Higher Computing Science - Computer Systems

bit

Binary Digit 0 or 1

Base 2

This is a numbering system that only has two digits. Binary - 0 OFF and 1 ON

denary/decimal

Base 10 numbering system featuring numbers from 0-9

Integer

All whole numbers (both positive and negative) and zero.

Mantissa

Representation of the decimal portion of a floating point number

Exponent

A number indicating how many times the decimal point has been shifted (+ve to the left, -ve to the right)

Address Bus

A uni directional bus that specifies the address location in memory to be read from/written to

ALU

Arithmetic and Logic Unit - Performs all the arithmetical and logical operations inside the processor

ASCII

American Standard Code for Information Interchange - 1 byte code used to represent characters.

Bit depth

bit depth is the size of the binary number needed to represent the colour of an individual pixel in an image

Cache Memory

A small piece of fast static memory use to hold the more commonly used instructions to increase performance

Control Characters

non -printable characters that have an effect on screen such as the tab key, enter key etc.

Data Bus

The data bus is used to carry data to and from the processor/memory.

Dynamic RAM

the more common type of memory. The dynamic refers to the fact that it constantly requires power to refresh its contents. Is it is less complex than static RAM

Fetch execute Cycle

The process of retrieving and executing an instruction from memory.

Floating Point Representation

The method used to store real numbers using a sign bit, mantissa and exponent.

Two's Complement

A system to represent negative integers in binary. The positive number in binary is found, this number is then inverted and then 1 is added to the right hand side of the number.

Unicode

A form of representing text using 8,16 or 32 bits. The larger storage requirements allow for more characters/languages to be represented

Vector Graphics

A method of representing graphics using the attributes of each shape. Such as x,y co-ordinates, radius etc.

Processor Core

A smaller part of a processor which is capable of executing a single task at a time. Modern processors can have multiple cores.

Concurrency

The ability for more than one task/instruction to be executed at the same time.

DOS

Denial of Service attack. DOS attack is an active attack in order to stop a website/online service from providing its services.

Formula for range for two's complement numbers?

from -ve2^n-1 to +ve(2^n-1)-1 where n is the number of bits

Mnemonic: MARE

If you increase the number of bits assigned to the mantissa, the accuracy of numbers you can store increases. If you increase the number of bits assigned to the exponent the range of numbers that you can store increases.

Disadvantage of ASCII

You can only encode a maximum of 256 characters, therefore not all languages can be encoded using ASCII.

Advantages of vector graphics:

1)Smaller file size to bit mapped graphics.

2)Resolution independent - same image quality regardless of the resolution.

3)Individual objects can be easily edited.

Disadvantage of Vector graphics:

They cannot represent photo-realistic images.

Control Unit

It controls the timing of the execution of instructions, ensuring they are completed in the correct order.

3 types of information that can be stored in a register

Data, Instructions and Addresses

interrupt

An instruction that forces the CPU to halt the execution of the current operation to handle the interrupt.

What are the steps in the fetch execute cycle?

1)The memory address of the next instruction is placed on the address bus.

2)Read line is activated.

3)The instruction is transferred to the processor on the data bus.

4)The instruction is decoded and executed.

What are the steps of the memory read operation?

1)The MAR is set up with the address that is to be accessed.

2)The address location is transferred on the address bus.

3)Read line activated.

4)The instruction is placed on the data bus and taken to the processor.

5)The instruction is then placed on the MDR

6)It is decoded and executed.

What are the steps of the memory write operation?

1)MAR set up with address location to be written to.

2)MDR set up with data to be written.

3)Address bus set up with address location, from MAR.

4)Data bus set up with data to be written, from MDR.

5)Write line is activated.

6)data on data bus is placed in memory location specified by the address bus.

Types of RAM

Dynamic RAM (DRAM) - typically called main memory, has to be refreshed regularly.

Static RAM (SRAM) - cache memory, very fast access time.

When does a cache hit occur?

When requested data or instruction is found within the cache memory, it is accessed within a couple of clock cycles.

When does a cache miss occur?

When requested data or instruction is not found within the cache, it will need to be accessed from memory which has a slower access time.

How do we optimise cache use?

1)CPU will fetch the next memory location to the one being asked for.

2)A hardware pre-fetcher can load 256 bytes after the line has been loaded.

3)This pre-fetch can be triggered when successive cache misses occur, so frequently used instructions will be cached.

Order memory size from smallest to largest:

Register, Cache, Memory, Backing storage

Order speed of access from fastest to slowest:

Register, Cache, Memory, Backing storage

How to increase system performance?

1)Increasing clock speed.

2)Increasing RAM

3)Adding multiple CPU cores.

4)Increasing data bus width.

5)Increasing cache memory.

How does increasing clock speed improve system performance?

Allows the CPU to execute more instructions per second. However cannot be increased too much since the processor will overheat.

How does increasing data bus width improve system performance?

It allows more data to be moved in a single operation, so less operations are required to complete tasks.

How does increasing cache memory improve system performance?

It allows to store more frequently used instructions and reduce the amount of memory read operations from RAM, which has a slower access time.

What are the 3 offences under the computer misuse act?

1)Gain unauthorised access to a computer system.

2)Gaining unauthorised access to a computer system with the intent to commit a further crime.

3)Unauthorised modification of data and files.

What are tracking cookies?

A small file placed on a user's computer that monitors their browsing history and habits, can be read/altered on next visits of websites. Used by marketers to create a user profile, to display ads that you are more likely to click on.

How does bandwidth consumption DOS attack work?

An attacker floods your server with packets of information that the server needs to respond to, consuming network bandwidth.

How does a Resource starvation DOS attack work?

A server has finite amount of resources such as backing storage and memory. An attack will try to exhaust these services. e.g. Continually creating new user accounts, Spamming an email server to use up all backing storage.

How does a DNS attack work?

A DNS server resolves the URL into an IP address. An attacker may try to cause the

DNS server to route traffic to a target server, can consume bandwidth of target server and denies user access to the website they want to visit.

DDOS Attack

Distributed Denial of Service Attack. Typically a virus/program/botnet installed on many computers (thousands) activate at the same time and flood a target with traffic to the point the server becomes overwhelmed.

What are the costs of a DOS attack?

1)Loss of revenue due to website downtime.

2)Determining the nature of the attack.

3)Payment for labour to repair the damages of the attack.

4)Payment for labour to implement new safeguards.

Reasons for DOS attacks:

1)blackmail - for money

2)political - you don't agree with their views

3)Grudge - ex.employee, ex partner.

4)Against a rival company.

What is symmetric encryption?

An encryption method in which the same key is used to encrypt and decrypt a message

What is asymmetric encryption?

An encryption method where two keys are used. A public key which is used to encrypt data and is made public. A private key which is used to decrypt messages and is kept secret.

What is a digital certificate?

A digital version of passport that confirms the identity of a person or organisation. This allows individuals and companies to feel secure in exchanging information as they can confirm each other's identities.

What is a digital signature?

A digital signature (not to be confused with a digital certificate) is a mathematical technique used to validate the authenticity and integrity of a message, software, or digital document..

How do you make a digital signature?

1)Obtain a message hash of the contents of the message.

2)Encrypt the hash with your private key.

3)attach it to the message.