Computing A-Level

For now it's gonna be for Oct 6th

What is a Computer?

An electronic device that takes input, processes data & delivers output

What is a program?

A set of instructions that the computer follows to process data

What does a CPU do?

Carries out instructions and stores the program in the memory

What are factors that affect performance of a computer? (3 marks)

Clock Speed, Number of Cores, Cache Size

What is clock speed?

The rate the processors sends electronic pulses to synchronize the CPU’s operations, measured in Hz

What does GHz mean? How many clock cycles per second?

Gigahertz, One Billion clock cycles per second.

What is the Cache?

Temporary storage of data & instructions being read & written from

What does the Cache do?

Stores copies of recent data & instructions. Much quicker to access than Main Memory.

What is a Core?

100% copy of a CPU.

What does a core contain?

Processing units, ALU, Accumulator, CU

What are disadvantages to cores?

They need to communicate with each other which takes time

Many programs are not designed to make use of multiple cores

What are properties of Von Neumann Architecture?

Shared memory space for instructions & data

Instructions & data are stored in the same format

A single CU or processor follow a linear FDE Cycle

One instruction at a time

Registers are used as fast access to instructions & data

What are properties of Harvard Architecture?

Instruction & data stored in seperate memory

Each has it’s own bus

Reading & Writing data can be done at the same time as fetching an instruction

Used by RISC processors

What does the Control Unit?

Decodes instructions

Sends signals to control how data moves around the CPU

What is a register?

Tiny areas of extremely fast memory located in the CPU

Enables FDE Cycle to operate

Designed for a specific purpose, where data or info is stored temporarily

What does the Current Instruction Register do?

Stores the address of the instruction currently being decoded or executed.

What does the Program counter do?

Holds the address of the next instruction

The value here is passed to the MAR

What can cause the value in the PC to change?

A jump instruction will cause it to the change the value it stores

What does the Memory Address Register do?

Holds the address of an instruction passed from the program counter

Holds the address of the data to be sent to the Memory Data register

What does the Memory Data Register do?

Holds data to be fetched from or to the memory or CPU

What does the Arithmetic Logic Unit do?

It performs calculations

Handles Bit Shifting

Manipulates data

Handles logical comparisons allowing a program to make decisions

What does the Accumulator do?

Holds the results of calculations

What are Buses?

Tiny wires that connect the internal components of a computer system.

Tiny wires that transfers binary between components

What is the Control Bus?

Set of wires that carry control signals to and from the processor and components of a computer

What is a Data Bus?

Set of wires that carry data to and from other components

What is an Address Bus?

Set of wires that carry the address of an instruction or data from cpu that’s read/written to the memory unit

What is Simple Instruction Multiple Data

Parallel processing is where a processor carries out a single instruction on multiple data items at the same time - often used by graphic processor

What is Multiple Instruction Multiple Data?

Multiple instructions are carried on multiple data items across several cores

What does Complex Instruction Set Computer do?

Aims to complete the task in a few lines of assembly as possible - this means the processor hardware & circuitry has to be more complicated so it can understand & execute a series of operations

CISC PROPERTIES

When executed, this instruction would load the 2 values into the register, multiply them together & store the result back out

MULT is an exmaple of a COMPLEX instruction

Complex Instructions might take more than one machine/clock cycle to execute

They’re found in desktops, computers & laptops

What does Reduced Instruction Set Computer do?

Aims to use simple instructions that’ll be executed within a single machine/clock cycle

RISC PROPERTIES

More RAM needed to store the assembly instructions. The process seems less efficient. Compiler has to do more work

However, each instruction can be completed in a single clock cycle.

Have fewer transistors

Less complex hardware = No room for general purpose registers & cache

They’re found in low-power portable devices: Smart TVs, thermostats, smart watches, smartphones, tablets, printers

Differences between CISC and RISC

Used in laptops/desktops in x86 processors made by Intel & AMD

Used in smartphones, tablets, embedded systems. Made by ARM

Has more complex hardware

Has more simple hardware

Large num of complex instruction of variable lengths

Smaller number of simple, standard instructions that are fixed length

Great energy requirements

Lower energy requirements

Doesn’t support pipelining

Suports pipelining

Compiler (software) does less work.

Compiler has to do more work

Small code size, Higher cycles per second

Large code size, Lower Cycles per second

Makes more efficent use of Ram

Multiple addressing modes usable

Heavy use of RAM (can cause bottlenecks if RAM is limited)

Fewer addressing modes available

What is a parallel system?

When a computer has multiple cores

Each core works on the same task, to complete it more quickly or each core can work on seperate tasks simultaneously

What do Multi-core Systems have?

It has more than one processing unit in a single processor which can independently process instructions at the same time

What are benefits to using Multi-core Systems?

• Multitasking

  • Each core can work on a different task - this is particularly effective when the user has multiple applications open at the same time

• Background tasks

  • When using a single core processor, a background task like anti-malware scans can slow down the user's other task. A multi-core processor can assign the background task to one core, to reduce the impact on the other task

• Improved responsiveness

  • If a program becomes unresponsive, it won’t slow the user's computer down as much if they're using multi-core as other cores will continue running their task

What are limitations to Multi-core Systems?

Limit on Max Speed: If a program can’t be parallelised, then there’ll be a cap on the max speed a multicore system can give.

Complex code: Harder to write code for parallel processing. Tasks have to be synchronised and data shared correctly.

Limited appliances: Not all tasks can run in parallel and must be executed serially

What is an Input Device?

Peripheral device that allows data to enter a computer system

What is an output device?

Peripheral Device that translates signals from the computer into a human-readable form

They enable the computer to communicate the results of data or commands

Examples of Input Devices

Keyboard, Mouse, Touchscreen, Microphone, Joystick

Examples of Output Devices

Monitor, Printer, Speakers, Headphones, Projector

Properties of ROM

Stores BIOS or firmware used to start computer & initialise the hardware components

Non-volatile: Keeps contents when computer is turned off. Can be read but not written to or modified.

Loads the Operating System

Properties of RAM

Stores data that is currently being used by the computer such as open apps and files.

Volatile: Loses all contents when computer is turned off.

Directly accessible = Processor can access any part of it equally as fast.

What is secondary storage?

A non-volatile storage that is used for the permanent collection of programs, data and instructions.

What do you consider when purchasing a Secondary Storage device?

Cost, Portability, Capacity, Performance needs

Optical Storage

Data is stored as pits and lands

Read by a laser

Stores data by burning pits via laser into surface of a disc

Advantages of Optical Storage

Durable (Especially to environmental phenomena)

Portable

Suitable for sharing music or movies

Disadvantages of Optical Storage

Low storage capacity

Slower read/write speeds

Easily scratched or damaged.

Magnetic Storage

Stores data via polarised metal

Polarity used to represent a 0 or 1

Advantages of Magnetic Storage

High storage capacity

Low cost per gigabyte

Disadvantages of Magnetic Storage

Moving Parts that can wear out overtime

Slower read/write than Flash Memory

Suspect to Physical Damage

Not Portable

Makes Noise

Flash Storage

Stores data in flash memory cells by removing or trapping electron in a floating gate transistor

Advantages of Flash Storage

Fast read/write speeds

No moving parts so less likely to fail due to physical shock

Silent

Disadvantages of Flash Storage

High cost per GB

Wears out after certain num of read/write cycles

What is primary storage?