GCSE Computer Science Paper 1: Principles of Computer Science

Decomposition breaks a problem into sub-problems. Removing unnecessary detail is abstraction.

Neighbouring single items are merged into ordered pairs: 15 38, 4 72, 23 56, 9 41. Pairs are merged into ordered fours: 4 15 38 72 and 9 23 41 56. Those two lists are merged by repeatedly taking the smaller front item: 4 9 15 23 38 41 56 72.

(0 + 7) // 2 = 3: item 23. 56 is larger, so first becomes 4. (4 + 7) // 2 = 5: item 41. 56 is larger, so first becomes 6. (6 + 7) // 2 = 6: item 56, found.

A AND NOT B is 1 only when A is 1 and B is 0. B AND C is 1 only when both are 1. P is 1 when either is 1.

Each pass multiplies result by number, then takes 2 off number: 1 x 6 = 6, 6 x 4 = 24, 24 x 2 = 48. number is then 0, so the loop ends and 48 is printed.

45 is 0010 1101. Inverting gives 1101 0010, and adding 1 gives 1101 0011. 1110 0110 is -128 + 64 + 32 + 4 + 2 = -26.

An arithmetic shift right copies the sign bit into the gap, keeping the number negative: 1110 1101. That is -76 divided by 4, -19.

Overflow happens when the result of an addition is too large to fit in the number of bits available, for example a result needing 9 bits in an 8-bit register. The final carry is lost, so the stored result is wrong.

1024 x 768 = 786,432 pixels. 786,432 x 24 = 18,874,368 bits. / 8 = 2,359,296 bytes. / 1024 = 2304 KiB. / 1024 = 2.25 MiB.

C, A, B each move three letters on: F, D, E. Moving K, H, O, O, R three letters back gives HELLO.

Lossless compression reduces the file size in a way that can be reversed, so the original data is restored exactly. Lossy compression permanently removes data that is judged less important, so it makes files smaller but the original can never be fully restored.

ASCII is a 7-bit code, which gives 128 characters.

3F = 3 x 16 + 15 = 63. 1010 is A and 1100 is C: AC.

Defragmentation reorganises, backup copies, compression shrinks and anti-malware protects.

The address bus carries the address of the memory location the CPU wants to use, one way, from the CPU to memory. The data bus carries the data or instruction itself between the CPU and memory, in either direction.

An embedded system is a computer built into another device to do one dedicated job.

RAM is where the programs and data currently in use are held while the computer runs, and its contents change constantly, but it is volatile. ROM is non-volatile, so it keeps the instructions needed to start the computer even when the power is off.

A compiler produces an executable, an interpreter works line by line, and an assembler translates assembly language mnemonics.

A low-level language gives the programmer direct control of the processor's registers and memory, so the code can be made to run very fast and use very little memory, which matters for device drivers and embedded systems.

File management is one of the operating system's jobs, along with memory, process, peripheral and user management.

A WAN connects networks over a large area, often using infrastructure owned by other companies.

Each device has its own cable to the central switch, so if one cable fails only that device is affected. But everything depends on the switch: if it fails, the whole network goes down, and all that cable makes it expensive to install.

POP3 downloads mail, SMTP sends it, HTTPS is secure web traffic and FTP transfers files.

HTTP is an application protocol, TCP works at the transport layer, IP at the internet layer and Ethernet at the link layer.

A firewall inspects every packet entering or leaving the network and compares it against a set of rules, blocking any traffic that is not allowed, such as connections from unknown addresses or to unauthorised ports.

Phishing is sending emails or texts that pretend to come from a trusted organisation, such as a bank, to trick people into clicking a link or giving away personal details. Staff can be trained to recognise suspicious messages, and email filters can block many of them.

Cables give faster, more reliable connections. Freedom to move and no cabling are advantages of wireless.

Misusing personal data breaks data protection law, sharing a film breaks copyright, and unauthorised access to change data breaks the Computer Misuse Act.

Open-source software is usually free, which saves the business money, and its source code can be modified to suit its needs. However, there is often no official support line, so the business may struggle to get help when something goes wrong.

Machine learning is a type of artificial intelligence in which a program improves at a task by being trained on data: it finds the patterns itself instead of being explicitly programmed with rules.

An AI system learns from the data it is trained on. If that data is unrepresentative, or reflects unfair decisions made in the past, the system learns the same patterns, so its decisions unfairly favour some groups of people over others.

Drone delivery could cut emissions, because small battery drones use far less energy per parcel than delivery vans, and they keep vans off congested roads. But the drones and their batteries must be manufactured from mined materials, charged and eventually disposed of, and hundreds of drones overhead would add noise that disturbs people and wildlife. Ethically, delivery drivers could lose their jobs, and drones carrying cameras raise privacy concerns as they fly over homes and gardens. Legally, the retailer must handle customers' addresses and any camera footage under the Data Protection Act 2018, and flights must follow aviation regulations about where and how high drones may fly. It must also be clear who is liable if a drone crashes and injures someone. On balance drones may suit light parcels in rural areas, but only with strict rules on safety, noise and privacy.