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AQA Computer Science

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Natalie Hughes

on 5 May 2016

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Transcript of AQA Computer Science

Data comes in various forms so they need to be handled differently
A constant is a named item or symbol that does not change its initial value

A Variable is a named value or symbol that can be changed as a program runs

Data types defines the range and allowed values a constant and variable can hold
examples include:
integer - whole numbers
real - numbers with decimals
boolean - only 2 values (yes/no)
text/string - all alphanumeric characters
character - more efficient alphanumeric characters
Constants, Variables and Data Types
Flowcharts and Pseudo Code
Sequencing, Selection and Iteration

Computers are all around us, how many can you see in the room now?

Mobile phone
Interactive whiteboard

A Computer System
One dimensional array stores numbers in one direction (horizontal only) eg imagine an array called 'array' with the following values:

1, 2, 3, 4, 5

array[0] would be 1, array[4] would be 5 (arrays are zero-indexed)

A two-dimensional array stores data in a data structure which is both horizontal and vertical eg imagine an array called '2Darray' with the following values:

1, 2, 3, 4
5, 6, 7, 8
9, 10, 11, 12

2Darray[0][1] would be the first column of the second row (again zero-indexed), so 5

2Darray[3][2] would be the fourth column of the third row, so 12

2Darray[2][3] would be out-of-range of the array and would cause
either a compile time or run-time error.
AQA Computer Science
A function returns a value and a procedure just executes commands.

The name function comes from maths. It is used to calculate a value based on input.

A procedure is a set of commands which can be executed in order.

The difference is only in the returning a value part.

Procedures and Functions
Parameters and Return Values
The function returns back to the code that called it if the number sent to the function is less than one.
Keyboard and concept keyboards
Trackball or mouse
Digital camera
Touch screen (output too)
Video digitiser
Graphics tablet
Input Devices
Sequence - handling instructions in a sequence and one at a time

Selection - when a decision needs to be made in an algorithm

Iteration - when a loop is used in an algorithm
A Parameter is a value that you pass to a routine.

e.g, if SQRT is a routine that returns the square root of a value, then SQRT(25) would return the value 5. The value 25 is the argument (parameter).

A return value is a statement that tells the program to leave the subroutine and return to the return address, directly after where the subroutine was called.

Output Devices
lights and mechanics (robot arm)
Output Devices
Central Processing Unit

The CPU (Central Processing Unit) is the part of a computer system that is commonly referred to as the "brains" of a computer. The CPU is also known as the processor or microprocessor.

The CPU is responsible for executing a sequence of stored instructions called a program. This program will take inputs from an input device, process the input in some way and output the results to an output device
CPUs are not only found in desktop or laptop computers, many electronic devices now rely on them for their operation. Mobile phones, DVD players and washing machines.

CPU Connections
To be useful, the CPU has to be connected to its adjacent components. These can be divided into two groups.The first is the main memory and the video memory (the graphics card memory). The second includes the 'slower devices like the hard disk, expansion cards, network and keyboard.

The common way of connecting to these groups is to connect the CPU via the north bridge,which is a chip on the motherboard. The north bridge then connects to the main memory and the graphics card memory. There is also another element that is connected to the north bridge, namely the south bridge, which handles the connections to the second group of the slower devices.

RAM (Memory)
AGP, PCIe (graphics)
APM/ACPI (Power Management)
AC97/HDA (Audio)
Other Devices
CPU Characteristics
Clock Speed
Clock Speed is measured in gigahertz (GHz) which represents how many times the clock ticks in one second. A 3 GHz processor means the internal clock ticks three billion times every second – very fast indeed!

A core is a single processor which can fetch and decode instructions. On a multiple core processor there are several processors each simultaneously fetching and executing instructions. Each processor is not faster but the overall rate of fetching and executing instructions is multiplied by the number of cores.
A processor with two cores is called a dual-core processor; a processor with three cores is called a triple-core processor; a processor with four cores is called a quad-core processor; and a processor with six cores is called a hex-core processor. For other numbers of core, the number itself is used, for example a ‘12-core processor’. The higher the number of cores, the better the performance of the computer.

CPU Characteristics cont...
Cache size
The CPU cache is a memory buffer that sits between the processor and the main memory. Before the CPU fetches anything from the main memory, it first checks whether it has been fetched before and is in the cache. If it is, then there is no need to fetch it again from the main memory. Fetching from the cache is much quicker than fetching from main memory.
The bigger the cache, the more space there is to store instructions that the processor needs, and so the more likely it is that when an instruction needs to be fetched, it will already be in the cache. Therefore, having a larger cache size on the CPU can improve the performance of the computer.

Typical cache sizes range from about 512 kB (or half a megabyte) to 8 MB.

Multicore processors normally have a separate cache for each core.

ROM is memory that cannot be changed by a program or user. ROM retains its memory even after the computer is turned off. For example, ROM stores the instructions for the computer to start up when it is turned on again.


RAM is a fast temporary type of memory in which programs, applications and data are stored. Here are some examples of what's stored in RAM:
the operating system
the graphical user interface (GUI)

If a computer loses power, all data stored in its RAM is lost.

Virtual Memory
If you load the operating system, an e-mail program, a Web browser and word processor into RAM simultaneously, 32 megabytes is not enough to hold it all. If there were no such thing as virtual memory, then once you filled up the available RAM your computer would have to say, "Sorry, you can not load any more applications. Please close another application to load a new one." With virtual memory, what the computer can do is look at RAM for areas that have not been used recently and copy them onto the hard disk. This frees up space in RAM to load the new application.
Secondary Storage
Secondary storage is a storage medium that holds information until it is deleted or overwritten regardless if the computer has power.

Types of Secondary Storage:
Hard disc
Optical disc
Flash drives (USB)
Flash memory cards
Old types
Floppy Disk
Magnetic Tape
Optical Media
Optical media - such as the compact disk (CD) - are storage media that hold content in digital form and that are written and read by a laser; these media include all the various CD and DVD variations.

Optical disk capacity ranges up to 6 gigabytes; that's 6 billion bytes compared to the 1.44 megabytes (MB) - 1,440,000 bytes - of the floppy.

One optical disk holds about the equivalent of 500 floppies worth of data.

Durability is another feature of optical media; they last up to seven times as long as traditional storage media.

How it Works
The disc is three layers: the big plastic disc part is on the bottom, a reflective surface is in the middle, and then the top part of the disc is where the art or label is, and this part actually protects the data itself. The data is kept in microscopic pits in the reflective surface.

A motor spins the disc in the drive super fast while a laser attached to a servo reads data off of it. This is why these drives tend to be pretty loud and draw a lot of power: a whole lot of stuff is moving.
Magnetic Media
Magnetic data storage media are so prevalent because they're inexpensive, can be made to retrieve data very quickly, and can be erased and re-written many times. The downside to using magnetic media for archival purposes is that the magnetic particles that store the data will eventually deteriorate, causing data loss. The lifespan of magnetically-encoded data is generally 10-20 years
Solid State Memory
SSD is a drive that uses non-volatile memory as a means of storing and accessing data, much like computer RAM. Unlike other storage devices such as hard drives, an SSD has no moving parts, which gives it advantages such as accessing stored information faster, produces no noise, often more reliable, and consume much less power than the traditional hard drive (HDD) found in computers.
An algorithm is a set of instructions for solving a problem that always finish and return an answer.
Algorithms are not computer code, so a programmer using 'C++' can use the same algorithm as someone programming in 'Java', they use the same steps, just coded differently.

Algorithms can take different forms:
pseudo code
Bits, Bytes and Nibbles
- short for BInary digiT is the smallest unit of data that can be stored by a computer, each bit is represented by a binary digit 1 or 0.

- contains 8 Bits, a single character on the keyboard such as the letter A takes up 1 byte of storage.

- is not very commonly used term . it is used to describe a group of 4 bits, 2 nibbles make a Byte. it is not common as microprocessors only use groups of 8 and higher, not many devices can make use of a nibble.
Kilo, Mega, Giga and Tera
- is generally thought of as 1000 bytes, however to be completely correct it is 1024 bytes. this would be equivalent to 1024 characters on the screen.

- is generally thought of as 1000 kilobytes, however to be completely correct it is 1048576 bytes or 1024 kilobytes.

- is generally thought of as 1000 megabytes, however to be completely correct it is 1024 megabytes. GB not Gb.

- is generally thought of as 1000 gigabytes, however to be completely correct it is 1024 gigabytes or just over 1 million megabytes.
A computer network is a number of computers linked together to allow them to share resources. Networked computers can share hardware, software and data.

Most computer networks have at least one server. A server is a powerful computer that provides one or more services to a network and its users. For example, file storage and email.


A LAN covers a small area such as one site or building, eg a school or a college.

Shows multiple workstations connected in a circle, including a server.


A WAN covers a large geographical area. Most WANs are made from several LANs connected together.

Advantages and Disadvantages
Sharing devices such as printers saves money.
Site (software) licences are likely to be cheaper than buying several standalone licences.
Files can easily be shared between users.
Network users can communicate by email and instant messenger.
Security is good - users cannot see other users' files unlike on stand-alone machines.
Data is easy to backup as all the data is stored on the file server.

Purchasing the network cabling and file servers can be expensive.
Managing a large network is complicated, requires training and a network manager usually needs to be employed.
If the file server breaks down the files on the file server become inaccessible. Email might still work if it is on a separate server. The computers can still be used but are isolated.
Viruses can spread to other computers throughout a computer network.
There is a danger of hacking, particularly with wide area networks. Security procedures are needed to prevent such abuse, eg a firewall.

In a bus network all the workstations, servers and printers are joined to one cable (the bus). At each end of the cable a terminator is fitted to stop signals reflecting back down the bus.
Shows one server, one printer, and five workstations, all connected to a single cable (the bus). At each end of the cable is a terminator to prevent the signal from reflecting.
A bus network, connecting several workstations, servers and printers.


Easy to install

Cheap to install as it doesn’t require much cable


If the main cable fails or gets damaged the whole network fails

Performance of the network will be slower because of data collisions if more workstations are added

Every workstation on the network sees all of the data on the network, this is a security risk

In a ring network each device (workstation, server, printer) is connected to two other devices, this forms a ring for the signals to travel around. Each packet of data on the network travels in one direction and each device receives each packet in turn until the destination device receives it.


This type of network can transfer data quickly, even if there are a large number of devices connected because the data only flows in one direction, so there won’t be any data collisions.


If the main cable fails or any device is faulty then the whole network will fail.

In a star network each device on the network has its own cable that connects to a switch or hub. A hub sends every packet of data to every device, whereas a switch only sends a packet of data to the destination device.

Very reliable – if one cable or device fails then all the others will continue to work
High performing as no data collisions can occur
Expensive to install as this type of network uses the most cable (network cable is expensive)
Extra hardware required (hubs or switches) which adds to cost
If a hub or switch fails all the devices connected to it will have no network connection
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