1.2.2 - Applications generation - OCR A-Level Computer Science

Virtual machine

Any instance where software is used to take on the function of a machine

What are 2 things a virtual machine might do?

Execute intermediate code or run an operating system within another.

What does an emulator do?

Tricks a program into thinking it’s running on native hardware whilst it’s actually running on a different machine.

What are 2 advantages of using virtual servers across a small number of physical servers?

• As demand on the network increases, virtual servers can be created to balance the load across physical devices

• If one stops working, the others can pick up the load

Application software

Can be used by the user to perform a specific function

Utility software

Maintains consistent and high performance of the computer system.

Generic application

Does not have a specific purpose

Specific application

Follows a specific purpose

Examples of application software (12)

Word processors, DTP, Spreadsheets, database management, presentation, multimedia, graphics manipulation, text and voice communication, social networking, email clients, web browsers, games

Examples of utility software (7)

• File management

• Antimalware

• Defragmentation

• Compression

• Backup

• File repair

• Device drivers

What is the purpose of file repair?

Corrects corrupted or damaged files by restoring the file to its original state

What is the purpose of data compression?

Reduces the file size so it takes up less space and downloads faster over the internet.

What is the purpose of defragmentation?

Reorganises files on a hard disk, putting fragments and free space back together

What is the purpose of antimalware?

Keeps your computer safe from malware including viruses, trojans, worms and spyware.

Open-source software

Software that users can freely distribute and modify with access to the source code

Benefits of open source software for users

• Software is free

• Wide community modification

Drawbacks of open source software for users

• Can be poorly supported

• Some features might not be well tested

Benefit and drawback of open source for creators

• Gets software out to a wider community

• Little to no financial gain

Proprietary/closed-source software

Software that users cannot modify due to it being protected by the Copyright Design and Patents Act, usually paid for and licensed per user/computer.

Benefit and drawback of proprietary for users

• Product is well supported, tested and professionally built

• Cost or ongoing subscription fee

Benefit and drawback of proprietary for creators

• They receive an income for their product

• Constant demand for more features and improvements from community

Machine code

The pure binary form of source code that machines can understand and execute

Translation

The process of converting source code into machine code

Interpreter

Translates code line-by-line into machine code

Compiler

Translates entire program all at once

Paging

A method of dividing physical memory into fixed size pages made to fit sections of memory.

Segmentation

A method of dividing memory logically into different sized segments, each one representing a complete section of a program.

Functions of an operating system (8)

• User management

• File management

• Memory management

• Scheduling

• Device drivers

• Security

• Platform for applications

• Utility software

Interrupts

The way a device signals to the processor that they require attention

Interrupt service routine

A program with a set of instructions that need to be fetched, decodes and executed to carry out the operations of the interrupt.

What happens to the values in registers when an interrupt is received?

They are pushed onto a stack and saved for later retrieval

Scheduler

Manages which processes to execute next and the length of time the processes can run for.

First come first serve (FCFS)

Processes are executed until completion in the order they arrive.

Shortest job first (SJF)

Executes the process that takes the shortest amount of time to execute first.

Round robin (RR)

Allocates each process a fixed time slice (or quantum) and sends processes to the back of the queue if they are not completed within that time slice.

Shortest remaining time (SRT)

Similar to SJF, however processes can be suspended if a higher priority process joins the queue.

Multi-level feedback queues (MLFQ)

Moves processes between multiple queues (on a multicore processor) of different priorities.

Multi-tasking operating system

Schedules each active program to receive a time slice of processing, giving the impression that they are running at the same time.

Multi-user operating system

Allows more than one person to use a computer or a server at a time, and manages the various permissions and access rights of users.

Distributed operating system

Coordinates multiple computers across a network to perform a single task, and presents them to the user as if they were a single system

Embedded operating systems

Run on dedicated hardware at maximum efficiency, using low powered processors and very little memory.

Real time operating systems

Processes are guaranteed to execute within a known time frame, and plenty of redundancy is built in to handle sudden increases in input.

Device driver

A piece of software that tells the operating system how to communicate with a peripheral device.

Advantages of compilers (3)

• No need for translation at run time

• Faster speed of execution

• Code usually optimised

Disadvantages of compilers (3)

• Programs will not run with syntax errors, which can make coding more difficult

• Code needs to be recompiled when changed

• Designed for a specific type of processor

Advantages of interpreter (3)

• Program will always run and stop at a syntax error, making coding easier

• Code does not need to be recompiled when changed

Disadvantages of interpreters (3)

• Translation is required at runtime

• Slower speed of execution

• Code is not optimised

Parse

A stage in the compilation process in which the compiler performs a different action on the source code

4 parses in the compilation process

• Lexical analysis

• Syntax analysis

• Code generation

• Code optimisation

Lexical analysis

Parse in the compilation process in which source code is read letter by letter, and separated into lexemes when a white space or special symbol is reached, which are input into a symbol table.

Syntax analysis

Receives the tokens from the lexical analyses and analyses their syntactical structure, checking it abides by the correct syntax of the programming language, and then creates an abstract syntax tree for the next stage if all the rules are followed.