOS)

First program that runs when a computer system is switched on

PC on start up

Points to location of BIOS

BIOS responsibility

To run various key tests before OS is loaded into memory including POST, checking CPU clock, memory and processor are operational, testing for external memory devices

POST

Power-on self test, ensures all hardware is correctly connected and functional

BIOS requirement

OS can only be loaded into RAM from hard disk after BIOS checks

Bootstrap/bootloader

Program that loads OS from hard disk into main memory

Device drivers

Computer programs provided by OS to allow OS to interact with hardware

Device driver function

Communicates request from peripheral hardware to OS to produce relevant output

Device driver specific

Specific to computer's architecture so different drivers must be used for different device types, also specific to OS

Virtual machine

Theoretical computer, program with same functionality of physical computer

Virtual machine function

Executing intermediate code (provides translator) and running one OS within another

Virtual machine use

To create development environment for testing programs on a different OS

Virtual machine advantage

Saves time and money of having to purchase multiple devices for testing

Virtual machine disadvantage

Slower to run code on

Virtual machine additional uses

Protection from malware (only affects VM), running incompatible software (reduces need for specific hardware)

Applications software description

Software designed to be used by the end user to perform one specific task

Applications software grouping

Categorised by function e.g. word processors, desktop publishing, presentation software

Applications software functioning

Accepts user input, processes input according to algorithms and generates output

Applications software requirements

Needs systems software to be able to run

Systems software description

Low-level software, responsible for computer running smoothly, interacting with hardware and providing a platform for applications software

Systems software examples

Utility programs, OS

Types of applications software

General purpose, special purpose, bespoke

General purpose software description

Can be used for a variety of tasks so attracts lots of users and is normally quite cheap

Special purpose software description

Can only be used for a single task, typically developed to fulfil a particular business need, fewer users so more expensive than general purpose

Bespoke software description

Developed to meet a user's specific requirements so very costly

Utility programs purpose

To ensure consistent high performance of OS

Examples of utility software

Compression, disk defragmentation, antivirus, automatic updating, backup, encryption, anti-malware

Compression

Used to compress large files to be transferred across internet or stored more space-efficiently

Disk defragmentation

Rearranges content of hard drive (not SSD) so files are stored together and can be accessed faster, increasing performance

Antivirus

Responsible for detecting potential threats to computer, alerting user and removing threats

Automatic updating

Ensures the OS is kept up to date, automatically installs updates when computer is restarted

Purpose of updates

To tackle bugs or security flaws, reducing vulnerability to malware and hacking threats

Backup

Creates routine copies of specific files at a specific frequency (both selected by user) so that they can be recovered

Full backup

Copies anything identified as important

Incremental backup

Only copies files that have been changed since last full backup

Encryption

Uses algorithm to convert plaintext into ciphertext so files can be transferred securely across networks and prevent access in case of hacking/theft, also decrypts files using encryption key

Anti-malware

Protects computer system by detecting and blocking threats, continuously monitors files and software being accessed by OS to detect signs of malicious intent

Malware

Software designed to cause damage to computer systems, corrupt or change files, steal data or cause disruption to services