Year 11 · Computer Science · Spring 2 – Summer 1
Final revision and the exams · Lesson 3 of 5
Command words and timing
Exam practice3.3 Data storage5.3 Cyber security7.3 The purpose of an algorithm
Year 11 · Computer Science · Final revision and the exams · Lesson 3 of 5
You will learn to
- Define the command words: state, identify, give, describe, explain, compare, calculate, evaluate and suggest.
- Sort the questions from one past paper by command word.
- Calculate how many minutes each question is worth from its marks.
- Use a fast start: underline, circle the marks, write the first point straight away.
Year 11 · Computer Science · Final revision and the exams · Lesson 3 of 5
Success criteria
You have got it when you can say:
- I can say what each command word asks me to do.
- I can tell a 'describe' answer from an 'explain' answer.
- I can work out roughly how long to spend on a question from its marks.
Year 11 · Computer Science · Final revision and the exams · Lesson 3 of 5
Retrieval starter
Questions from before. Write your answers, then show them.
- Last lesson
A good primary key is…
A A surnameB A date of birthC A unique ID number
C. A unique ID number
- Last unit
X = (A AND B) OR C with A = 0, B = 1, C = 1 gives…
A 1B 0C Cannot tell
A. 1
- Earlier this year
Which of these is lossy?
A PNGB JPEGC ZIP
B. JPEG
Year 11 · Computer Science · Final revision and the exams · Lesson 3 of 5
Key words (1 of 9)
- CPU
- The central processing unit, which processes instructions and data by fetching, decoding and executing them.
- ALU
- Arithmetic logic unit: does calculations and logical comparisons.
- Control unit
- Manages the fetch–decode–execute cycle and sends control signals.
Year 11 · Computer Science · Final revision and the exams · Lesson 3 of 5
Key words (2 of 9)
- Cache
- Small, very fast memory in or near the CPU that holds frequently used data and instructions.
- Instruction set
- The list of all the instructions a particular CPU can carry out.
- Sensor
- An input device that measures a physical property and sends the reading to a computer.
- Volatile
- Loses its contents when the power is switched off.
Year 11 · Computer Science · Final revision and the exams · Lesson 3 of 5
Key words (3 of 9)
- Virtual memory
- Part of secondary storage used as extra RAM, with pages moved between them.
- NIC
- Network interface card: the hardware that connects a device to a network.
- IP address
- A number that identifies a device on a network, such as 151.101.0.81.
- Internet
- The worldwide network of connected computer networks: the physical infrastructure.
Year 11 · Computer Science · Final revision and the exams · Lesson 3 of 5
Key words (4 of 9)
- URL
- Uniform resource locator: the text address of a web page, made of protocol, domain name and file name.
- HTTP / HTTPS
- The rules for sending web pages; HTTPS adds encryption using SSL.
- Web browser
- Application software that fetches web pages and renders HTML so you can see them.
Year 11 · Computer Science · Final revision and the exams · Lesson 3 of 5
Key words (5 of 9)
- DNS
- Domain Name System: servers that find the IP address that matches a domain name.
- Digital currency
- Money that exists only electronically.
- Brute-force attack
- Trying every possible password or key until the right one is found.
- DDoS
- Distributed denial of service: flooding a server with requests from many computers so real users cannot reach it.
Year 11 · Computer Science · Final revision and the exams · Lesson 3 of 5
Key words (6 of 9)
- Malware
- Software designed to cause harm, such as a virus, worm, Trojan horse, spyware, adware or ransomware.
- Phishing
- A fake message that tricks the user into clicking a link and giving personal details on a fake site.
- Pharming
- Malicious code or a tampered DNS server redirects the user to a fake website, even if they type the correct URL.
Year 11 · Computer Science · Final revision and the exams · Lesson 3 of 5
Key words (7 of 9)
- Social engineering
- Manipulating people into breaking normal security rules.
- Firewall
- Hardware or software that checks traffic in and out against rules and blocks anything that breaks them.
- Proxy server
- A server between the user and the internet that filters requests, hides the user's IP address and caches pages.
Year 11 · Computer Science · Final revision and the exams · Lesson 3 of 5
Key words (8 of 9)
- SSL
- Secure Sockets Layer: a protocol that uses certificates and encryption to secure data between browser and server.
- Algorithm
- A precise, ordered set of steps that solves a problem.
- Program development life cycle
- The four stages of making a program: analysis, design, coding and testing.
Year 11 · Computer Science · Final revision and the exams · Lesson 3 of 5
Key words (9 of 9)
- Pseudocode
- A structured, code-like way of writing an algorithm that is not tied to one language.
- Verification
- A check that data has been copied or typed in exactly as the original.
- Check digit
- An extra digit calculated from the other digits, used to catch typing errors.
- Normal data
- Test data that should be accepted.
The explanation · step 1 of 26 · part 1 of 3
What each command word wants
The command word is the first word of the question. It tells you what kind of answer to write.
The explanation · step 1 of 26 · part 2 of 3
What each command word wants (continued)
| Command word | What it wants | Example question |
|---|---|---|
| State | Say it in clear words. No reason is needed. | State one function of an operating system. |
| Identify | Name it or pick it out. | Identify the type of error in line 4. |
| Give | Write an answer from memory or from the source. | Give two examples of application software. |
| Describe | Say what it is like or what happens. Give the main features. | Describe how a blockchain records transactions. |
| Explain | Say why or how, with reasons. | Explain why a computer needs both RAM and ROM. |
| Compare | Say what is the same and what is different. | Compare serial and parallel transmission. |
| Calculate | Work it out from the numbers. Show your working. | Calculate the file size of the image in KiB. |
| Evaluate | Judge how good or useful it is. Give both sides, then a conclusion. | Evaluate the use of robots in a car factory. |
| Suggest | Use what you know in a new situation. More than one answer can be right. | Suggest a suitable primary key for this table. |
The explanation · step 1 of 26 · part 3 of 3
What each command word wants (continued)
In a 'compare' answer, each point mentions both things. Words like 'whereas' and 'but' help.
Compare: 'Serial sends one bit at a time, whereas parallel sends several bits at once.'
The explanation · step 2 of 26 · part 1 of 2
Marks tell you the time
Paper 1 and Paper 2 are each 75 marks in 105 minutes.
Spend about 1¼ minutes per mark. That leaves about 10 minutes to check.
| Marks | About how long |
|---|---|
| 1 | 1 minute |
| 2 | 2½ minutes |
| 4 | 5 minutes |
| 6 | 7½ minutes |
| 15 (the Paper 2 scenario) | 30 minutes: it is a whole program |
The explanation · step 2 of 26 · part 2 of 2
Marks tell you the time (continued)
When a question's time is up, put a star by it and move on. Come back if there is time.
The explanation · step 3 of 26
Start fast
- Underline the command word.
- Circle the marks.
- Write the first point straight away, even if it is simple.
- Add points until you have as many as there are marks.
- Move on.
Why write a simple point first?
A blank page slows you down. One written point gets your thinking going, and it may earn a mark.
3.3 Data storage · The explanation · step 4 of 26 · part 1 of 3
Primary storage: RAM and ROM
Primary storage is memory the CPU can access directly. It includes RAM and ROM.
RAM (random access memory) holds the data and programs currently in use, including parts of the operating system. It can be read and written. It is volatile: everything is lost when the power goes off.
3.3 Data storage · The explanation · step 4 of 26 · part 2 of 3
Primary storage: RAM and ROM (continued)
ROM (read-only memory) holds the start-up instructions (the bootloader firmware) that run when you switch on. It can only be read. It is non-volatile: it keeps its contents without power.
| RAM | ROM | |
|---|---|---|
| Volatile? | Yes: loses data without power | No: keeps data without power |
| Read / write? | Read and write | Read only |
| What it holds | Programs and data in use now | Start-up instructions (bootloader) |
| Size in a laptop | Large, e.g. 16 GiB | Small, e.g. a few MiB |
3.3 Data storage · The explanation · step 4 of 26 · part 3 of 3
Primary storage: RAM and ROM (continued)
Both are needed. RAM is fast and changeable, so programs can run and data can change. But RAM is empty at switch-on, so ROM must hold the instructions that start the computer and load the operating system.
3.3 Data storage · The explanation · step 5 of 26
Secondary storage
Secondary storage is non-volatile storage that the CPU does not access directly. Data is copied into RAM first.
It is needed to keep files, programs and the operating system when the power is off. It also holds far more than RAM, for less money per gigabyte.
There are three types: magnetic, optical and solid-state.
3.3 Data storage · The explanation · step 6 of 26 · part 1 of 2
Magnetic storage: the hard disk drive
A hard disk drive (HDD) has metal or glass platters coated with a magnetic material. They spin very fast, for example 7,200 times a minute.
A read/write head on a moving arm floats a tiny distance above each surface. To write, it magnetises tiny areas in one direction for 1 and the other direction for 0. To read, it senses the direction.
3.3 Data storage · The explanation · step 6 of 26 · part 2 of 2
Magnetic storage: the hard disk drive (continued)
Each platter is divided into tracks (circles) and sectors (slices of each track). Data is found by moving the head to the right track, then waiting for the right sector to spin past.
| Advantages | Disadvantages |
|---|---|
| Large capacity, cheap per gigabyte | Moving parts can break if dropped |
| Can be rewritten many times without wearing out | Slower to access data than solid-state (head must move, disk must spin) |
| Good for servers and backup | Heavier, noisier and uses more power |
The explanation · 20 more steps
More on the lesson page
The lesson page has the rest of the explanation, step by step:
- Optical storage: CD, DVD and Blu-ray
- Solid-state (flash) storage
- Virtual memory
- Cloud storage
- Brute-force attack
- Data interception
- Distributed denial of service (DDoS)
- Hacking
- Malware: virus, worm and Trojan horse
- Malware: spyware, adware and ransomware
- Phishing and pharming
- Social engineering
- Solutions: access levels, anti-malware and authentication
- Solutions: updates, checking messages and links
- Solutions: firewalls and privacy settings
- Solutions: proxy servers
- Solutions: SSL
- Matching threats to defences
- Stating the purpose
- Describing the processes
Year 11 · Computer Science · Final revision and the exams · Lesson 3 of 5
Quick quiz · 1 of 3
With even parity, the 7 data bits 0110100 need the parity bit…
- A 1
- B 0
- C It cannot be worked out
A. 1
0110100 has three 1s, which is odd. A 1 makes the count even.
Year 11 · Computer Science · Final revision and the exams · Lesson 3 of 5
Quick quiz · 2 of 3
Parity can…
- A Detect and fix any error
- B Detect two flipped bits
- C Detect one flipped bit but not fix it
C. Detect one flipped bit but not fix it
Two flips keep the count even, so they slip through, and parity never says which bit flipped.
Year 11 · Computer Science · Final revision and the exams · Lesson 3 of 5
Quick quiz · 3 of 3
In asymmetric encryption the message is locked with the…
- A Same key that unlocks it
- B Private key
- C Public key
C. Public key
Anyone can lock with the public key. Only the private key unlocks.
Year 11 · Computer Science · Final revision and the exams · Lesson 3 of 5
Exit ticket
Answer on paper before you go. Then check.
ARQ stands for…
A Address resolution questionB Average rate queryC Automatic repeat request
C. Automatic repeat request The receiver acknowledges; if no acknowledgement arrives before the timeout, the sender resends.
The register that holds the address of the next instruction is the…
A Memory address register (MAR)B Accumulator (ACC)C Program counter (PC)
C. Program counter (PC) PC points at the next instruction. MAR holds the address being used right now.