AQA Computing Unit 2 Last Minute Revision

Note I've not included the entire specification in this, but only the bits I needed to revise. But you're welcome to include it in your revision programme.

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Computing: Last Minute Revision
Hardware is the electrical components/circuits a computer is assembled from; the platform upon which the
software is executed.
Software is sequences of instructions called programs which are understood and executed by the
Moore's Law: The number of transistors than can be integrated into a chip for the minimum possible costs
doubles every 18-24 months.
The Central Processing Unit (CPU) is used to describe the processor AND main memory ONLY.
A peripheral is a computer device NOT PART OF THE CPU (i.e. anything but the processor and main memory);
can be INTERNAL (e.g. CD-Drive) or EXTERNAL (e.g. Keyboard).
I/O devices (peripherals) send/receive/store data by communicating with the CPU via an I/O Controller.
An I/O controller consists of:
- Electronics that interface the controller to the system bus;
- Electronics appropriate for sending control signals to the peripheral;
- A set of data, command and status registers.
The processor communicates with the I/O controller through a port in the controller called the I/O port.
Main Memory (or Immediate Access Store IAS) is memory which is directly addressable by the processor
(i.e. Does not require an I/O controller to communicate with it, e.g. RAM and ROM).
Memory not directly addressable by the processor (i.e. requires an I/O controller for communication and so is
a PERIPHERAL) is called Secondary Storage (e.g. DVD-R, Flash Memory etc).
The System Bus/External Bus (consists of Data, Address and Control Buses) connects the processer, main
memory and I/O controllers together.
The address bus is the ONLY UNIDIRECTIONAL BUS, the rest are bidirectional
Examples of control signals carried by the control bus:
- CLOCK SIGNAL for timing purposes;
- RESET SIGNAL to initialise components;
- MEMORY READ/WRITE to assert whether the memory location being accessed is being read or written
- I/O SIGNALS to indicate the processor wants to use an I/O controller and NOT a main memory, when they
both may have the same memory addresses.
When the processor, main memory and I/O controllers are integrated on a single chip, it's called a
Components of a processor:
- PROGRAM CONTROL UNIT, this fetches instructions from memory, decodes and executes them one at a
time (i.e. It's responsible for carrying out the fetch-execute cycle);
- ALU performs the arithmetic and logical operations on data;
- Registers are fast memory locations inside the processor (or I/O controllers) which can be dedicated or
general purpose;

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Internal clock: derived either directly or indirectly from the system clock;
- Internal buses link the PCU, ALU and the registers together;
- Logic gates used for flow control.
General-Purpose registers are not assigned a specific role by the processor designer; programmers may
use these.
Dedicated registers are assigned a specific role by the processor designer; programmers may use some
but not all of these.…read more

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A hyperlink links one web page to another.
Hypertext is a body of text and graphics stored in a machine readable form and structured so that a reader
can cross refer between related items of data.
A web browser displays a web page by rendering its HTML elements.…read more

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Type Selectors select HTML elements to follow a style rule. Their general format is:
selector { property : value }
Where selectoris the HTML tag to be styled, and propertyis the attribute of the tag to which the
value is to be assigned.
Examples of type selectors:
An embedded style sheet is placed within the head section of the HTML page as follows:
An external style sheet is much better, it is created separately using the `.…read more

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Protects intellectual property;
Copyrighted work can only be used/copied with owner's permission (Exception for education purposes);
Design right gives you protection from someone copying your original design;
A patent protects how an invention works or what it does. It cannot be obvious to others with
knowledge in the field. An exception this is mathematical or scientific discoveries;
A trademark protects the name or logo used to identify a business or product.…read more

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Under this act, it is an offence to intercept a message sent via a public or private telecoms system
But security and intelligence services are exempt from this act.
A code of conduct is not a law. If you break the code of conduct you cannot be criminally convicted but
the organisation may follow a disciplinary procedure.…read more


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