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WJEC A2 Computing Unit 4.1 Cheat Sheet by

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Primary Storage

RAM (Random Access Memory)
RAM is vola­tile and relatively slow compared to other methods of primary storage. It is useful for storing parts of programs while they are being run.
ROM (Read Only Memory)
ROM is not volatile and read-only. This means that once data has been programmed into it, it cannot be overwr­itten.


RAM Cache
Also known as L2 or L3 cache, this is a memory chip between the CPU and main memory where small sections of data are stored.
These are extr­emely fast since they are small and close to the CPU.
Disk Cache
This is a section of memory between the CPU and the disk where data can be tempor­arily stored before being transf­erred to RAM.

Harvard Archit­ecture

In the Harvard Archit­ect­ure, data and instru­ctions are stored seperately and accessed via separate buses. The Harvard Archit­ecture is generally used by RISC proces­sors.

Fetch-­Execute Cycle

The processor sends the address held in the PC to the address bus. The data is sent back to the CIR and the PC is increm­ented.
The processor runs the instru­ction in the CIR. It then fetc­hes the next instru­ction.

Graphics Processing Unit

GPUs are able to run a single instru­ction on multiple pieces of data in parallel. This is known as SIMD - Single In­str­uction Mu­ltiple Data.
GPUs are frequently used for processing screen data, since operations can be performed on each pixel in parallel.

Parallel Processing

Parallel Proces­sing involves setting two or more processors to perform a single task. The task is split into thre­ads which can run concur­rently.
1. More instru­ctions can be processed in a given amount of time since they can be executed concur­rently.
2. Tasks can be shared to reduce bottle­necks
1. It is difficult to write programs for multi-core proces­sors.
2. Results from different processors need to be combined at the end of the program which may take more time.
3. Not all tasks can be parall­elized.
4. Concur­rency may introduce new software bugs.

Types of Network

Local Area Network (LAN)
Connected locally, usually on the same site.
Wide Area Network (WAN)
Connected via satellite links or any other method, may be spread out across towns or countries
Storage Area Network (SAN)
A dedicated network for large scale data storage. Usually provides a disc array of high capacity and perfor­mance.
Metr­opo­litan Area Network (MAN)
Networks that provide WAN services in a city.
Personal Area Network (PAN)
Links personal devices such as phones.

Network Hardware

Network Interface Card (NIC)
1. NICs take data that the computer sends it and transmits it through any connected network cable via the approp­riate protocol.
2. The most popular one is Ethe­rnet, or for wireless the 802.11 protoc­ol..
1. Allows more than one computer to be connected in a network by connecting them all to a hub.
2. If a cable breaks, it only affects the computer connected to the hub through that cable.
3. This is generally very slow for large networ­ks, since the more traffic that is generated means the more work that the hub has to do.
4. When a hub receives a packet, it transmits the packet to all connected ports.
1. Unlike hubs, a switch can route packets from one port to another. This is known as rout­ing.
2. Switches hold a MAC address table to determine which computer is connected to which port.
1. Routers perform the same as switches, except instead of using MAC addresses to determine where to send packets they use IP addresses.
2. These are used for WANs or very large networks.
Wireless Interface Card (WIC)
1. This connects to a WAP or Wi­reless Access Point.
2. Most WICs use the 802.11 protocol to commun­icate with the WAP, in combin­ation with security protocols such as WEP or WPA2.


Buses are groups of parallel wires which connect the processor to I/O contro­llers or memory. There are three types of bus:
Data Bus
The data bus is bidi­rec­tio­nal and carries data between the CPU and the main memory. The width of the data bus is usually the same as the CPU word size, so either 32 or 64 are common.
Address Bus
This bus only goes from the processor to memory. This is used by the processor to retrieve data from memory. The address bus carries the memory address of the next instru­ction or data item to be received through the data bus.
Control Bus
This is a bidi­rec­tio­nal bus that sends control signals to the registers, data and address bus. This helps to ensure that everything is kept in sync.

CPU Components

ALU (Arith­metic and Logic Unit)
Carries out mathem­atical and logical functions. It is sent an opcode and an oper­and and carries out the required process.
Control Unit
Supervises the fetc­h-e­xecute cycle. The control unit also decodes instru­cti­ons.

Factors Affecting Perfor­mance

Clock Speed
This is how many ticks per second the CPU is able to perform.
Bus Width
This determines how much data can be transf­erred along each bus in a single tick.
Word Length
How many bits can be processed at a time. This is usually either 32 or 64 bits.
Multiple Cores
A multi-core CPU will be able to run two different instru­ctions at the same time. This is useful for tasks that can be para­lle­liz­ed.
Cache Memory
Frequently used inform­ation can be placed into a temporary area of memory close to the CPU. This is much faster to read from.

Amdahl's Law

This is where T(n) is the time taken on n threads and B is the fraction of the algorithm that must be sequen­tial.


Hardware such as prin­ters can be shared
Viru­ses can be spread across a network to all computers
Software can be shared
Hack­ers may be able to access a network
Data can be shared, e.g from a file server
Network failure means that nobody can use their computer
Computers may comm­uni­cate with each other via messaging
Networks may be slower than standalone computers
Complex cabling may be expe­nsive to install

Network Models

This is where the client frequently requests data from a central entity, the server. This is the most common type of network since it seperates functions, such as storage or printing.
This is where every computer is a client. This is cheaper to implement and also useful for sharing files frequently between computers.

Addres­sable Memory

Memory is made up of a number of uniquely identi­fiable addr­essable cells.
Memory is organized system­ati­cally such that data that is related to one process is stored in one block. This speeds up the time to access memory, since the computer has a vague idea of where it is.
A memory map can be created to show which programs are stored at which address.


Regi­sters are located in the CPU and hold only a few bits of data at a time. However, they are very fast to access.
There are many types of registers in the CPU:
Status Register
This keeps track of the status of many parts of the computer.
Inte­rrupt Register
This stores details of any interrupt signals sent to the processor. If a bit is set in the interrupt register, the CPU must respond depending on its severity.
Current Instru­ction Register (CIR)
Stores the currently executed instru­ction
Program Counter (PC)
Stores the memory location of the next instru­ction
Memory Buffer Register (MBR)
Holds the data that has just been transf­erred between memory
Accu­mulator (ACC)
Stores the results of the ALU

I/O Contro­llers

I/O Contro­llers are able to translate signals from external devices into something unders­tan­dable by the CPU.
Also, I/O Contro­llers will buffer data that is sent between the processor and the external device so that the processor does not have to wait.

von Neumann Archit­ecture

The von Neumann archit­ect­ure describes a computer with one control unit that sequen­tially works through instru­ctions. Instru­ctions and memory are bundled together. This means that instru­ctions can't be fetched while data is being sent along the bus, causing the von Neumann Bottle­neck

Input and Output Devices

Optical Character Recogn­ition (OCR)
This converts printe­d(often hand-w­ritten) media into editable text documents via scanning.
Optical Mark Recogn­ition (OMR)
The computer reads selections from a predefined form. This is often used for multip­le-­choice tests.
Magnetic Ink Recogn­ition (MICR)
A computer uses magnets to read data from a strip of semi-m­agnetic material. This is used most commonly for cheques since the reader can be very expensive.
Touch Screens
There are two types of touch screen.
1. Resi­stive touch screens are made up of two thin transp­arent sheets that transmit a signal when they touch.
2. Capa­citive touch screens are more common and use the fact that the human body conducts electr­icity to determine when a touch has occurred.
Voice Input
The computer detects commands spoken by the user into a microp­hone.
Voca­bulary Dictat­ion
The computer attempts to detect all words spoken by the user. This is used frequently by people with RSI.

Cloud Storage

Cloud storage is off-site storage that is accessible anywhere and usually provided by a third party company.
This is useful for a few reasons:
1. Removes the need to install and upgrade software
2. Removes the need to hire specialist staff
3. Removes the need to back up data.
However, there are drawbacks:
1. Handing control of data to another party may be risky
2. Risk of losing access to the service and having no means to recover

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Coler997 Coler997, 18:09 13 Jun 18

Can you post more for the rest of the topics in the unit, my class are really struggling

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