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AS Computing Unit 1 Cheat Sheet by

computing

RAM vs Cache Memory

RAM
Cache Memory
Fast read and write access
Faster read and write access
Volatile
Volatire
Used to store data and currently running programs
Used to store frequently used data
Necessary as most data is stored on slower storage media
Resides close to CPU

Defrag­men­tation of SSDs

SSD uses direct access, so there woudl be no improv­ement in read times
May perform "­tri­m" command which imrpoves speed of write operations
SSD uses NAND based flash memory. Defrag­men­tation may shorten its lifespan.

FTP vs UDP

FTP
UDP
Breaks data into packets
Breaks data into packets
Can re-send dropped or damaged packets
Does not re-send dropped or damaged packets
Packets can be re-ass­embled if they arrive out of order
 
FTP not useful for streaming media, as receiving new packets is more important than receiving old ones
UDP useful for streaming media, as receiving new packets is more useful than receiving old ones

Floating Point vs Integer

Inte­ger
Floating Point
Numbers are stored accurately
Non-in­tegers can be stored
Requires less complex processing
Greater range of numbers can be stored
Allows for exact repres­ent­ation of 0
 
Takes up less storage space
 
 

Von Neumann Archit­ecture

Clock
Manages Fetch/­Dec­ode­/Ex­ecute cycle
ALU
Processes data involving arithmetic and logical operations
Registers
A small/­fas­t/t­emp­orary storage addressed by mechanisms other than main memory
Buses
Connects all internal components of a computer
Cache Memory
Fast, easily accessed memory close to the CPU.

Star Network

If one cable breaks, network will still operate.
Faults are easy to detect.
Data has to pass through hub, so it is very secure.
Easy to add new stations.

Handsh­aking

Exchange of signals between devices to establish readiness to commun­icate
Example: Establ­ishing a printer's readiness to print

Packet Switching

Data split into packets
Each packet can be transm­itted on different routes
Packets may arrive out of order
More secure than circuit switching
Makes more efficient use of data lines
Contains source address, order number of packet and parity bit.
 

Storage Devices

External HDD
Fast data transfer
Cost-effective cost per unit of storage
Portable and can be stored securely
Third party storage provider
Fast transf­er(­dep­ending on network speed)
Could be more expensive than HDD due to company fees
Data is stored securely
Flash memory drive
Very fast access
Cost effective cost per unit of storage
Very portable
Magnetic tape
Can have very slow access speed
Allows for very large amounts of storage

Network Protocols

HTTP
Transf­erring multimedia web pages over the Internet
DHCP
Assigning dynamic IP addresses to devices on a network.

Data Transm­ission

Simplex
In one direction only
Half-D­uplex
In both direct­ions, but not simult­ane­ously
Duplex
In both directions simult­ane­ously

Overflow Areas

Physical location of a piece of data is calculated using a hashing algori­thm
Calcul­ation is carried out on the primary key
Data collision occurs when two items are hashed to the same location
In this case, the data is sent to the overflow area.
When there are many hash collis­ions, access to the overflow area may be slow
A new hashing algorithm is required and a larger file may be needed.

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