Year 11 · Computer Science · Spring 2 – Summer 1
Final revision and the exams · Lesson 4 of 5
Last full Paper 1
Exam practice1.1 Number systems3.1 Computer architecture5.3 Cyber security
Year 11 · Computer Science · Final revision and the exams · Lesson 4 of 5
You will learn to
- Complete a full Paper 1 in 1 hour 45 minutes, without a calculator.
- Use the timing plan: about 1¼ minutes per mark, with 10 minutes to check.
- Correct every lost mark using the mark scheme in the corrections lesson.
Year 11 · Computer Science · Final revision and the exams · Lesson 4 of 5
Success criteria
You have got it when you can say:
- I can finish the paper and check it in the time given.
- I can show my working on every calculation, with units.
- I can write a full-mark correction for every question where I lost marks.
Year 11 · Computer Science · Final revision and the exams · Lesson 4 of 5
Retrieval starter
Questions from before. Write your answers, then show them.
- Last lesson
An IDE provides…
A The operating systemB Only the keyboardC An editor, a run button and a debugger
C. An editor, a run button and a debugger
- Last unit
A record is…
A One columnB The whole tableC One row: everything about one thing
C. One row: everything about one thing
- Earlier this year
Sending one bit at a time down one wire is…
A SerialB ParallelC Duplex
A. Serial
Year 11 · Computer Science · Final revision and the exams · Lesson 4 of 5
Key words (1 of 12)
- Bit
- A single binary digit, 0 or 1.
- Nibble
- A group of 4 bits, which matches one hexadecimal digit.
- Byte
- A group of 8 bits.
- Denary
- The base 10 number system we use every day, with digits 0 to 9.
Year 11 · Computer Science · Final revision and the exams · Lesson 4 of 5
Key words (2 of 12)
- Binary
- The base 2 number system computers use, with digits 0 and 1.
- Hexadecimal
- The base 16 number system, with digits 0 to 9 and A to F, used to write binary in a short form.
- Overflow
- An error when a result is too big to fit in the number of bits available.
- Logical shift
- Moving all the bits left or right, losing bits off one end and filling the other end with 0s.
Year 11 · Computer Science · Final revision and the exams · Lesson 4 of 5
Key words (3 of 12)
- Two's complement
- A way to store negative numbers, where the leftmost bit has a negative place value (−128 in 8 bits).
- Character set
- A list of characters a computer can use, each with its own binary code.
- Colour depth
- The number of bits used to store the colour of each pixel.
Year 11 · Computer Science · Final revision and the exams · Lesson 4 of 5
Key words (4 of 12)
- CPU
- The central processing unit, which processes instructions and data by fetching, decoding and executing them.
- Microprocessor
- A CPU made as an integrated circuit on a single chip.
- Von Neumann architecture
- A computer design where instructions and data are stored together in the same memory.
Year 11 · Computer Science · Final revision and the exams · Lesson 4 of 5
Key words (5 of 12)
- ALU
- Arithmetic logic unit: does calculations and logical comparisons.
- Control unit
- Manages the fetch–decode–execute cycle and sends control signals.
- Register
- A tiny, very fast store inside the CPU that holds one item.
- Bus
- A set of parallel wires that carries addresses, data or control signals.
Year 11 · Computer Science · Final revision and the exams · Lesson 4 of 5
Key words (6 of 12)
- Cache
- Small, very fast memory in or near the CPU that holds frequently used data and instructions.
- Clock speed
- The number of cycles the CPU does each second, measured in hertz.
- Core
- A complete processing unit; a multi-core CPU can process several instructions at once.
- Instruction set
- The list of all the instructions a particular CPU can carry out.
Year 11 · Computer Science · Final revision and the exams · Lesson 4 of 5
Key words (7 of 12)
- Embedded system
- A computer system inside a larger device that does one dedicated job.
- Sensor
- An input device that measures a physical property and sends the reading to a computer.
- NIC
- Network interface card: the hardware that connects a device to a network.
- MAC address
- A unique 48-bit address given to a NIC at manufacture, written in hex.
Year 11 · Computer Science · Final revision and the exams · Lesson 4 of 5
Key words (8 of 12)
- 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.
- URL
- Uniform resource locator: the text address of a web page, made of protocol, domain name and file name.
Year 11 · Computer Science · Final revision and the exams · Lesson 4 of 5
Key words (9 of 12)
- 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.
- DNS
- Domain Name System: servers that find the IP address that matches a domain name.
- Digital currency
- Money that exists only electronically.
Year 11 · Computer Science · Final revision and the exams · Lesson 4 of 5
Key words (10 of 12)
- 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.
- Malware
- Software designed to cause harm, such as a virus, worm, Trojan horse, spyware, adware or ransomware.
Year 11 · Computer Science · Final revision and the exams · Lesson 4 of 5
Key words (11 of 12)
- 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.
- Social engineering
- Manipulating people into breaking normal security rules.
Year 11 · Computer Science · Final revision and the exams · Lesson 4 of 5
Key words (12 of 12)
- 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.
- SSL
- Secure Sockets Layer: a protocol that uses certificates and encryption to secure data between browser and server.
1.1 Number systems · The explanation · step 1 of 37 · part 1 of 2
Why computers use binary
A computer is made of billions of tiny switches called transistors. Each switch is either off or on.
We write off as 0 and on as 1. A number system with only two digits is called binary.
One binary digit is a bit. Eight bits together make a byte.
Every extra bit doubles the number of different patterns you can make.
1.1 Number systems · The explanation · step 1 of 37 · part 2 of 2
Why computers use binary (continued)
Two states are very reliable. A switch is clearly on or clearly off, so the computer rarely misreads a value.
1.1 Number systems · The explanation · step 1 of 37
The maths: How many patterns can n bits make?
1.1 Number systems · The explanation · step 2 of 37 · part 1 of 3
Place value: denary and binary
Denary is our everyday number system. It has ten digits, 0 to 9, and each column is worth 10 times the column to its right.
Binary works the same way, but each column is worth 2 times the column to its right.
For 8 bits the column values, from left to right, are 128, 64, 32, 16, 8, 4, 2, 1.
To turn binary into denary, add up the column values that have a 1 under them.
1.1 Number systems · The explanation · step 2 of 37 · part 2 of 3
Place value: denary and binary (continued)
Try the toy: switch on the 128 bit. What happens to the denary value? Now make 255.
| 128 | 64 | 32 | 16 | 8 | 4 | 2 | 1 |
|---|---|---|---|---|---|---|---|
| 0 | 1 | 0 | 1 | 1 | 0 | 1 | 0 |
1.1 Number systems · The explanation · step 2 of 37 · part 3 of 3
Place value: denary and binary (continued)
Always write the column values above the bits first. Most mistakes come from adding the wrong column.
There is something to try for this step in the IGCSE library.
1.1 Number systems · The explanation · step 3 of 37 · part 1 of 2
Denary to binary
Start at the left column, 128. Ask: does 128 fit into my number?
If it fits, write 1 and take 128 away. If it does not fit, write 0.
Move to the next column and repeat with what is left. Stop when you reach the 1 column.
Always write all 8 bits, including the 0s at the front.
1.1 Number systems · The explanation · step 3 of 37 · part 2 of 2
Denary to binary (continued)
Each column is used once at most, so taking away the biggest column first always gives the right pattern.
1.1 Number systems · The explanation · step 4 of 37 · part 1 of 2
The biggest number n bits can hold
With n bits you get 2ⁿ patterns. One of those patterns is 0.
So the biggest number is one less than the number of patterns: 2ⁿ − 1.
8 bits hold 0 to 255. 16 bits hold 0 to 65,535.
A register is a small store inside the processor. An 8-bit register cannot hold anything bigger than 255.
1.1 Number systems · The explanation · step 4 of 37 · part 2 of 2
The biggest number n bits can hold (continued)
8 bits give 256 different values, but the largest value is 255, because counting starts at 0.
1.1 Number systems · The explanation · step 6 of 37 · part 1 of 2
Hex to denary and denary to hex
Hex to denary: multiply each digit by its column value, then add. Change letters to numbers first (B = 11, F = 15).
Denary to hex: divide by 16. The whole-number answer is the left digit. The remainder is the right digit.
Change any digit from 10 to 15 into its letter.
1.1 Number systems · The explanation · step 6 of 37 · part 2 of 2
Hex to denary and denary to hex (continued)
A remainder of 10 to 15 must be written as a letter. Writing "128" for C8 is a common slip.
1.1 Number systems · The explanation · step 6 of 37
The maths: Hex to denary
The explanation · 32 more steps
More on the lesson page
The lesson page has the rest of the explanation, step by step:
- Hexadecimal: base 16
- Binary and hex using nibbles
- Why hexadecimal is used
- Adding binary: the four rules
- A full 8-bit addition
- Overflow
- Logical left shift
- Logical right shift
- Two's complement: negative numbers
- Denary to two's complement
- The CPU and the microprocessor
- Von Neumann architecture
- The five registers
- The three buses
- The fetch–decode–execute cycle
- The instruction set
- Cores, cache and clock speed
- Embedded systems
- 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
Year 11 · Computer Science · Final revision and the exams · Lesson 4 of 5
Quick quiz · 1 of 3
Which bus carries the actual data?
- A Address bus
- B Control bus
- C Data bus
C. Data bus
Address bus: where. Data bus: what. Control bus: when and which way.
Year 11 · Computer Science · Final revision and the exams · Lesson 4 of 5
Quick quiz · 2 of 3
The ALU does…
- A Arithmetic and logic
- B Fetching instructions
- C Storing files
A. Arithmetic and logic
Add, subtract, compare. The control unit does the fetching and decoding.
Year 11 · Computer Science · Final revision and the exams · Lesson 4 of 5
Quick quiz · 3 of 3
Cache is…
- A The hard disk
- B Small, fast memory beside the CPU for recently used data
- C The program counter
B. Small, fast memory beside the CPU for recently used data
Bigger cache means fewer slow trips to RAM.
Year 11 · Computer Science · Final revision and the exams · Lesson 4 of 5
Exit ticket
Answer on paper before you go. Then check.
Which memory is volatile?
A ROMB RAMC An SSD
B. RAM Volatile means wiped when the power goes. RAM is; ROM and SSDs are not.
A solid-state drive…
A Has no moving partsB Uses a laserC Uses spinning platters
A. Has no moving parts Lasers are optical (DVD). Platters are magnetic (hard disk).