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Learn about why testing is needed, types of testing, types of test data and types of error. Based on the J277 OCR GCSE Computer Science specification (first taught from 2020 onwards). 3.2: Testing Exam Board: OCR Specification: J277 The main purpose of testing is to ensure that a program works correctly no matter what input has been entered by the user. Other reasons to test a program include ensuring the user requirements have been met , errors have been removed and the program doesn't crash while running . Types of Testing Iterative Testing Iterative testing takes place during program development . The programmer develops a module , tests it and repeats this process until the module works as expected . Final Testing Final testing, also known as terminal testing , takes place after development and before the program is released to the end user. This testing takes place once all modules have been individually tested to ensure the whole program works as originally expected. Programming Errors Syntax Error Logical Error A syntax error is a mistake in the grammatical rules of the programming language , such as an incorrect spelling of a command word. A syntax error will prevent the program from being compiled and executed . Examples: Incorrect Spelling: pront ( "hello" ) Incorrect punctuation: print ( "hello" ( A logic error is a mistake made by the programmer - the program runs without crashing but will display the wrong output . Examples: Incorrect calculation: total = num1 - num2 print (total) Incorrect variable printed: age = 16 name = "Steve" print ( "Nice to meet you" , age) Test Data Test data is used to test whether a program is functioning correctly . It should cover a range of possible and incorrect inputs , each designed to prove a program works or to highlight any flaws . Four types of test data are: Q uesto's Q uestions 3.2 - Testing: 1. Give 3 reasons why programs are tested . [ 3 ] 2. What is the difference between iterative and final testing ? [ 2 ] 3a. What is a syntax error ? Give an example . [ 2 ] 3b. What is a logical error ? Give an example . [ 2 ] 4. State and describe the four types of test data . [ 6 ] Normal data - Sensible data that the program should accept and be able to process . Boundary data - Data at the extreme boundary of any data ranges. Invalid data - Data of the correct data type that does not meet the validation rules (e.g. outside of the range). It should not be accepted . Erroneous data - Data of the wrong data type that the program cannot process and should not accept . 3.1 - Defensive Design Theory Topics 4.1 - Boolean Logic

Learn about different forms of cyber attacks online including malware, SQL injection, DoS attacks and social engineering. Based on the J277 OCR GCSE Computer Science specification (first taught from 2020 onwards). 4.1: Security Threats Exam Board: OCR Specification: J277 Watch on YouTube : Malware Social Engineering, Brute-Force & DoS Interception & SQL Injection Types of Malware Malware is any type of harmful program that seeks to damage or gain unauthorised access to your computer system. Virus A virus can replicate itself and spread from system to system by attaching itself to infected files . A virus is only activated when opened by a human . Once activated, a virus can change data or corrupt a system so that it stops working . Worm A worm can replicate itself and spread from system to system by finding weaknesses in software . A worm does not need an infected file or human interaction to spread. A worm can spread very quickly across a network once it has infiltrated it. Trojan A trojan is a harmful program that looks like legitimate software so users are tricked into installing it . A trojan secretly gives the attacker backdoor access to the system . Trojans do not self replicate or infect other files. Spyware Spyware secretly records the activities of a user on a computer. The main aim of spyware is to record usernames, passwords and credit card information . All recorded information is secretly passed back to the attacker to use. Keylogger A keylogger secretly records the key presses of a user on a computer. Data is stored or sent back to the attacker. The main aim of a keylogger is to record usernames, passwords and credit card information . Keyloggers can be downloaded or plugged into the USB port . Ransomware Ransomware locks files on a computer system using encryption so that a user can no longer access them. The attacker demands money from the victim to decrypt (unlock) the data . ? ? ? ? Attackers usually use digital currencies like bitcoin which makes it hard to trace them. SQL Injection SQL ( Structured Query Language ) is a programming language used for manipulating data in databases . A SQL injection is when a malicious SQL query (command) is entered into a data input box on a website. If the website is insecure then the SQL query can trick the website into giving unauthorised access to the website’s database . An SQL injection can be used to view and edit the contents of a database or even gain administrator privileges . DoS Attack A DoS (Denial of Service ) attack is when a computer repeatedly sends requests to a server to overload the system . A server overload will slow the system and may take websites offline temporarily. A DDoS (Distributed Denial of Service ) attack is a coordinated attack using a botnet of infected systems to overload a server with requests . A botnet is a large group of devices controlled and used maliciously by an attacker. Brute-Force Attack Every possible combination is tested in order from start to finish . This is not a quick method but it should break the password eventually and can be sped up if multiple computer systems are used at the same time. Social Engineering Social engineering means to trick others into revealing their personal data by posing as a trusted source . For example, impersonating an IT technician of a school via email and asking for a student's username and password . Data Interception This is when data packets on a network are intercepted by a third party (e.g. a hacker) and copied to a different location than the intended destination. Software called packet sniffers are used to intercept and analyse data packets. Q uesto's Q uestions 4.1 - Security Threats: 1. What is malware ? [ 2 ] 2a. Describe three characteristics of a virus . [3 ] 2b. Describe three characteristics of a worm . [3 ] 2c. What is a trojan ? [ 3 ] 2d. Describe how spyware and keyloggers work. [ 4 ] 2e. Explain how ransomware works and why it is difficult to trace attackers . [ 3 ] 2f. In your opinion, which malware do you think is the most dangerous and why ? [ 2 ] 3. Describe what an SQL injection is and how an attacker would use it. [ 3 ] 4a. Describe what a DoS attack is and its impact . [2 ] 4b. Describe how a DDoS attack is different to a DoS attack . [2 ] 5a. Describe a brute-force attack. [ 2 ] 5b. Describe social engineering and give an example of when it might be used . [2 ] 5c. Describe interception . [2 ] 3.2b - Protocols & Layers Theory Topics 4.2 - Preventing Vulnerabilities

Learn about the three main types of secondary storage - magnetic, optical and solid-state. Also, learn about the characteristics of secondary storage media including reliability and durability. Based on the J277 OCR GCSE Computer Science specification (first taught from 2020 onwards). 2.2: Secondary Storage Exam Board: OCR Specification: J277 Watch on YouTube : Secondary Storage Six Characteristics Magnetic Storage Optical Storage Solid State Storage Secondary storage is non-volatile storage used to save and store data that can be accessed repeatedly. Secondary storage is not directly embedded on the motherboard (and possibly even external ) and therefore further away from the CPU so it is slower to access then primary storage . Storage Characteristics you should know: CAPACITY : The maximum amount of data that can be stored on the device. DURABILITY : The strength of the device, to last without breaking . PORTABILITY : How easy it is to carry the device around . ACCESS SPEED : How quickly data on the device can be read or edited . COST : The average price it costs to purchase the storage device. RELIABILITY : The likelihood of the device continuing to perform well over time . Magnetic Storage A magnetic hard disk drive (HDD ) is the most common form of secondary storage within desktop computers. A read/write head moves nanometres above the disk platter and uses the magnetic field of the platter to read or edit data. An obsolete (no longer used) type of magnetic storage is a floppy disk but these have been replaced by solid state devices such as USB sticks which are much faster and have a much higher capacity. Another type of magnetic storage that is still used is magnetic tape . Magnetic tape has a high storage capacity but data has to be accessed in order (serial access ) so it is generally only used by companies to back up or archive large amounts of data . Magnetic Storage Characteristics (Hard Disk Drive): ✓ - Large CAPACITY and cheaper COST per gigabyte than solid state . ✓ - Modern external HDDs are small and well protected so they are DURABLE and PORTABLE , however because of the moving parts, they should not be moved when powered on because it can damage the device. X - Slower ACCESS SPEED than solid state but faster than optical storage . Optical Storage Optical storage uses a laser to project beams of light onto a spinning disc, allowing it to read data from a CD , DVD or Blu-Ray . This makes optical storage the slowest of the four types of secondary storage. Disc drives are traditionally internal but external disc drives can be bought for devices like laptops. Magnetic Disks are spelled with a k and Optical Discs have a c. Optical Storage Characteristics: X - Low CAPACITY : 700 MB (CD ), 4.7 GB (DVD ), 25 GB (Blu-ray ). X - Not DURABLE because discs are very fragile and can break or scratch easily. ✓ - Discs are thin and very PORTABLE . Also very cheap to buy in bulk. X - Optical discs have the Slowest ACCESS SPEED . Solid State Storage There are no moving parts in solid state storage. SSD s (Solid State Drives ) are replacing magnetic HDDs (Hard DIsk Drives) in modern computers and video game consoles because they are generally quieter , faster and use less power . A USB flash drive ( USB stick ) is another type of solid state storage that is used to transport files easily because of its small size. Memory cards , like the SD card in a digital camera or a Micro SD card in a smartphone , are another example of solid state storage. Solid State Characteristics: X - More expensive COST per gigabyte than magnetic . ✓ - Usually DURABLE but cheap USB sticks can snap or break . ✓ - The small size of USB sticks and memory cards mean they are very PORTABLE and can fit easily in a bag or pocket. ✓ - Solid State storage have a high CAPACITY and the fastest ACCESS SPEED because they contain no moving parts . Q uesto's Q uestions 2.2 - Secondary Storage: 1. Rank magnetic , optical and solid-state storage in terms of capacity , durability , portability , speed and cost . For example, magnetic has the highest capacity , then solid-state, then optical. This could be completed in a table . [15 ] 2. Justify which secondary storage should be used in each scenario and why it is the most appropriate: a. Sending videos and pictures to family in Australia through the post . [ 2 ] b. Storing a presentation to take into school . [ 2 ] c. Storing project files with other members of a group to work on together . [ 2 ] d. Backing up an old computer with thousands of files to a storage device. [ 2 ] 2.1 - Primary Storage Theory Topics 2.3 - Data Units

Learn about the tools of an integrated development environment (IDE) including the editor, error diagnostics and run-time environment. Based on the J277 OCR GCSE Computer Science specification (first taught from 2020 onwards). Exam Board: OCR Specification: J277 5.2: Integrated Development Environment An IDE (Integrated Development Environment ) provides programmers with the following facilities (tools ) to help create programs : Editor The editor is software that allows a programmer to enter and edit source code . Editor features may include: Automatic formatting (e.g. automatic indentation). Automatic line numbering (this helps to identify exactly where an error has occurred). Automatic colour coding (e.g. Python IDLE turns loop commands orange and print commands purple). Statement completion (e.g. offering to auto-complete a command as the user is typing.) Error Diagnostics & Debugger Break point The programmer selects a specific line and the program displays the variable values at that point . The code can then be executed one line at a time to find exactly where the error occurs. This process is called single-stepping . Variable Watch / Watch Window cost Displays the current value of a selected variable . A variable can be watched line-by-line to see how the value changes . Trace Logs the values of variables and outputs of the program a s the code is executed line by line . Both tools are used to display information about an error when it occurs, such as the line it occurred on and the error type (e.g. syntax ). These tools may also suggest solutions to help the programmer to find and fix the error . Compilers & Interpreters Both tools convert the source code written by a programmer into machine code to be executed by the CPU. A compiler converts the entire source code into executable machine code at once . After compilation, the program can be run again without having to recompile each time. An interpreter converts source code into machine code line by line . An interpreter must reinterpret the code each time the program is required to run . See 5.1 for both types of translators. A runtime environment allows a program to run on a computer system. It checks for runtime errors and allows users to test the program . A runtime error occurs as the program is being executed , such as dividing a number by zero . A commonly used example is the Java Runtime Environment . This allows programmers to design a program on one platform ( using the programming language Java ) which allows the finished program to then be run on many others systems . A runtime environment enables the tools above such as a trace and breakpoint to be used. Run Time Environment Q uesto's Q uestions 5.2 - Integrated Development Environment: 1. Describe the purpose of each type of IDE tool : a. Editor b. Interpreter c. Compiler d. Error Diagnostics / Debugger e. Break point f. Variable Watch / Watch Window g. Trace h. Runtime Environment [ 2 each ] 5.1 - Languages & Translators Theory Topics

Learn about how to perform binary addition and binary shifts. Based on the J277 OCR GCSE Computer Science specification (first taught from 2020 onwards). 2.4b: Binary Addition & Shifts Exam Board: OCR Specification: J277 Watch on YouTube : Binary Addition Overflow Errors Binary Shifts Issues with Shifts Binary Addition Binary addition is a method of adding binary values without having to convert them into denary. How to add binary numbers: What is an overflow error? An overflow error occurs when a binary value is too large to be stored in the bits available . With a byte (8 bits ) the largest number that can be held is 255 . Therefore any sum of two binary numbers that is greater than 255 will result in an overflow error as it is too large to be held in 8 bits . What is binary shift? Binary shift is used to multiply and divide binary numbers . The effect of shifting left is to multiply a binary number. The effect is doubled by each place that is shifted . x The effect of shifting right is to divide a binary number. ÷ Shifting by 1 has an effect of 2 . Shifting by 2 has an effect of 4 . Shifting by 3 has an effect of 8 . For example, shifting left by 2 places has an effect of multiplying by 4 . Another example: Shifting right by 3 places has an effect of diving by 8 . How to shift a binary number: Binary Shifts Watch on YouTube Watch on YouTube Q uesto's Q uestions Binary Addition: 1. Explain what an overflow error is. [ 2 ] 2. Add together the following binary values. If an overflow error occurs you must state one has occurred. a. 010110012 and 010001012 [2 ] b. 110110112 and 010111012 [2 ] c. 001101102 and 011010112 [2 ] d. 110110112 and 010101112 [2 ] e. 011011012 and 110101102 [2 ] Binary Shifts: 1a. Draw a diagram to show the effect of multiplying and dividing a binary number . [2 ] 1b. Draw a diagram or table to show the effect a shift has for each place from 1 to 4 . For example, a shift of 1 place has an effect of 2. [4 ] 2. State the effect of the following shifts: a. Shift right by 2 places. b. Shift left by 1 place. c. Shift left 3 places. d. Shift right by 4 places. [ 1 each ] 3. Shift the following binary numbers and state the effect of the shift: a. 10101011 : Shift left by 2 places. b. 11101100 : Shift right by 3 places. c. 00001011 : Shift right by 2 places. d. 01101110 : Shift left by 1 place. [ 2 each ] 2.4a - Number Systems Theory Topics 2.4c - Character Storage

Learn about network devices such as a switch, router, modem and NIC. Also learn about internet terms and services including DNS and the Cloud. Based on the J277 OCR GCSE Computer Science specification (first taught from 2020 onwards). 3.1b: Network Hardware + The Internet Exam Board: OCR Specification: J277 Watch on YouTube : The Internet Network Hardware DNS Servers The Cloud Network Devices When sending data across a network, files are broken down into smaller parts called data packets . Whole files are too large to transfer as one unit so data packets allow data to be transferred across a network quickly . Each packet of data is redirected by routers across networks until it arrives at its destination. Data packets may split up and use alternative routes to reach the destination address. When all the packets have arrived at the destination address the data is reassembled back into the original file. Wireless Access Point A Wireless Access Point provides a link between wireless and wired networks . It creates a wireless local area network that allows WiFi-enabled devices to connect to a wired network. Examples of a wireless access point in a public space could be a WiFi or Bluetooth hotspot , for example a WiFi hotspot in a coffee shop or airport to provide access to the internet. A wireless access point may be a separate device or built into another device such as a router. Router Routers are used to transfer data packets between networks . Routers receive data packets and use the IP address in the packet header to determine the best route to transmit the data. Data is transferred from router to router across the internet towards the destination. A router stores the IP address of each computer connected to it on the network and uses a list called a routing table to calculate the quickest and shortest route to transfer data. Switch A switch is used to connect devices together on a LAN . It receives data packets from a connected node, reads the destination address in the packet header and forwards the data directly to its destination. A switch will generate a list of the MAC addresses of all devices connected to it when it receives data , and must scan for a matching destination address before sending. An alternative to a switch is a hub but a hub is slower and less secure as it forwards a copy of received data to all connected nodes . Network Interface Controller / Card A Network Interface Controller (NIC ) commonly also known as a Network Interface Card is an internal piece of hardware that is required for the computer to connect to a network . The card includes a MAC address which is used when sending data across a LAN . An ethernet cable is plugged into the network card to allow data to be exchanged between the device and a network. A NIC used to be a separate expansion card but is now typically embedded on the motherboar d . Transmission Media Although not technically a device, the communication channel along which data is transferred will affect performance . Three common types of transmission media include: Ethernet cables - used typically on a LAN to transfer data between nodes and hardware such as switches. Examples include Cat5e and Cat6. Fibre Optic cables - very fast but more expensive and fragile cables typically used to send data quickly along a WAN . Data is sent as pulses of light . Coaxial cables - older , slower , copper cables that are not used as much in modern times as they can be affected by electromagnetic interference . The Internet The internet is a global network of interconnected networks . The world wide web (WWW ) is not the same as the internet. It is a way of accessing information , using protocols such as HTTPS to view web pages . Servers provide services on the internet , such as a web server which responds to the web browser (client) request to display a web page . The web server processes the client request to prepare the web page and return it so the web browser can display it to the user . A website must be hosted (stored) on a web server so that it can be accessed by others using the internet . A unique domain name (e.g. csnewbs.com) must be registered with a domain registrar – this is a company that checks the name is valid and not already taken . What is the Internet? DNS Servers A DNS ( Domain Name System ) server stores a list of domain names and a list of corresponding IP addresses where the website is stored. The first thing to understand is that every web page has a domain name that is easy for humans to remember and type in (such as www.csnewbs.com ) as well as a related IP address (such as 65.14.202.32) which is a unique address for the device that the web page is stored on. The steps taken to display a web page: 1. A domain name is typed into the address bar of a browser . 2. A query is sent to the local DNS server for the corresponding IP address of the domain name . www.facebook.com 3. The local DNS server will check if it holds an IP address corresponding to that domain name. If it does it passes the IP address to your browser . 66.220.144.0 4. The browser then connects to the IP address of the server and accesses the web site . If the local DNS server does not hold the IP address then the query is passed to another DNS server at a higher level until the IP address is resolved. If the IP address is found, the address is passed on to DNS servers lower in the hierarchy until it is passed to your local DNS server and then to your browser. Cloud Storage The cloud refers to networks of servers accessed on the internet . Cloud computing is an example of remote service provision . Cloud servers can have different purposes such as running applications , remote processing and storing data . When you store data in 'the cloud', using services such as Google Drive or Dropbox, your data is stored on large servers owned by the hosting company . The hosting company (such as Google) is responsible for keeping the servers running and making your data accessible on the internet . Cloud storage is very convenient as it allows people to work on a file at the same time and it can be accessed from different devices. However, if the internet connection fails , or the servers are attacked then the data could become inaccessible . Cloud Storage Characteristics: ✓ - Huge CAPACITY and you can upgrade your subscription if you need more storage. ✓ / X - Cloud storage is difficult to rank in terms of PORTABILITY , DURABILITY and ACCESS SPEED because it depends on your internet connection. A fast connection would mean that cloud storage is very portable (can be accessed on a smartphone or tablet) but a poor connection would make access difficult . ✓ - Cloud storage is typically free for a certain amount of storage. Users can then buy a subscription to cover their needs - Dropbox allows 2 GB for free or 2 TB for £9.99 a month. Q uesto's Q uestions 3.1b - Network Hardware & Internet: 1a. Explain how a switch works. [ 2 ] 1b. Describe the purpose of a router . [ 2 ] 1c. State what WAP stands for and why it is used . [ 2 ] 1d. State what NIC stands for and why it is required . [ 2 ] 1e. State the differences between the three main types of transmission media . [ 3 ] 2a. State what the internet is and how it is different to the world wide web . [ 2 ] 2b. What is web hosting ? [ 2 ] 3a. What is a DNS server ? [ 2 ] 3b. Describe, using a mix of text and icons / images , how a DNS server is used to display a web page . [5 ] 3c. Describe how a DNS server searches for an IP address if it is not found on the local DNS server . [ 2 ] 4a. Describe what cloud computing is. [ 2 ] 4b. State two advantages and two disadvantages of the cloud . [ 4 ] 3.1a - Network Types & Performance Theory Topics 3.2a - Wired & Wireless Networks

Based on the 2015 OCR Computer Science A-Level specification. Exam Board: OCR 2.1: Operating Systems Specification: A-Level 2015 An instruction set is a list of all the instructions that a CPU can process as part of the FDE cycle . CPUs can have different sets of instructions that they can perform based on their function. The two most common instruction sets are the simpler RISC (Reduced Instruction Set Computer ) and more complicated CISC (Complex Instruction Set Computer ). Instruction Sets This page is still being updated. Graphical Processing Unit What is cache memory? Cache memory is temporary storage for frequently accessed data . Cache memory is very quick to access because it is closer to the CPU than other types of memory like RAM . Multicore & Parallel Systems What is cache memory? Cache memory is temporary storage for frequently accessed data . Cache memory is very quick to access because it is closer to the CPU than other types of memory like RAM . Multicore & Parallel Systems What is cache memory? Cache memory is temporary storage for frequently accessed data . Cache memory is very quick to access because it is closer to the CPU than other types of memory like RAM . Q uesto's Q uestions 2.1 - Operating Systems: 1. What is cache memory ? [ 2 ] 1.3b - Memory & Storage Theory Topics 2.2a - Applications & Utilities

Learn about how images are represented in a computer system, including file size, resolution, colour depth and metadata. Based on the J277 OCR GCSE Computer Science specification (first taught from 2020 onwards). 2.4d: Image Storage Exam Board: OCR Specification: J277 Watch on YouTube : Pixels & Resolution Colour Depth & Metadata Image File Size Bitmap Images Bitmap images are made of pixels - single-colour squares - arranged on a grid . Each pixel is assigned a binary value which represents the colour of that pixel. The quality of a bitmap image depends on the total amount of pixels , this is known at the image resolution . Because it is made of pixels, scaling a bitmap image up will result in a visible loss of quality . Most images on computers are bitmaps, such as photos and screenshots . Vector Images Vector images are drawn by the computer following precise mathematical instructions to create lines and objects . Vectors are usually smaller in file size compared to bitmaps because each pixel in a bitmap is stored as an individual binary value. Vectors can be scaled up without any loss of quality and are typically used for logos and animations . How to Calculate the File Size of a Bitmap File Size = Resolution x Colour Depth The resolution of an image is the width in pixels multiplied by the height in pixels. The colour depth (also known as bit depth ) is the number of bits that are used to represent each pixel's colour . 1 bit represents 2 colours (0 or 1 / black or white). 2 bits will allow for 4 colours, 3 bits for 8 colours, 4 for 16 etc. A colour depth of 1 byte (8 bits ) allows for 256 different colours . Remember you must multiply the colour depth , not the number of available colours (e.g. 8 not 256). The RGB (Red , Green , Blue ) colour model uses 3 bytes (a byte of 256 red shades , a byte of 256 green shades and a byte of 256 blue shades ) that together can represent 16.7 million different colours. Example Height = 6 bits Resolution = height x width Resolution = 8 x 6 = 48 bits -------------------------- Colour Depth = 1 bit (only 2 colours) -------------------------- File Size = Resolution x Colour Depth File Size = 48 x 1 = 48 bits File Size in bytes = 48 ÷ 8 = 6 bytes File Size in kilobytes = 6 ÷ 1000 = 0.00 6 kilobytes Width = 8 bits Look carefully at the exam question to see if the examiner is expecting the answer in bits, bytes or kilobytes . Always calculate the file size in bits first then: Divide the file size in bits by 8 to convert to bytes . Divide the file size in bytes by 1000 to convert to kilobytes . Metadata for Images Metadata is additional data about a file . Common image metadata includes: Height and w idth in pixels Colour depth Resolution Geolocation Date created Last edited File type Author details Metadata is important, For example, the dimensions must be known so the image can be displayed correctly . Metadata for a picture taken on a smartphone: width in pixels, e.g. 720 height in pixels, e.g. 480 Q uesto's Q uestions 2.4d - Image Storage: 1. Describe three ways that bitmap and vector images are different. [ 6 ] 2. Define the terms image resolution and colour depth . [2 ] 3. How many colours can be represented with a colour depth of... a. 1 bit [ 1 ] b . 5 bits [ 1 ] c. 1 byte [ 1 ] 4. How is the file size of an image calculated? [2 ] 5a. An image file has a width of 10 pixels , a height of 8 pixels and a colour depth of 2 . What is the file size in bytes ? [3 ] 5b. An image file has a width of 120 pixels , a height of 120 pixels and a colour depth of 1 . What is the file size in kilobytes ? [3 ] 5c. An image file has a width of 32 pixels , a height of 21 pixels and a colour depth of 1 . What is the file size in bytes ? [3 ] 6. State what is meant by metadata and give three examples of metadata for a graphics file. [ 3 ] 2.4c - Character Storage Theory Topics 2.4e - Sound Storage

Learn about how to convert between the denary (decimal), binary and hexadecimal number systems. Based on the J277 OCR GCSE Computer Science specification (first taught from 2020 onwards). Exam Board: OCR 2.4a: Number Systems Specification: J277 Watch on YouTube : Binary and Denary Hexadecimal Number System Ranges Binary to Denary Denary to Binary Binary to Hexadecimal Hexadecimal to Binary Denary to Hexadecimal Hexadecimal to Denary What is binary? By now you should know that computer systems process data and communicate entirely in binary . Topic 2.3 explained different binary storage units such as bits (a single 0 or 1), nibbles (4 bits) and bytes (8 bits). Binary is a base 2 number system. This means that it only has 2 possible values - 0 or 1 . What is denary? Denary (also known as decimal ) is the number system that you've been using since primary school. Denary is a base 10 number system. This means that it has 10 possible values - 0, 1, 2, 3, 4, 5, 6, 7, 8, and 9 . Binary & Denary Convert from binary to denary: Convert from denary to binary: Hexadecimal What is hexadecimal? Hexadecimal is a base 16 number system. This means that it has 16 possible values - 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E and F . Hexadecimal is used as a shorthand for binary because it uses fewer characters to write the same value . This makes hexadecimal less prone to errors when reading or writing it , compared to binary. For example, 100111101011 in binary is 9EB in hexadecimal. Hexadecimal only uses single-character values. Double-digit numbers are converted into letters - use the table on the right to help you understand. Binary to hexadecimal: Hexadecimal to binary: Converting from denary to hexadecimal / hexadecimal to denary To convert from denary to hexadecimal or from hexadecimal to denary , it is easiest to convert to binary first . However, it is possible to convert directly from denary to hexadecimal or directly from hexadecimal to denary . The videos below explain both methods . Denary to hexadecimal: Hexadecimal to denary: Watch on YouTube Watch on YouTube Watch on YouTube Watch on YouTube Watch on YouTube Watch on YouTube Q uesto's Q uestions 2.4a - Number Systems: 1. Explain why hexadecimal numbers are used as an alternative to binary . Use an example . [ 3 ] 2. Convert the following values from binary to denary : a. 00101010 b. 11011011 c. 01011101 d. 11101110 e. 01011111 [1 each ] 3. Convert the following values from denary to binary : a. 35 b. 79 c. 101 d. 203 e. 250 [1 each ] 4. Convert the following values from binary to hexadecimal : a. 11110101 b. 01100111 c. 10111010 d. 10010000 e. 11101001 [1 each ] 5. Convert the following values from hexadecimal to binary : a. C2 b. 8A c. DE d. 54 e. F7 [1 each ] 6. Convert the following values from denary to hexadecimal : a. 134 b. 201 c. 57 d. 224 e. 101 [1 each ] 7. Convert the following values from hexadecimal to denary : a. 32 b. A5 c. 88 d. C0 e. BE [1 each ] Click the banners below to try self-marking quizzes (Google Forms) on these topics. Binary to Denary: Denary to Binary: Binary to Hexadecimal: Hexadecimal to Binary: 2.3 - Data Units Theory Topics 2.4b - Binary Addition & Shifts

Learn about the three factors that affect computer performance - cache memory, clock speed and the number of cores. Based on the J277 OCR GCSE Computer Science specification (first taught from 2020 onwards). 1.2: CPU Performance Exam Board: OCR Specification: J277 Watch on YouTube : Clock Speed Cache Memory Number of Cores The performance of a computer system is affected by three main factors: Cache Memory What is cache memory? Cache memory is temporary storage for frequently accessed data . How does cache memory improve performance? Cache memory is closer to the CPU than RAM , meaning that it can provide data and instructions to the CPU at a faster rate . A computer with more cache memory (e.g. 8GB instead of 4GB) should have a higher performance because repeatedly used instructions can be stored and accessed faster . What is the limitation of cache memory? Cache memory is costly, so most computers only have a small amount . Clock Speed What is clock speed? Clock speed is the measure of how quickly a CPU can process instructions . Clock speed is measured in Gigahertz (GHz) . A typical desktop computer might have a clock speed of 3.5 GHz . This means it can perform 3.5 billion cycles a second . How does clock speed improve performance? The faster the clock speed, the faster the computer can perform the FDE cycle resulting in better performance because more instructions can be processed each second . How does overclocking and underclocking affect performance? Default clock speed: 3.5 GHz Underclocking Overclocking 3.9 GHz 3.1 GHz Overclocking is when the computer's clock speed is increased higher than the recommended rate. This will make the computer perform faster, but it can lead to overheating and could damage the machine . Underclocking is when the computer's clock speed is decreased lower than the recommended rate. This will make the computer perform slower but will increase the lifespan of the machine . Number of Cores What is a core? A core is a complete set of CPU components (control unit, ALU and registers). Each core is able to perform its own FDE cycle . A multi-core CPU has more than one set of components within the same CPU. How does the number of cores improve performance? In theory, a single-core processor can execute one instruction at a time , a dual-core processor can execute two instructions, and a quad-core can execute four instructions simultaneously . Therefore, a computer with more cores will have a higher performance because it can process more instructions at once . What are the limitations of having more cores? If one core is waiting for another core to finish processing, performance may not increase at all. Some software is not written to make use of multiple cores , so it will not run any quicker on a multi-core computer. Q uesto's Q uestions 1.2 - CPU Performance: Cache Size & Levels 1a. What is cache memory ? [ 2 ] 1b. Describe two ways that more c ache memory will mean performance is higher . [ 4 ] 1c. Explain why most computers only have a small amount of cache memory. [ 1 ] Clock Speed 2a. What is clock speed ? What is it measured in? [ 2 ] 2b. Explain how a higher clock speed improves performance . [ 2 ] 2c. Explain the terms 'overclocking ' and 'underclocking ' and explain the effects of both on the performance of a computer. [ 4 ] Number of Cores 3a. What is a core ? [ 2 ] 3b. Explain why a quad-core processor should have a higher performance than a dual-core processor . [ 3 ] 3c. Explain two reasons why having more cores doesn't necessarily mean the performance will be better . [ 2 ] 1.1b - Registers & FE Cycle 1.3 - Embedded Systems Theory Topics

Learn about key registers used in the fetch - execute cycle such as the program counter and current instruction register. Based on the J277 OCR GCSE Computer Science specification (first taught from 2020 onwards). 1.1b: Registers & The F-E Cycle Exam Board: OCR Specification: J277 Watch on YouTube : Registers in the FE Cycle Reading instructions The fetch - execute (F-E) cycle is performed by the CPU millions of times every second. This cycle is how the CPU processes data and instructions for each program or service requiring attention . Important Registers A register is a small storage space for temporary data in the CPU . Each register has a specific role . There are four essential registers used in the F-E cycle : Program Counter (PC) A register that tracks the RAM address of the next instruction to be fetched . Memory Address Register (MAR) Stores the RAM address of the current instruction (or data ) the CPU needs to access . Memory Data Register (MDR) The MDR stores the instruction that has been transferred from RAM to the CPU . Accumulator (ACC) The ACC stores the result of mathematical or logical calculations . The PC , MAR and MDR are used in the fetch stage , whereas the ACC is used in the execute stage . The PC and MAR store an address (a memory location in RAM ) whereas the MDR and ACC store data (a value ). Fetch - Execute Cycle The essential idea of the F-E cycle is that instructions are fetched from RAM , to be decoded (understood) and executed (processed) by the CPU . 1. The Program Counter (PC ) register displays the address in RAM of the next instruction to be processed . This value is copied into the Memory Address Register (MAR ). 0054 2. The PC register is increased by 1 . This prepares the CPU for the next instruction to be fetched. 0055 3. The CPU checks the address in RAM which matches the address held in the MAR . 0054 4. The instruction in RAM is transferred to the Memory Data Register (MDR ). MDR 5. The instruction is decoded by the contol unit which splits the instruction into an opcode (an action ) and an operand ( data or address ). 6. The instruction i s executed (run). Any result of an execution is stored in the Accumulator (ACC ) register, such as calculations made by the ALU . ACC 7. The cycle repeats by returning to the first step and checking the program counter for the address of the next instruction . Q uesto's Q uestions 1.1b - Registers & The F-E Cycle: 1 . What is the purpose of the registers ? [1 ] 2 . Describe the purpose of each register : a. The Program Counter (PC) [ 2 ] b. The Memory Address Register (MAR) [ 2 ] c. The Memory Data Register (MDR) [ 2 ] d. The Current Instruction Register (CIR) [ 2 ] e. The Accumulator (ACC) [ 2 ] 3. Draw a diagram with icons and words to show the steps of the Fetch - Execute cycle . [7 ] 1.1a - The CPU Theory Topics 1.2 - CPU Performance

Learn about the benefits of compression and the differences between lossy and lossless compression. Also, learn how compression ratios work. Based on the J277 OCR GCSE Computer Science specification (first taught from 2020 onwards). 2.5: Compression Exam Board: OCR Specification: J277 Watch on YouTube : Compression Benefits Lossy Compression Lossless Compression What is compression? To compress a file means to make its size smaller . Benefits of compression include: Files take up less storage space (so more files can be stored). Files can be transferred quicker (because they are smaller). Files can be read from or written to quicker . There are two methods that are used to compress files: Lossy and Lossless . Lossy Compression Lossy compression uses an algorithm (set of instructions) to analyse a file and remove data that cannot be heard or seen by humans . For example, a lossy algorithm would analyse the sound waves of an audio file and remove any frequencies which humans cannot hear. This process reduces the size of the file . Further lossy compression will remove data that humans can see / hear . For example, the dog image to the right has been strongly compressed using a lossy algorithm and some data has clearly been removed. Lossy compression removes the data permanently , so the file can never return to its original form . Lossy compression is often used with images , audio and video to reduce the file size, for example to send over the internet. Lossless Compression Lossless compression reduces the size of a file without permanently removing any data . Because of this, the file is returned to its original form when decompressed, so no quality is lost . A file that is compressed with a lossless algorithm is usually larger than a file compressed with a lossy algorithm because no data has been permanently removed. Lossless compression is used with files that would not work if data was removed, for example executable files (e.g. programs and games) or word documents . Remember that lossy and lossless compression do not just refer to images. Below is an audio file that has been compressed with lossy compression . Data has been removed so the audio quality has decreased. 197 KB 81 KB 43 KB Q uesto's Q uestions 2.5 - Compression: 1. Describe 3 benefits of compressing a file . [ 3 ] 2. Describe the differences between lossy and lossless compression . [4 ] 3. A student needs to compress a Microsoft Word document to send in an email. Suggest which type of compression they should use and why . [ 2 ] 2.4e Sound Storage Theory Topics 3.1a - Network Types & Performance