GCSE Computer Science Paper 2: Computer Systems

AQA GCSE Flashcards

Hardware

Hardware are the physical/mechanical components that make up a computer/device.

Software

The programs/instructions that are executed by the computer. They are installed onto the computer.

What are embedded systems?

An embedded system is a computer that exists within a larger mechanical or electronical device. Embedded systems are used when the whole computer is not viable or they only have one specific purpose. The processing power in such a device is considerably less than a modern PC, but these devices are much cheaper.

- microwave

- printer

-calculator

Logic Gates

NOT - the output is the reverse of the input

AND - both inputs need to be 1 for the output to be 1

OR - one or both inputs need to be 1 for the output to be 1

XOR- only one should be 1 for the output to equal 1, NOT BOTH

Different types of software

- System software

- Operating systems (windows, android, iOS)

- Utility software (antivirus and compression software), library routines and programming language translators

- Application software

- Mobile apps, desktop applications such as browsers and word processing packages

System Software

Programs that are needed for effective communication with hardware and for launching application software to enable the computer to function.

Application Software

Programs that are launched from the operating system by the user to perform a specific task. It is the software used for end-user tasks. eg. word processing software, spreadsheet software, communication software, social media software

Operating System

A piece of system software that acts as an interface between the user and the hardware, managing all hardware and other software. (If another piece of software is to be launched, it will be launched from the operating system.) eg. Windows, Linux, iOS

Functions of utility software

Compression - reducing the size of a file so that it can be stored using less space or transmitted more quickly

Defragmentation - moving separate parts of a file physically together to speed up disk access

Backing up - creating a copy of files, either on the same disk, on a backup device or in the cloud. Backing up can either be full or incremental: full back-up involves creating a copy of all files and incremental back-up involves creating a copy only of files that have been created or edited since the last back up.

Encryption - Allowing for data to be scrambled in order to prevent unauthorised individuals understanding any files that they see. This may be used for secure storage or secure transmission.

Functions of the operating system

Processor management

- manages the CPU

- it decides which processes will be executed

- if multiple processors are running it decides which one the processor should handle next

- it decides how long a time slice should be given (how long before the processor's attention switches to the next process)

Memory management

-it controls which parts of the memory are being used by the processor

- it loads programs and data from back up store to main memory

- removes unneeded programs and data to make room for more

- manages virtual memory where part of secondary storage is used as an overflow area for main memory

Input/Output device management

- acts as a go between passing data from input into the application software into an output

- manages (installation of) device drivers, which are programs telling the operating system how to communicate with attached input/output devices

- manages peripherals

Application management

-controls access to applications

-prevents users from using or accessing unauthorised programs by processing requests from application software for resources such as a network connection or remotely stored file

Security management

- manage multiple user accounts, keeping users' data separate

-automatically backs up data, thereby increasing its security

- handles usernames and passwords to prevent unauthorised access: user authentication, users must prove who they are to access the system

-protects against malware

- recognises one user as an administrator who would have greater access rights

Utility Software

Programs that keep the computer functioning efficiently, perhaps by freeing up storage space, removing viruses or ensuring that files are backed up. eg. antivirus, firewall, backup

Von Neumann Architecture

Von Neumann architecture is based on the stored-program computer concept, where instruction data and program data are stored in the same memory. This design is still used in most computers produced today.

Major components of the CPU

- Arithmetic Logic Unit (ALU)

- Control Unit (CU)

- Bus - data bus, address bus, control bus

- Registers

- Clock

Arithmetic Logic Unit (ALU)

The part of the CPU that performs arithmetic (-, +, /,*), relational (=, <, >) and logic (AND, NOT, OR) operations.

Control Unit (CU)

CPU component that decodes instructions, controls the execution of instructions in the correct sequence, and regulates and controls processor timing using regular pulses from the system clock.

Bus

Collections of wires that transmit data between computer components.

Data Bus

Moves data back and forth between the CPU and memory and input/output devices- sends and receives data.

Address Bus

Carries the addresses of data (but not the data) between the processor and memory.

Control Bus

Carries control signals/commands from the CPU (and status signals from other devices) in order to control and coordinate all the activities within the computer (such as READ and WRITE).

Registers

Small, high-speed, short-term storage locations for small, specific pieces of data within the CPU itself. (One register would store data just retrieved from memory; another would store the memory location of where the data came from.)

Clock

A clock generates a pulse billions of times each second, synchronising the activities within the CPU. Every action performed by the CPU must be during a clock pulse. The clock frequency is the number of clock cycles which occur every second.

Factors affecting the performance of CPU

Number of cores - (a core executes instructions and so more cores means multiple instructions can be executed at the same time. Each core can fetch/execute its own instructions, increasing the speed at which instructions can be executed.)

Clock speed - (increasing the clock speed increases the amount of clock pulses per second, so more processes done per second which leads to more instructions to be completed/executed)

Cache size - (larger the cache the faster the performance because it is the fastest form of memory- cache is located closer to the CPU. More cache= more data/instructions held closer to the CPU, so faster access to frequently needed instructions/data.)

Cache type - (L1 cache is located directly in the CPU and so it's faster so maximises the use of L1 cache over L2 or L3 cache)

How does the fetch-decode-execute cycle work?

Fetch - The clock pulses and the program counter register transmits the instruction location, address, along the address bus to memory. Memory responds by sending the content of that address along the data bus, back to the processor and is stored in the instruction register. Instructions are fetched to the CPU from main memory.

Decode - The instruction is decoded and read by the control unit to work out what it is; the operation needed to be executed is identified.

Execute - The instruction is then executed which may involve data being read from/written to main memory . The program counter updates to show the location of the next instruction and the cycle begins again

Different types of Primary Memory

RAM- Random Access Memory

ROM- Read Only Memory

Cache - copies of RAM that are accessed very frequently. Very small but very fast - located in and very close to the CPU

Secondary Storage

Long-term storage in a computer system, necessary because main memory is volatile and will run out of storage space. The three types of backing store are optical, magnetic and solid state. All of these devices are required to store vast numbers of 1s and 0s but they do so in different ways.

ROM

Read Only Memory. It is non-volatile, so content cannot be edited or deleted. ROM contains the bootstrapping instructions - initial steps in initialising the operating system.

RAM

Random Access Memory. It is volatile. The more RAM, the more instructions/programs can be loaded from secondary storage into RAM so they can be executed by the processor.

Optical Storage

Optical storage media, including CDs, DVDs and Blu-Ray disks are written to and read from using lasers. The disks rotate at high speed, and the laser head moves across the disk, allowing the laser to shine across the disk. Tiny pits of the disk reflect lights differently to lands, and this reflected light is interpreted into 1s and 0s representing the data stored on the disk. This data is stored on a single spiral track.

+ one disk is very cheap

+ read-only so it is difficult to accidentally overwrite data

+ easily portable

+ takes up very little physical space

- slow write speeds

- unprotected disk is vulnerable to being scratched/ damaged

- low data capacity

Magnetic Storage

Magnetic disks are round and built into rectangular housing. The surface of the disks are divided into concentric circles called tracks and sectors. Tracks are circular in shape and the straight lines bisecting them divide the disk into segments. The disk spins very quickly, allowing a read-write head to access the data in a particular segment. Within each sector, billions of magnetic particles exist which are either magnetised (1) or not magnetised (0).

+ cheapest storage medium per megabyte

+largest storage

+ less cumbersome than multiple optical disks

- slower access speeds than SSDs

- data is all on one device which can be lost, unlike optical disks

- not very portable or durable

- lots of mechanical parts

Solid State Storage

Solid state storage devices (flash drives) use electronic circuits and no moving parts to store data electronically. With no moving parts, these devices are both sturdier and quieter that magnetic or optical devices. SSD (solid state disks) use electrical circuits which unlike those of RAM are non-volatile.

+ fastest access speeds

+ no moving parts makes it harder to damage, very durable

- less storage capacity than traditional hard disks

- most expensive per megabyte

- limited number of times each bit can be written to

Cloud Storage

Cloud storage involves storage on remote computers usually managed by other organisations. When a file is saved or loaded, it is transmitted across the internet, and multiple backup files often exist around the world. (Cloud storage uses magnetic and increasingly, solid state storage.)

+ multiple copies means that it is less likely to be damaged

+ capacity on your machine is freed up

+ easy to use since most companies offer apps for desktop and mobile devices to manage their data

+ companies offer a high data storage capacity

+ convenient to use since users can access their data from anywhere with an internet connection.

- often comes with a subscription fee

- speed is limited by your internet connection, and you cannot access your data without an internet connection.

- security risk if the company experiences a data breach, since malicious users could access your personal data.

Translators

Convert both high-level source code and low-level assembly code into binary machine code ahead of execution.

Interpreter

Converts a program written in a higher level language (source code) into machine code ready to be processed by the CPU. The program is translated line by line as the program is running.

+ easy to write source code -the program will always run and only stop when it finds a syntax error

+ not designed for a specific type of processor

- software needs to be translated at run time, so speed of execution is slow

- code is neither optimised nor kept secret

Assembler

Translates an assembly-language program into machine code ready to be processed by the CPU. It changes labels and variables into memory addresses.

+ software does not need to be translated at run-time, so quick speed of execution

+ code is optimised and source code is kept secret

- designed for a specific type of processor

- hard to write in assembly due to the complexity and limited range of commands

Compiler

Translates source code into object code and then into machine code ready to be processed by the CPU. The whole program is translated into machine code before it is run.

+ code does not need to be recompiled following changes, making it easier to try out commands

+ software does not need to be translated at run-time, so quick speed of execution

+ code is optimised and source code is kept secret

- the program will not run with syntax errors, which can make writing the code more difficult

- designed for a specific type of processor