computer science wjec gcse - THEORY ONLY

What does CPU stand for

Central Processing Unit

what does a CPU consist of

Arithmetic logic unit (ALU), control unit, internal memory, registers

What are Vou Nemann architectures?

A form of CPU architecture which stores a program in memory as instructions and executes them sequentially using the ALU, CU and registers

what are CPU harvard architectures?

architecture that sotres data on another media e.g. magnetic tape. Not used anymore

What does a CPU do?

The main component in a computer for processing data and instructions

What is a control unit and what does it do

the controller (control unit) sends and receives signals from all parts of the computer, and ensures all processes take place at the right time and in the correct order.

What does the ALU do

processes and manipulates data. Performs simple calculations and operations on data. Also able to perform comparisons on data e.g. IF

what are registers?

a storage location found on the CPU where data is temporarily stored.

what does the accumulator (ACC) do?

stores the result of mathematical or logical calculations performed in the ALU

What does the program counter do?

A register that tracks the RAM address of the next instruction to be fetched

what does the MAR do in the FDE cycle?

A register that tracks the RAM address of the instruction that is to be fetched

What does the MDR do in the FDE cycle?

The MDR stores the instruction that has been transferred from RAM to the CPU

what does the CIR do in the FDE cycle?

A register that stores the instruction that has been fetched from RAM, and is about to be decoded / executed.

what is internal memory?

fast access temporary storage on the CPU

what does the internal memory do?

Data is moved from registers to internal memory when it is not being actively used. Data from internal memory can then be written to RAM or called back into registers for further processing.

What is a bus in the CPU?

Buses allow data to be transferred to different parts of the computer

What does the address bus do?

A uni-directional bus that Carries addresses generated by the processor from processor to main memory or other Input/output devices.

What does the data bus do?

A bi-directional bus that carries data from main memory to processor. Data can be read and or written from.

What does the control bus do?

Carries control signals or commands from the CPU to control and co-ordinate all the activities within the computer.

Instructs which data can be travelling to / from memory.

Control signals are sent along the control bus.

Describe the Fetch part of the FDE cycle

1. Program Counter displays the address in RAM of the next instruction to be processed, which is copied into the MAR

2. PC register increment increased by 1; prepares CPU for the next instruction to be fetched

3. CPU checks the address in RAM which matches address in MAR

4. instructions in RAM is transferred to the MDR

5. Instructions in MDR copied into the CIR

Describe the decode cycle in the FDE cycle

The control unit authenticates the instruction in the current insturction register, and instruction is decoded to determine the action that needs to be carried out

Describe the execute cycle of the FDE cycle

Actions that happen during the execution depend on the action itself.

If it's a calculation instruction, ALU performs it and stores in ACC.

what is Cache memory in the CPU

fastest access type of memory for CPU; expensive and small

How does the Cache memory function?

Cache memory acts as an intermediary between the processor and the main memory (RAM) and as programs are executing the cache holds commonly used instructions

How does more cache memory improve CPU performance

Increasing cahce size reduces the number of memory to disk transfer and therefore speed up processing of these instructions (fastest form of memory to be fetched by CPU)

What is clock speed

the speed at which a processor operates. It is the number of FDE cycles a CPU can operate per second.

How does a higher clock speed affect CPU performance

Fastest clock speed = faster the computer is able to run the FDE cycle and therefore process more instructions.

Define overclocking

Setting a processor to run faster than its original design however causing the CPU to shorten its lifespan as CPU overheats and uses more power

Define underclocking

Setting a CPU to run slower than its original design. Results in less power consumption, less heat produced and increase battery life of device.

What does cores mean?

describes the processing components in the CPU

How does having multiple cores instead of 1 affect CPU performance?

Multi cores have several processing components within the same CPU. A dualcore will be able to perform 2 instructions at once, whereas a singular core can only carry out 1.

Disadvantage of multi-cores?

Sometimes instructions from one core require instructions from another core in order to be operated, so speed would be similar or equal to that of a singular core

RISC and CISC stands for:

Reduced instruction set computer

complex instruction set computer

Describe a RISC processor

1. Process a limited number of relatively simple instructions

2. To carry out complex commands, problem broken down into a longer list of simpler instructions

3. Requires less circuitry than CISC to decode and execute, so less power consumption and less heat is also generated

Describe a CISC processor

1. Process a large number of complex instructions

2. Understands and carries out complex tasks with only a few instructions without needing to break them down.

3. Requries more circuitry to decode, so more power consumption and therefore more heat generated

what is an input device

sends information to a computer for processing

what is an output device

a device which receives information from a computer in order to present it in human form

What is RAM used for?

temporary storage of currently running programs and data. Contains a large number of store locations, each of which is identified by a unique address

Properties of RAM

Fast form of memory, however data is volatile (data lost when power is switched off)

what is ROM used for

permanent storage of data; data in each store location cannot be changed. Usually used to store the BIOS

Properties of ROM

non-volatile (data isnt lost when power switched off)

what is a disadvantage of using ROM

BIOS that is stored in ROM cannot be changed.

What is flash memory

a form of memory used to store permanent data, however data can be read / written to. This is an alternative to storing the BIOS which can be upgraded, however slower than ROM

Order of speed for different types of memory (fastest to slowest)

1. cache memory

2. ROM

3. RAM

4. Flash memory

Why is secondary storage used

when data is no longer being actively used but the user wishes to retreive said data at a later time. It takes longer to store data than it is to access data from memory.

How does optical storage work>?

Uses lasers which are projected onto CD/DVD/BLU-RAY disc and if light is reflected back, then data is read as as 1. If not , it is read as a 0. Lasers are used to read and write info on a disk.

How does magnetic storage work?

data stored on a magnetic medium e.g. disc / tape by writing data used a write-head. Data can then be read by the read-head.

Why are Solid State used?

involves no moving parts, very portable, low power consumption, high speed access.

Order of secondary memory from fastest to slowest

1. flash drive

2. normal hard drive

3. DVD/Blu-ray disc

4. magnetic tape

Order of secondary memory from most durable to least

1. Flash drive

2. dvd/blu-ray disc

3. magnetic tape

4. normal hard drive

Order of secondary memory from largest typical capacity to smallest

1. normal hard drive 320mb - 8TB

2. flash drive 2gb - 512 gb

3. magnetic tape 200gb - 400gb

4. dvd/blu-ray - 620mb (cd), 9gb (dvd), 50gb (blu-ray)

Describe the motherboard

the mother board is the main circuit board of a computer. Mounts other components of the PC together.

Describe the GPU

a microprocessor that performs the calculations needed to produce graphic images on screen

differences between an integrated CPU and a dedicated GPU

integrated GPU uses the computers RAM whereas dedicated GPU's has its own video memory.

integrated GPU's therefore use power and less heat.

Describe sound cards

enable the computer to output soudn through speakers, record sound form a microphone and manipulate sound stored on a disk

they convert analogue input signals into digital data and reverse this process for audio output.

What is an embedded system

a combination of software and hardware that performs a specific task rather than a general-purpose computer that is designed to carry out multiple tasks

what is firmware?

software written for an embedded system

where are instructions stored in an embedded system?

flash memory / read-only memory

most embedded systems are ____?

reactive towards conditions such as temperature, weight, vibration and air quality.

advantages of a network (be able to name atleast 3)

share hardware

share software

share data/files

send emails

central backup

centrally controlled security!

can access data from any computer

disadvantages of a network (name atleast 3)

network manager may need to be employed - expensive

security problems - files sent between computers could spread a virus

hackers can gain access to data more easily

all workstations on the network are affected if the server is down

intitial costs of servers, communication devices etc - expensive.

what is a LAN?

Local area network where computer systems are all located relatively close to each other.

what is a WAN?

Wide area netowkr which computers sytems are all located relatively far from each other. E.g. countries

If protocols cannot be agreed between computer systems what would happen?

they would not be able to communicate with each other.

describe a bus network

nodes all connected to a single cable which data sent called bus

terminators on each end of bus to ensure network functions correctly

bus carries data packets, which arrive at each node and authenticates destination address. If wrong address, data moved to another node.

advantages of a bus network

easy to implement and add more computer systems

quick to set up

cost-effective - less cabling

disadvantages of a bus network

difficult to troubleshoot the bus

low security - all nodes on bus can see all data transmissions

data collisions more likely causing network to slow down. (when 2 nodes send a data packet at the same time)

If there is a problem with the main cable or connection, entire network goes down

Describe a ring network

computer systems connected in a ring; data packets sent around ring, passed from one computer system to the next until it arrives at destination

advnatages of a ring network

data quickly transferred without bottlenecks

transmission of data relatively simple; travel uni-directional

adding additional nodes hasa very little effect on bandwidth

prevents network collisions

disadvantages of a ring network

if any of the computer systems fail, ring is broken and data cannot be transmitted efficiently

problem with the main cable = entire network goes down

difficult to troubleshoot ring

describe a star network

each computer system connected to a central node known as a hub

advantages of a star network

good performance

easy to setup

possible to add more computer systems without taking the network down

better security

minimal network collisions

disadvantages of a star network

expensive - more cabling required

extra hardware required e.g. hub

describe a mesh network

each computer system is directly connected to as many other nodes as possible.

advantages of a mesh network

data can be transmitted from diff nodes simulataneously

mesh topologies can withstand high traffic

if one node fails, there are alternatives present

high levels of security and privacy

a fault can be diagnosed easily

disadvantages of a mesh network

installation and configuration can be difficult as netowkr grows

expensive - cabling costs

set up and maintenance is difficult

high chance that many of the network connections will be redundant.

typical network connections

1. copper cable (100Mb/s to 1 Gb/s)

2. wifi (54-108mb typically)

3. fibre-optic connection (1-10GB/s)

describe circuit switching

provides a temporary but dedicated link between 2 nodes, where no other data can be transmitted along same route.

advantages of circuit switching

reliable; once connection established, it is fast and error-free (generally)

disadvantages of circuit switching

takes time to establish connection

should anywhere on the route fail, the connection will be broken and all data will be lost.

why was packet switching made?

to overcome the problems that came with circuit switching

describe packet switching

Packet switching involves breaking data down into small packets and using a switch or router, packets are delivered to a specified destination.

Each packet transmitted over a network and may take diff routes to its destination. When the packets arrive, data is re-assembled.

What will a packet typically contain? (name atleast 4)

The source address

The data address

The data itself

Re-assembley info

a checksum to authenicate data i.e checks it hasnt been corrupted

other tracking information

what is routing?

the method of selecting paths along which packets are sent on a computer network.

what is an IP address?

address which is allocated to a computer system on a network, usually by a DHCP

what are IP addresses used for?

to uniquely identify computer systems on a network, thus allowing communication between them. In routing tables, the corresponding IP address of a unique MAC address is stored and updated as necessary.

what is a routing table?

A routing table resides in a router's memory to determine the networks that the router knows how to send the data to.

what is a DNS

a distributed database that matches IP addresses to computer system resources.

Describe how a browser obtains an IP from a DNS

1. Browser checks cached host file to check if it already holds IP address

2. if not, local ISP DNS server queried for IP address

3. if ISP DNS does not hold IP address, query passed onto another DNS server at a higher level

4. repeat until IP resolved

5. Address passed on to DNS servers lower in hierarchy

6. When full address resolved, IP addressed passed onto browser

7. Browser connects to the IP address and is then passed into browser

8. browser connects IP address of server and downloads website.

describe a protocol

A set of rules and standards that two nodes use for communication.

What does a protocol involve?

handshaking - 2 devices establish their readiness to communicate

how the sending device will indicate that it has finished sending a message

how the receiving device will indicate that it has received a message

type of error checking used

agreement on the data compression method to be used

describe the TCP protocol

TCP - transmission control protocol allows packets to be sent and received between computer systems

describe the IP protocol

protocol that sets out the format of packets and an addressing system

describe the HTTP protocol

a protocol used to transfer multimedia web pages over the internet

describe the FTP protocol

protocol used when copying a file from one location to another via a network / internet.

allows broken communications to resume transferring a file rather than having to restart.

what is a protocol stack?

a set of protocols that work together to provide networking capabilities

why are protocols stacked?

allow for new protocols and different network architectures to work correctly.

Lowest layer will deal with physical interaction of hardware, user applications interacting with top layer and each higher layer adding additional features.

Name the layers involved in the TCP/IP 5 layer protocol stack model

PHYSICAL layer

DATA LINK layer

NETWORK layer

TRANSPORT layer

APPLICATION layer

what does the physical layer do in the TCP/IP 5 layer model?

transmits the raw data, consists of hardware such as switches and routers, deals with all aspects of setting up and maintaining a link between communicating computers

what does the data link layer do in the TCP/IP 5 layer model?

sends data from network layer to physical layer, divides data into data frames, handles acknowledgements setn from receive and ensures that incoming data has been received correctly by analysing bit patterns in frames.