Computing A-Level

For now it's gonna be for Dec 10th

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 Virtual Storage?

Storing & retrieving data over the internet in the cloud instead of a local storage device

Advantages to Virtual Storage

Data can be accessed from any device at any time as long as there’s internet

Data can easily be shared without the need for media transfer

Storage is considered “limitless” 

Disadvantages to Virtual Storage

Can become quite expensive

If connectivity is poor, access times can be slow

No internet connection = No access to files

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 a multicore processor

A single chip containin 2 or more duplicate CPUs, each can FDE instructions

What is parallel processing?

The processing of program instructions by diving them between multiple processors or proessor cores

How is parallel processing achieved?

Processing Cores carrying out a single job split into tasks, with each task able to execute on any unit.

The main processing core fetches & decodes an instruction & then farms off various parts of the program to other processing units

Advantages of Parallel Processing

Problems which are large and complex can be solved more easily

If a task can be divided into subtasks that can be executed simultaneously, the total execution time can be reduced

Parallel processing allows for better use of computer resources as multi-core or multiple processors can be used more effectively

Disadvantages of Parallel Processing

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

It is more difficult to debug a parallel program than a serial program due to the precise timing of specific events 

Limit to the maximum speed improvement that can be made using parallel processing if a part of the program can’t be parallelised

What does the Operating System do?

Provides the interface between the user & the hardware

Managing the hardware and software resources of a computing system

What does the Application Software do?

User doesn’t interact directly with hardware but they use this such as word processors, spreadsheets or graphic programs to accomplish a given task. OS provides a platform on which that Application Software can run

What does the User Interface do?

Allows the users to interact with the computer & hardware

(GUI, Command Line Interface)

What does Resource Management do?

Manage the computer's resources, including the CPU, memory, disk drives, and printers

They allocate resources to specific tasks and ensure that they are used effectively

What does File Management do?

Manages the storage, retrieval, and manipulation of data files

Security in an Operating System

Operating systems provide various security features such as password-protected system accounts, a firewall, virus scanning and file encryption

Providing Utilities in an Operating System

Utility programs help with system maintenance and security

Some utility programs include: file encryption, file compression, disk defragmentation, system backup, disk cleanup

What is Paging?

Paging is a method of chunking the primary memory into equal-sized blocks

Data stored in memory will lead to the smooth running of applications

When an application is launched, data will be moved from the hard disk into Pages for faster access

As users move between applications, memory is dynamically allocated

Pages will be taken away from applications not in active use and granted to applications that are in active use

What is Segmentation?

Segmentation is a method of chunking memory into blocks that correspond to different types of data needed by an application

Segments are not all the same size; they are sized depending on their allocated data

Segmentation is space-efficient due to only allocating space depending on the amount an application needs

What is Virtual Memory?

If a computer is running low on primary memory, it can make secondary storage act as an 'extension' of the main memory

Virtual memory creates an illusion of a larger memory and enables applications to continue to multitask

However, accessing data in virtual memory is considerably slower compared to RAM

What is an Interrupt?

A signal to the processor that stops its current task and performs a different task temporarily

When an interrupt occurs, the processor suspends the current program execution and transfers control to an interrupt service routine

What is the purpose of an Interrupt?

Real-time Event Handling: hardware errors and signals from input devices e.g. hard disk failure

Device Communication: alerts from external devices e.g. printer jams and network errors

Multitasking: suspending processing in one application so that the user can switch to another

What’s the process of an interrupt?

1. Interrupt Request (IRQ)

   • An external device or software generates an interrupt, signalling the processor to stop its current task

   • The interrupt controller passes this to the interrupt handler for assessment

2. Interrupt Acknowledge

   • The interrupt handler decides if the interrupt needs to be dealt with now or later

   • If yes, the current contents of the processor registers are saved in memory

3. Interrupt Service Routine (ISR) Lookup

   • The processor fetches the corresponding Interrupt Service Routine (ISR) associated with the interrupt type

4. ISR Execution

   • The processor transfers control to the ISR and executes the routine to handle the specific interrupt

5. Interrupt Exit

   • After the ISR completes, the processor restores the content of the registers from step 2

   • The fetch-decode-execute cycle is resumed

What is scheduling?

Deciding which tasks to process, for how long, and in what order is achieved through scheduling algorithms

A CPU is responsible for processing tasks as fast as possible

Different algorithms are used to prioritise and process tasks that need CPU time

What is pre-emptive scheduling?

Allocates the CPU for a specific time quantum to a process

Allows interruption of processes currently being handled

It can result in low-priority processes being neglected if high-priority processes arrive frequently

What is Non-Pre-emptive scheduling?

Once the CPU is allocated to a process, the process holds it until it completes its burst time or switches to a 'waiting' state

A process cannot be interrupted unless it completes or its burst time is reached

If a process with a long burst time is running, shorter processes will be neglected

What is Round-Robin?

Pre-Emptive Scheduling Algorithm

Equally distributing processor time amongst all processes

Each process is given a time quantum to execute

Processes that are ready to be worked on get queued

If a process hasn’t been completed by the end of its time quantum, it will be moved to the back of the queue

Advantages to Round Robin?

All processes get a fair share of the CPU

Good for time-sharing systems

Predictable, as every process gets equal time

Disadvantages to Round Robin

Choosing the right time quantum can be difficult

This can lead to a high turnaround time and waiting time for long processes

What is First Come First Served?

Non-Pre-Emptive Scheduling Algorithm

Prioritises processes that arrive at the queue first

The process currently being worked on will block all other processes until it is complete

All new tasks join the back of the queue

Advantages to First Come First Served

Simple and easy to understand

Fair in the sense that processes are served in the order they arrive

Disadvantages to First Come First Served

This can lead to poor performance if a long process arrives before shorter processes

High-priority tasks wait for their turn in the queue

What is Shortest Job First?

Non-Pre-Emptive Scheduling Algorithm

Where all processes are continuously sorted by burst time from shortest to longest

When new processes arrive on the queue, they are prioritised based on their burst time in the next cycle

Shorter jobs are placed at the front of the priority queue

Longer jobs have lower priority, so they are placed at the back

Advantages to Shortest Job First

Minimises waiting time

Efficient and fast for short processes

Disadvantages to Shortest Job First

Requires knowing the burst time of processes in advance

Long processes can starve if short processes keep arriving

What is Shortest Remaining Time?

Pre-emptive Scheduling Algorithm

Processes with the shortest remaining time are higher priority

Time quantum is set, and if a task doesn’t complete in time, it will be re-queued for further processing

Before the next cycle starts, all processes are inspected and ordered by the shortest remaining time to complete

Advantages for Shortest Remaining Time

Ideal for jobs that have shorter burst times

It is preemptive, so it can be aligned with CPU for best performance (time quantum)

Disadvantages for Shortest Remaining Time

Like SJF, it requires knowing the burst time of processes in advance

High context switching overhead due to preemption

What is Multi Level Feedback Queue

Pre-emptive Scheduling Scheduling Algorithm

Shorter and more critical tasks are processed first

Multiple queues are used so that tasks of equal size are grouped together

All processes will join the highest priority queue but will trickle down to lower priority queues if they exceed the time quantum

Advantages to Multi Level Feedback Queue

Smaller tasks are prioritised

Creates a prioritisation system where similar-sized tasks are queued together

Disadvantages to Multi Level Feedback Queue

More complex than other algorithms

Setting the correct parameters (e.g., number of queues, ageing rules) can be complex

What is a virtual machine?

A virtual machine (VM) is a software-driven computer that runs within a physical machine

It mimics a complete computer system, including virtual CPU, memory, storage, and network interface