AQA Computing Unit 2 – REVISION
Hardware and Software:
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Hardware is the physical components of a computer system.
Software is a sequence of instructions executed by the hardware.
Buses:
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Data Bus – Bidirectional, Used to transport data between the three components of the three
box model(Processor, main memory and I/O)
Address Bus – Unidirectional, Used to address memory and I/O locations
Control Bus – Birdirectional, used to transport control signals between the three
components of the three box model(Processor, main memory and I/O)
Clock speed, Word length and Bus width:
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The clock speed of a processor is the frequency at which the processor executes
instructions.
Word length is the number of digits in a binary word.
Bus width is the number of signal wires or lines allocated to the bus.
Improving computer performance:
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Increasing the data bus width increases the number of bits that can be transferred at one
time.
Increasing the address bus width increases the number of memory locations that can be
accessed.
Increasing the clock speed allows instructions to be executed at a faster rate.
Addressable Memory:
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Each main memory location has a unique address code.
To read or write to memory, the processor writes the address to the address bus, then uses
the control bus to indicate a read or a write operation. The data is transferred to or from the
selected address over the data bus.
The three buses work together – collectively, they are called the system bus.
The Stored Program Concept:
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A program must be resident in main memory for it to be executed. Instructions are fetched
one at a time in sequence from main memory, and are executed in sequence by the
processor.
Components of a processor:
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Program control unit – fetches program instructions from memory, decodes them and then
executes them one at a time
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Arithmetic and logic unit – performs arithmetic and logical operations. Boolean logic and
shift operations
Registers – very fast memory location in the processor. General or dedicated.
Internal clock – system clock
Internal buses – several buses link the control unit, the ALU and the registers
Logic gates – used for flow control
General purpose registers + Dedicated registers
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A general purpose register is a register that is not assigned a specific role by the processor
designer.
A dedicated register is a register that is assigned a specific role by the processor designer.
o Examples:
 Program Counter – points to the next instruction to be fetched and
executed
 Accumulator – holds the result of the current set of calculations.
 Current Instruction Register – holds the current instruction to be executed
while it is decoded and executed.
 Memory Address Register – holds the address of the memory location
currently being accessed by the processor
 Memory Buffer Register – holds the data item being transferred to or from
the memory location currently being accessed by the processor.
Fetch-Execute Cycle
1.
2.
3.
4.
5.
Contents of the Program Counter are copied to the Memory Address Register(MAR)
Program Counter incremented by 1
Contents of memory addressed by MAR are copied to the Memory Buffer Register (MBR).
Contents of MBR are copied to the Current Instruction Register (CIR).
Content of CIR decoded and executed.
Peripheral:
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A peripheral is a hardware device not directly connected to the processor.
A peripheral is hardware device that is not part of the CPU.
EXAMPLES: Hard disk, mouse, keyboard, CD rom drive
Protocol:
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A protocol is a set of agreed rules/signals used when exchanging data between two
computers.
EXAMPLES: HTTP, HTTPS, FTP, TELNET, POP3/SMTP
What is system software?:
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Provides an interface between user and computer.
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Software that performs the tasks needed to operate the hardware.
Layer of software which enables the user to operate the computer
Utility Software:
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Utilities are programs that perform a very specific task related to working with computers.
EXAMPLES: Disk defragmenter, disk clean-up, file compression and anti-virus software
Library programs:
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A program library is a collection of compiled routines that other programs can use
Different software applications may share run time libraries
EXAMPLE: Word processor and Spreadsheet software sharing the same print routine.
Operating systems:
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Operating systems act as an interface between user and hardware and provide the user with
a virtual machine.
Operating systems provide a software platform on which other programs can run.
EXAMPLES: Windows, Linux, MAC OX,
What is application software?:
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Application software allows users to perform non-computer tasks such as writing a letter.
General Purpose Software:
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General purpose software is software that can carry out many tasks
General purpose software is software that has many purposes.
EXAMPLES: Word processing software, Spreadsheet software, Database software
Special purpose Software:
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Special purpose software supports one specific task.
EXAMPLES: Stock control, payroll, web browsers
Bespoke Software:
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Bespoke software is software that is written to a particular customer’s needs.
Software written for a specific customer need
EXAMPLE: Air traffic control.
Bespoke vs Special purpose:
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Special purpose – Immediately available, Lower costs as development costs are shared
amongst many, Regular upgrades available.
Bespoke – Only way to obtain software that does exactly what they want, no unnecessary
features,
Software translators:
Assembler
Process
Use
Reason
Compiler
Process
Use
Reason
Interpreter
Process
Use
Reason
Translates second generation assembly language into
machine code.
One-to-one.
Running an assembly language program.
Required to run assembly language.
No other options for assembly language.
Translates high level languages into machine code.
One-to-many.
Checks the syntax of the program, and then translates all of
the code in one go and produces an output file which can
then be run directly without the compiler being present.
Where tested software is to be distributed to a user.
Where the program needs to be as fast as possible.
Where it cannot be easily changed.
Speed of execution.
Simplicity of product.
Cannot be changed.
Translates high level languages into machine code one line
at a time.
Checks the syntax of each line and then translates it into
machine code and executes the program on the computer.
Software under development.
Software run in a controlled environment .
Software debugging.
Line by line analysis.
Limitations of assembly code:
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Very easy to make mistakes
Very difficult to understand
Hard to find errors and debug
Compiler vs Interpreter:
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Compiler optimises the code
No need to distribute the source code
Object code can be installed on target computer
High level language vs assembly language:
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High level languages are problem orientated
The syntax is more like English.
Easier to write.
Easier to debug.
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Less tedious to program
Translators:
Assembly codeMachine code = ASSEMBLER
High level languageMachine code = COMPILER
Declarative
WHAT?
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Language that specifies what the problem to be solved is/what needs to be done
Language that does not say how to solve a problem/what algorithm to follow
Language that does not specify the order in which to carry out actions to solve problem
Imperative:
WHAT?
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Instructions are executed in a programmer defined sequence.
Relationships between different programming generations:
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One assembly language instruction will translate into one machine code instruction.
o 1-to-1
One high level language statement corresponds to many low level language statements.
o 1-to-many
One high level language statement will translate into many machine code instructions.
o 1-to-many
Difference between Internet, Intranet and WWW:
World-Wide Web:
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a system of interlinked hypertext documents;
accessed via the Internet;
using HTTP protocol (to retrieve webpages);
Web pages not restricted to intranet;
Internet:
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global network;
A network of interconnected computer networks / computers;
using a globally unique address space;
Packet-switched network;
using end-to-end communication protocol // Internet Protocol // TCP/IP;
public network;
Intranet:
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a private (computer) network;
available to a closed community // only within an organisation;
that uses Internet Protocols;
to share part of an organisation’s information (with its members);
Similarities between Internet and Intranet:
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Both use the same protocol
Both are client-server based
Both have similar purpose
HTTPS vs HTTP:
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HTTPS is secure
HTTPS is encrypted
HTTPS uses port 443. HTTP uses port 80.
FQDN vs IP address:
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It is easier to remember a FQDN
More recognizable
IP addresses are less memorable
DRM – Preventing sharing:
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Encrypt the file
File limited to certain hardware devices
Limit the number of times it can be used/played
Robots
WHAT?
A robot is a mechanical, moveable structure
1)
2)
3)
4)
That can sense its surroundings/environment.
Can manipulate things
That is programmable
Artificially created
Machines/Robots – Good or bad?
Good:
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Bad:
Make precise/complex calculations
More consistent than humans
Perform repetitive tasks
Can work in dangerous conditions
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Lack creativity
Poor at coping with unexpected/emergency situations
Shape detection/Image recognition
Cannot think for themselves
Registers:
A register is a very fast memory location in the processor.
Code of conducts:
WHAT?
A code of conduct is an outline of the rules that an employee must follow. It also outlines the
disciplinary actions they face if they breach it.
WHY?
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Ensures employees are aware of legal requirements
Sets out good practice for employees
Relate legal requirements to the work that the employee does.
Client ports:
A client port is a port that is assigned a temporary address.
Biometric properties:
A biometric property is a biological property of a person that can be used to identify them.
EXAMPLES:Fingerprint, Iris scans, Voice pattern
How does RFID work?
1.
2.
3.
4.
The reader sends radio frequency energy/wave; to the antenna of the RFID tag in the object
The RFID tag is energised by the reader
The transponder (in the RFID tag) sends the data signal
The reader near the exit receives the data signal