Unit 3: Computers Edexcel GCSE Computer Science

von Neumann architecture

Computer design where processing instructions are stored in memory; the stored program concept

Central Processing Unit (CPU)

Hardware that carries out the computer's processing

Bus

Group of connections between components in a computer

ALU

performs calculations, logical operations and comparisons on values held in registers

Register

high speed memory in the CPU used to hold single data items

Control unit

sends read/write controls signals to the RAM via the control bus and decodes instructions

Clock

sends constant electrical pulses to control the speed of the FDE cycle

Main memory / Random Access Memory (RAM)

Temporary storage for data and instructions

Volatile memory

Memory that is lost when the power is switched off, RAM is volatile

Fetch - Decode - Execute

Steps carried out repeatedly by the CPU: instructions are fetched from the memory, then they are processed and the cycle repeats...

Read-only memory (ROM)

Non-volatile memory, programs that run when the computer is turned on are kept in ROM

Non-volatile memory

Data is not lost when the power is switched off

Input - Process - Output model

An input is received, the instruction are processed, an output is given

John von Neumann

Developed the idea that a computer could store instructions in memory

Cache

Very fast Random Access Memory (RAM), stores frequently used instructions to speed up processing

Secondary storage

Where data and instructions are permanently stored

Secondary storage: magnetic

There is magnetic coating on the surface of the disk and the state of the magnet can be changed to represent a 0 or 1

Secondary storage : optical

These storage devices have data represented as pits and lands on the disc surface, these are read by a laser (CDs, DVDs and Blu-Ray discs)

Secondary storage: solid-state

These have no moving parts, uses semiconductor chips to store data (SD cards, microSD cards, USB flash drives)

Secondary storage: cloud storage

Data is stored in data centres accessible over the internet (Dropbox and Microsoft OneDrive)

Software

The programs that run on a computer. There are two types: application software and system software

Application software

Examples include Microsoft Office (Word, Excel, etc) and Adobe Creative Cloud (Photoshop, Illustrator, etc)

The 2 types of system software

Includes utility software and operating system software

Types of Utility software

(FABDD)

File repair

Anti-malware

Backup

Disc defragmentation

Data compression

Operating system software

OS software manages the tasks running on a computer (examples include Windows, Linux and Apple's Mac OS)

High-level programming language

Examples include Python, C and Visual Basic, instructions resemble human languages

Translator

Converts source code into machine code

Compiler

Translator that translates the whole program in one go

Interpreter

Translator that translates the program one line at a time

Embedded system

A computer system with a dedicated function within a larger electrical or mechanical system.

Internet of Things

The connection via the Internet of computing devices embedded in everyday objects, enabling them to send and receive data.

Describe how Scheduling Algorithms work

• All processes are held in a queue

• Processes are prioritised

• Processes are allocated time slices

• Processes are switched out at the end of their time slice

• Unfinished processes are put to the back of the queue

Scheduling algorithms: round robin

Each process is assigned a time slice of the CPU. When the time slice is up, if the process is not complete it goes to the back of the queue

Virtual Memory

A portion of secondary storage functioning as additional RAM

Paging algorithm

The OS swaps processes out of RAM into virtual memory and back again when it's their turn to use the CPU

Hardware

The machines, wiring, and other physical components of a computer

Pipelining

Technique that allows the CPU to work on more than one instruction at a time

Multitasking

An operating system feature that allows more than one application to run at a time.

User management

• User permissions can be edited so that they can access their own files

• User accounts can be created so that multiple people can use a device

User interface

The visual elements of a program through which a user controls an application. Often abbreviated UI.

WIMP

Windows, Icons, Menus, Pointers

File repair software

Software used to restore corrupt files

Backup software

automates the process of backing up files so that data can be recovered if something goes wrong

data compression

reduces the size of files so that they take up less storage space and need less time to transfer

defragmentation software

moves all the blocks of data for one file together and all the free space is moved together - speeds up access to files

anti-malware software

detects, quarantines and removes malware from a computer

Methods of identifying vulnerabilities in software

Audit trails

code reviews

Audit trails

• record of all the changes that have been made to a program and who has made the changes

Code reviews

checks of source code by other programmers to help find vulnerabilities and help make the code more efficient

Compiler advantages

Faster execution

Original source code is hidden

Compiler disadvantages

Program will not run with syntax errors - more difficult to write

Code needs to be recompiled if it is changed

Interpreter advantages

easy to write source code as the program will always run until an error is found

code does not need to be recompiled when its changed

Interpreter disadvantages

slower execution

translation software required to run the code

Which bus is unidirectional

address bus

Which busses are bidirectional

data bus and control bus

Role of the control bus

carries the control signals sent by the control unit to other components

Role of the address bus

carries the address of a memory location in RAM, of the data or instruction to be fetched or stored

Role of the data bus

carries the data or instruction fetched from the RAM or written to the RAM

Purpose of a mnemonic

make an instruction easier to remember (e.g. INP means INPUT)

Defragmentation

when file blocks are rearranged on the disk

Fragmentation

Files are written to the disk in blocks, blocks can be dispersed across the disk

Describe 2 ways how a compiler and interpreter are different

A compiler produces a stand-alone executable file whilst an interpreter is needed each time the code is run

A compiler reports errors after translation whilst an interpreter reports errors as they occur

Describe 2 ways an OS manages processes

Uses a scheduling algorithm to share processing time between processes

Allocates sections of RAM to share memory between processes

Describe two ways that a high level language and low level language are different

High-level languages have instructions that look like English whereas a low-level language uses binary code

High-level languages are general purpose but a low-level language is machine-specific

3 Aspects of a compiler

Translates the entire source file to machine code in one go

Shows all syntax errors at the end of translation

Produces a single executable file

Characteristics of embedded systems

• smaller in size

• smaller storage

• lower power consumption

Type of vulnerabilities

• inadequate validation (that fails to prevent incorrect data input)

• inadequate authentication (that fails to prevent hackers from accessing private information)

• bad programming practise (e.g. confusing variable names)

Robust software

software that does not contain vulnerabilities

types of low level language

assembly language

machine code

assembly language

program instructions represented as mnemonics

machine code

program instructions represented in binary

Define translation

Converting human readable code to binary/machine code