Computer Science - Topic 3: Computers

What is the stored program concept?

Both data and program instructions are stored, temporarily in main memory, and then fetched, decoded and executed by the processor

What is memory?

The part of a computer that stores data

What is Von Neumann architecture?

The processor architecture that uses the stored program concept

What are the 5 key elements of Von Neumann Architecture?

Data and instructions are both stored as binary digits
Data and instructions are both stored together in the same RAM
Instructions are fetched from memory one at a time and in order - serially
The processor decodes and executes an instruction, before cycling around to fetch the next instruction
The cycle continues until no more instructions are available

State 5 special registers that Von Neumann Architecture uses for processing

Program Counter
Memory Address Register (MAR)
Memory Data Register (MDR)
Current Instruction Register (CIR)
Accumulator (ACC)

What is the FDE cycle?

The fetch-decode-execute cycle is a method used by the CPU to carry out a single instruction contained in a computer program

Describe the 7 steps in the FDE cycle

The memory address held in the program counter is copied into the MAR
The address in the program counter is then incremented by one
The program counter now holds the address of the next instruction to be fetched
The processor sends a signal containing the address of the instruction to be fetched along the address bus to the computer's memory
The instruction held in that memory address is sent along the data bus to the MDR
The instruction held in the MDR is copied into the CIR
The instruction held in the CIR is decoded and then executed. The results of processing are stored in the ACC
The cycle then returns to step one

Describe the additional step in the FDE cycle if the instruction is to transfer data held in the ACC back to RAM

The intended memory address is copied into the MAR
The data to be transferred is copied into the MDR and then transferred to the specified address using the address bus and data bus

Describe the additional step in the FDE cycle if the instruction is for the program to jump to a different place in the program

The memory address of the new next instruction to be fetched is copied into the program counter
The process then restarts at step one

What is hardware?

The physical components of a computer

What is software?

The programs and other operating information used by a computer.

What are the two main functions of the CPU?

To process data and instructions
To control the rest of the computer system

Central Processing Unit

The brain of the computer that processes program instructions

State 6 components found in the CPU

Control Unit (CU)
Arithmetic Logic Unit (ALU)
Registers
Cache
Buses
Clock

What are 3 functions the CU provides?

Fetches, decodes and manages the execution of the instructions
Issues control signals that control hardware
Moves data around the system

What is the purpose of the ALU?

Performs calculations and makes logical decisions (performs logical and arithmetic operations)

What are registers?

Small amounts of high speed memory contained within the CPU

How are registers used?

They are used by the processor to store small amounts of data that are needed during processing

State 3 pieces of data that might be stored in registers

The address of the next instruction to be executed
The current instruction being decoded
The results of calculations

What is cache?

A small amount of high speed memory built directly within the processor

What is the purpose of cache and why is this useful?

It is used to temporarily hold data and instructions that the processor is likely to reuse

Why is cache useful?

It allows for faster processing as the processor does not have to fetch data from the RAM

What is the purpose of the clock in the CPU?

The clock sends out a regular electrical pulse which synchronises all the components

What clock speed and its unit?

The frequency of the pulses send out by the clock; the speed of the CPU
Indicates the number of FDE cycles that can run each second
Measured in Hz

What is a bus?

A high speed internal connection

What are buses used for?

Used to send control signals and data between the processor and other components

State the three types of buses

Address Bus
Data Bus
Control Bus

Describe the function of the address bus

It carries memory addresses from the processor to other components such as RAM and input/output devices

Describe the function of the data bus

It carries the actual data between the processor and other components

Describe the function of the control bus

It carries control signals from the processor to other components

How does clock speed affect the performance of the CPU?

The more pulses per second, the more FDE cycles that can be performed and the greater the number of instructions that are processed in a given space of time

State 3 factors that affect the performance of processors

Clock Speed
Cache Size
Number of Cores

How does cache size affect the performance of the CPU?

The bigger its cache, the less time a processor has to wait for instructions to be fetched

How does the number of cores affect the performance of the CPU?

The more cores a CPU has, the greater the number of instructions it can process in a given space of time

What is a core?

A processing unit within a CPU that is capable of fetching, decoding and executing its own instructions

State the 3 types of memory

Read Only Memory (ROM)
Random Access Memory (RAM)
Cache memory

State 4 properties of main memory

Volatile
Instructions must be loaded into here before the CPU can fetch them
Stores open operating files, application, documents
Instructions can be fetched from here to the CPU for execution

State 3 properties of secondary storage

Non volatile
Not accessible by the CPU
Store documents and installed applications

Define volatile and non-volatile

When power is switched off, volatile memory no longer retains any data
Non-volatile memory stores data even when not powered

State a property of ROM

ROM can be read from, but not written to

What does ROM store?

Instructions and data that are needed for the computer to run e.g. BIOS
These instructions and data are usually programmed by the computer's manufacturer and cannot be overwritten

Is ROM and example of main memory or secondary storage?

Secondary storage

State 3 properties of RAM

RAM can be read from and written to
The contents of RAM can be changed at any time, simply by overwriting them with other data and/or instructions
RAM can also be upgraded easily

Is RAM and example of main memory or secondary storage?

Main memory

What does RAM store?

Data and instructions that are currently in use. This includes:
Operating system and any open documents and programs that are running

Why is it beneficial to have more RAM?

The more RAM a computer has, the more data and programs it can hold simultaneously

State 3 types of secondary storage devices and an example of each

Magnetic Media e.g. hard disk drive
Optical Media e.g. CD
Solid State e.g. SSD

Describe how is data stored on magnetic media?

Magnetic media uses magnetic fields to magnetise tiny individual sections of a metal spinning disk
Each tiny section represents one bit.
A magnetised section represents a binary 1 and a demagnetised section represents a binary 0

How is data read from magnetic media?

As the disk is spinning, a read/write head moves across its surface.
To read data, the head makes a note of whether the section is magnetised or not

How is data written on magnetic media?

As the disk is spinning, a read/write head moves across its surface.
To write data, the head magnetises or demagnetises a section of the disk that is spinning under it.

State 3 advantages of magnetic media

Fairly cheap
High in capacity
Durable

State 5 disadvantages of magnetic media

They are susceptible to damage if dropped - have moving parts
They are vulnerable to magnetic fields- a strong magnet might erase the data the device holds
Can be noisy
Slow(ish)
Requires more power

Describe how data is stored on optical media

The disc surface is divided into tracks, with each track containing many flat areas and hollows
The flat areas are known as lands and the hollows as pits

How is data read from optical media?

Optical devices use a laser to scan the surface of a spinning disc
When the laser shines on the disc surface, land reflects the light back, whereas pits scatter the laser beam.
A sensor looks for the reflected light.
Reflected light (land) represents a binary 1
No reflection (pits) represents a binary '0'

State 3 types of optical media

ROM media - pre written, cannot be changed
Read (R) media - comes black, can be written to once
Read/Write (RW) media - can be written to more than once

What is flash memory?

A type of non-volatile memory that can be read from and written to

State 4 advantages of solid state drives

Faster access times
Durable - as they have no moving parts
Portable - small and require little power
Make no noise

State 2 disadvantages of solid state devices

Expensive
Smaller in capacity

How is data stored on solid state devices?

In each memory cell in a SSD, electrons are being stored on different layers on charge traps
Each of these traps give a binary value depending the number of electrons
These electrons stay there for decades which is how they are stored

What happens when data is released from solid state devices?

If the data is deleted, these electrons are let go and removed from the trap

What is cloud storage?

Storing data at a remote location online
When files and data are sent to the cloud, they are being sent to a server connected to the internet

Describe 3 advantages of cloud storage

Data can easily be accessed from anywhere with an internet connection
The business running the cloud storage service manages backups and security
Additional storage can be added easily without having to invest in additional hardware locally

Describe 3 disadvantages of cloud storage

Hackers could take advantage of the fact that data can be accessed from anywhere with an internet connection
If the internet connection fails, files cannot be accessed
It relies on other people correctly storing and backing up data

State 6 factors affecting the choice of secondary storage

Cost
Capacity
Speed of Access
Portability
Durability
Reliability

What is a general purpose computer?

A computer designed to be able to carry out a wide range of instructions

Give 8 examples of things that general purpose computers can do

Access the internet
Browse the world wide web (WWW)
Use word processing software
Play games
Communicate via email and social media
Design and build web pages
Store and retrieve data
Play videos and music

What is an application?

A software program that allows a user to perform a specific task

Give 6 examples of general purpose computers

Desktops
Laptops
Tablets
Smartphones
Game Consoles
Media systems in cars

Describe how data flows in general purpose computers

Data is input, processed, then either output or sent to secondary storage

What is an embedded system?

A computer system that is self contained and has a specific purpose and it cannot be altered to perform a different task

What is the purpose of and embedded system?

To control the device and to allow a user to interact with it

Give 7 examples of embedded systems

Central heating systems
Engine management systems in vehicles
Domestic appliances, such as dishwashers and TVs
Digital watches
Electronic calculators
GPS systems
Fitness trackers

What is included in an embedded system?

Input Devices
Output Devices

Are embedded system programmable?

Embedded devices are not usually programmable by a user - the programming is usually done beforehand by the manufacturer.
However, it is often possible to upgrade the software on an embedded device

Describe 3 advantages of embedded systems

Their limited number of functions means they are cheaper to design and build
They tend to require less power. Some devices run from batteries
They do not need much processing power. They can be built using cheaper, less powerful processors

What is system software?

A type of computer program that runs a computer's hardware programs and application programs

What is an operating system?

A collection or suite of programs that manages and controls the computer

What are the 4 main functions performed by an operating system?

File management
Process management
Hardware management
User interface management

State 5 common operating systems

Microsoft Windows
macOS
Linux
Android
iOS

Describe file management

The operating system controls where each piece of data is stored in RAM and also stops applications accessing the memory from another application
When memory becomes full it uses part of the hard drive as virtual memory

How does the file management system help user find and manage data (files)?

The file management system does this by creating a virtual file structure of the physical components. It is a hierarchical structure starting with the root directory

State 7 pieces of metadata assigned to a file by the file management system

Date created
Date modified
Last date accessed
Last backup
File creator user ID
Access permissions
File size

State 6 functions that users can perform on files

Create
Name/rename
Save
Copy
Move
Delete

Describe process management

Instructions are executed by the CPU, but the operating system controls which instructions are fed in to be executed. The operating system makes sure that the time of the processor is fairly shared by using suitable scheduling algorithms

What is multiprocessing?

The ability to use multiple processing devices to allocate use of main memory and CPU

State 3 similarities between the several different scheduling algorithms

They all use a queue to hold waiting processes
They all use the clock cycle to time the execution of processes
They all switch between processes

State 3 different scheduling algorithms

First come, first serve (First in, first out)
Round robin
Priority based (shortest job first)

Describe first come first serve algorithm (first in first out)

It is based on a first in, first out (FIFO) queue, where the processes arrive, are executed, then leave the queue in order.

State an advantage of first come first serve algorithm

It is simple and easy to understand

State 2 disadvantages of first come first serve scheduling algorithm

The process with less execution time suffers
Results in lower CPU utilisation

Describe round robin scheduling algorithm

Each process is given a fixed amount of time, then it is switched out and sent to the back of the queue. The next process then has its turn, and so on

State 2 advantages of round robin scheduling algorithm

Every process gets an equal share of the CPU
There is no starvation of jobs

State 2 disadvantages of round robin scheduling algorithm

The average waiting time is long
Priority cannot be assigned to any process

Describe Priority based scheduling algorithm

Each process is given a priority based on several things, including how much time it needs and how much memory it will use
A process with a higher priority will be executed before a process with a lower priority

State 2 advantages of shortest job first scheduling algorithm

Shortest jobs are favoured
It gives the minimum average waiting time for a given set of processes

State a disadvantage of shortest job first scheduling algorithm

Longer jobs may be ignored if shorter processes keep coming

Describe Memory management (Hardware management)

When you save data to your hard drive the operating system will find an available section of storage and send the data to be written to that location. The fact that you can have folders inside folders with files in them is also managed by the OS

Explain how resources are managed by operating systems

The operating system allocates the required resource to the processes that are currently running
The operating system ensures that any data and/or instructions in main memory do not conflict with each other, and allocates the memory that the process requires

Explain how input/output devices are managed by operating systems

Data has to be transferred between input devices, the CPU and main memory, and output devices
Device drivers will need to be installed for each new device. Device drivers allow devices created by different manufacturers to communicate

Describe peripheral management (user interface management)

The operating system captures data from input devices and provides data to the output devices to display to the user. It does this by having the correct device drivers installed
The operating system uses abstraction to hide complexity from the user