Computer Systems AQA 8520 GCSE

Logic Gates

Not - Opposite, OR - either, AND - both.

System Software

Performs tasks to do with managing the computer. i.e. Operating system

Application Software

Performs tasks that the user wants to do. i.e. write a letter, play a game.

Utility Software

Provides extra functionality that makes it easier to use. i.e. Computer security, file organisation, system maintenance.

Operating System

Provides: User Interface, manages main memory, allows processor management, manages peripherals, manages applications, provides security.

Firewall

Prevents unauthorised access to computer/network from Internet, filter access to unsuitable sites.

Disk Defragmentation

Reorganises files so they are stored together.

CPU

Responsible for all of a computers processing.

Von Neumann architecture

Computer programs and the data the programs are using are stored in the same memory.

ALU

Performs arithmetic operations and logical operations (comparisons).

Control Unit

Co-ordinates and controls all the operations carried out by the computer.

Clock

Controls the timing of the processor.

Bus

Set of wires connecting two or more components of the computer. 3 main buses are Address Bus, Data Bus and Control Bus.

Fetch-execute cycle

Fetches an instruction whose address is held in the PC from memory (RAM or cache). The PC is updated. The control unit decodes the instruction. The instruction is executed this may use the ALU and the cycle begins again.

Clock speed

The amount of instructions executed in 1 second. Measured in MHz or GHz.

Cache

small amount of very fast, expensive memory located very close to the CPU that can be accessed faster than regular main memory (RAM).

Primary storage

RAM and ROM.

ROM

Read-only Memory - permanently stores data even when computer is off. Read-only. Often contains boot up instructions.

RAM

Random Access Memory aka Main Memory. Very fast. Volatile. Usually contains: part of operating system in use, software currently in use, data software is using.

RAM and ROM differences

RAM is volatile, ROM is non-volatile. RAM is read/write, ROM is read-only.

Virtual Memory

Used as an extension to RAM by storing it on the hard drive which is a lot slower. Prevents crashing but usually a sign you need more RAM.

Secondary Storage

Non-volatile place to store data and programs. Hard Disk Drive, Solid State Drive, CD, DVD, BluRay, USB Flash, Removable HDD.

Magnetic Storage

Such as Hard Disk Drive or Floppy Disk, mechanical parts move over the disks surface to read and write data magnetically.

Optical Storage

Such as CD, DVD or BluRay. Data is stored as pits and lands burnt into a spiral track circulating outwards from the centre. A laser beam passes over the pits and lands, the level of reflection is measured.

Solid State Drives

Use non-volatile flash memory to store information. Very fast burst read/write speeds due to data being physically close and easy to recover. No mechanical or moving parts.

Flash memory

Large electric current used to force electrons through a barrier and trap them on the other side. They remain on the other side until flashed with a new current.

Cloud Storage

Saving data in an off-site storage system maintained by a third party, for example Dropbox or Google Drive. Accessible via the Internet.

Embedded Systems

A system with a single microprocessor that includes RAM, ROM and a CPU. Used to control a device with simple controls or one use. E.g. Washing machine, microwave. No OS. Program is held in ROM.