Computing: F451, Technical Bits

Arithmetic Logic Unit 

• Arithmetic or logical comparisons are carried out here.
• Is a gateway to the proccesor, all inputs and outputs pass through before being redirected to other parts of the system.

Control Unit

• Fetches instruction, decodes it and manages the execution.
• Sends control signals to other parts of the processor using busses.

Immediate Access Store

• The main memory of the computer.

Registers ensure progress of instructions before and during their execution.

Program Counter

• Knows exactly where the next instruction is stored in the computer memory, relays this information to the processor.

Memory Address Register

• The address read from the Program Counter is sent here.

Memory Data Register

• Memory searched for the address held in the MAR and the instruction held at that address is then placed in the MDR.

Current Instruction Register

• Sends part of the instruction to the computer so the processor can carry out the instructions. The other tells the processor where it can read the data required for the action from.

These are pathways used by the processor to send signals to part of the computer.

Control Bus

• Sends instructions from the control unit to other parts of the processor.

Data Bus

• Carries data from one register to another.

Address Bus

• Transmits the location address to where the data is headed - as it travels round the computer if that address matches where the data is, the data is 'grabbed'.

A LAN, Local Area network, is a collection of computers/peripherals on one site, connected through cable or wireless.

A WAN, Wide Area Network, connects over long distances. The most common being the internet. 

Hardware Required

• Computers require a Network Interface Card
• Often uses a central server with shared files.
• A router needed to connect to a WAN.

Software Required

• Network operating system and software.
• Data compression, encryption and error correction required.
• Browser to view inter/intranet pages.
• Firewalls when connected to the internet.

Serial Data Transmission: One wire, bit by bit. Only advantage is that allt he bits are kept together.

Parallel Transmission: one wire for each bit in a byte.

Simplex: one-way data communication.

Duplex: two-way data communication.

Half-duplex: two way, but only one at a time (for example, baby monitor).

Bit Rate the number of bits transmitted within a timeframe.

Packet Switching

• Groups data into blocks and sends across the network.
• Any route is used, packets are rebuilt by the received.

Circuit Switching

• Decides path before data transmission starts - uses this path for whole message.
• Very fast rates, but only a few messages can be sent at once.
• Easier to hack as data stays together - unlike packet switching. This also means a break in transmission if the connection breaks.

A protocol sets rules used by two devices communicating with each other, such as: bit rate; data type; error checking used. A protocol has logical parts (those which apply to data) and physical parts, those which set the methods for communication.

Echoing Back

• A set of data is 'echoed' back after transfer. It is checked against the original data to ensure it is correct. Repeated until the echo matches the original.
• Time/bandwidth consuming.

Check sums

• The original bytes are added together and sent with the file. The bytes received can then be added together and the number compared with the one sent. If different, it is likely the data should be resent.

Parity check 

• The eighth bit of a byte is reserved for parity. This digit it used to ensure every byte has an odd or even number of 1's in. This is useful because it is easy to find a corrupt byte, yet the data does not need to be bounced backwards and forwards between devices.