O Level Computer Science (2210) — MCQs & Practice Questions

A computer is built from electronic switches, each of which has only two reliable states — on and off, or high and low voltage. Binary has exactly two digits, so each digit maps directly onto one switch, making the hardware simple and resistant to electrical noise.

Place values 16, 8, 4, 2, 1: 16 + 8 + 0 + 2 + 0 = 26.

It is far shorter and easier to read — one hex digit replaces four bits, so a 32-bit address takes 8 hex characters instead of 32 binary ones — while converting between the two needs no arithmetic at all.

1. 2 in hex is 0010 in binaryfour bits per hex digit[1]
2. C is 12 in decimal, which is 1100A=10, B=11, C=12, D=13, E=14, F=15[1]
3. Together: 00101100₂concatenate the groups, do not add them[1]
4. Decimal: 32 + 8 + 4 = 44or 2×16 + 12 = 44 directly from hex[1]

2C₁₆ = 00101100₂ = 44₁₀

1. Adds column by column from the right, carrying where a column totals 2[1]
2. Result within 8 bits: 01000010 with a carry out of the leftmost column[1]
3. In decimal 202 + 115 = 317, which exceeds the 8-bit maximum of 255[1]
4. Overflow occurs — the true answer needs 9 bits, so the stored 8-bit result is incorrectthe consequence must be stated[1]

Carry out of the final column; overflow occurs because 317 > 255.

1. 2⁷ = 128 characters[1]
2. Urdu uses a different script with many additional letterforms, and 128 slots are already taken by English letters, digits and punctuation — so Unicode is used insteadnaming Unicode expected[1]
3. 2²⁴ = 16 777 216 coloursaccept "about 16.7 million"[1]
4. Total pixels = 100 × 200 = 20 000[1]
5. Each pixel takes 24 bits = 3 bytes, so 20 000 × 3 = 60 000 bytesconverting bits to bytes is the step most often missed[1]
6. 60 000 / 1024 = 58.6 KBaccept 58.6 or 60 KB if 1000 bytes per KB is stated[1]

(a) 128, too few for a second script (b) 16 777 216 (c) about 58.6 KB

Advantage: it represents over a million characters, covering every writing system including Urdu and Chinese, where ASCII covers only 128. Disadvantage: each character may need up to four bytes rather than one, so files are larger.

2⁸ = 256 colours.

Sample rate is the number of measurements taken per second, in hertz. Sample resolution is the number of bits used to store each measurement. The first controls accuracy in time, the second accuracy in amplitude.

1. Total pixels = 1024 × 768 = 786 432[1]
2. Total bits = 786 432 × 16 = 12 582 912[1]
3. Bytes = 12 582 912 ÷ 8 = 1 572 864dividing by 8 is the step most often missed[1]
4. Kilobytes = 1 572 864 ÷ 1024 = 1536 KB[1]

1536 KB (1.5 MB)

1. Bits = 44 100 × 16 × 30rate × resolution × seconds[1]
2. = 21 168 000 bits[1]
3. Bytes = 2 646 000, so ÷ 1 048 576 = 2.52 MBaccept 2.65 MB using 1 000 000[1]
4. Stereo records two channels, so the file size doubles to about 5 MB[1]

About 2.5 MB mono, doubling to about 5 MB in stereo

1. Reduce the resolution — fewer pixels means a smaller file, at the cost of visible detail[1]
2. Reduce the colour depth — fewer bits per pixel, at the cost of colour accuracy and possible banding in smooth gradientsaccept compression as a third method[1]
3. The file size is width × height × colour depth, and only the depth has changed[1]
4. 24 is three times 8, and the size is directly proportional to the depth, so the file is three times as largethe proportionality is the mark[1]
5. Consecutive codes mean sorting alphabetically is the same as sorting numerically[1]
6. And converting between upper and lower case is a fixed addition or subtraction of 32, both of which would be far harder with arbitrary assignments[1]

(a) fewer pixels or fewer bits per pixel (b) size is proportional to depth, and 24 is 3 × 8 (c) sorting and case conversion become arithmetic

It uses less storage space, and it transmits faster over a network while using less bandwidth — which reduces cost on a metered connection.

Lossless reduces the file size in a way that allows the original to be reconstructed exactly. Lossy discards some data permanently, giving a much smaller file that can never be restored to the original.

Lossy compression alters the data, and in a program every bit matters — changing even one instruction could stop the program running or make it behave incorrectly. Lossy methods rely on human perception tolerating small changes, and a computer executing code has no such tolerance.

1. Runs identified: five R, three G, eight B[1]
2. Encoded as 5R 3G 8B — six items instead of sixteen characters[1]
3. It works poorly when adjacent values rarely repeat, such as in a photograph where neighbouring pixels differ slightly[1]
4. In that case each run has length 1 and the encoding stores a count as well as the value, making the file larger than the originalthe enlargement is the key insight[1]

5R3G8B; it fails on data with little repetition, where it can increase the file size.

1. Magnetic uses magnetised regions on spinning platters — large capacity and cheap per gigabyte[1]
2. Optical uses pits read by laser — portable and cheap, but small capacity and slow[1]
3. Solid-state advantage: no moving parts, so it is much faster, silent and resistant to being dropped[1]
4. Disadvantage: higher cost per gigabyte, and each memory cell tolerates only a limited number of writesthe write-endurance point is the stronger one[1]

Magnetic cheap and large, optical portable, solid-state fast but dearer with limited write endurance.

1. Photographs: lossy compression such as JPEG[1]
2. The reduction is large and the discarded detail is not noticeable in a photograph viewed normally, so the loss costs nothing in practice[1]
3. Database: lossless compression, or none at all[1]
4. Every value must be recoverable exactly — a changed digit in a mark or a date is simply wrong data, with no perceptual tolerance to exploit[1]
5. Lossless compression works by removing repetition, and once the repetition is gone there is nothing further to remove[1]
6. Compressing an already-compressed file usually achieves nothing and can make it slightly larger, since the compression data itself must be storedaccept that repeated lossy compression degrades quality instead[1]

(a) lossy — the loss is imperceptible (b) lossless — every value must be exact (c) once repetition is removed there is nothing left to compress

Any two of: the sender's address, the receiver's address, the packet number, and the total number of packets making up the file.

The bits travel at very slightly different speeds along different wires, so they arrive out of step — a problem called skew — and the data can no longer be read correctly. Long parallel cables also suffer crosstalk between adjacent wires.

A keyboard to a computer: data travels from the keyboard to the machine and never in the other direction, so only one direction is ever used.

1. The file is divided into packets, each with a header containing the addresses and its packet number[1]
2. Packets travel independently and may take different routes; the receiver reassembles them in order using the packet numbers[1]
3. Advantage: a single corrupted or lost packet can be resent on its own rather than restarting the whole file[1]
4. Advantage: many transfers share the same links, and a broken route can be avoided rather than stopping transmission[1]

Split into numbered packets, routed independently, reassembled by number — allowing resends and rerouting.

1. (i) Serial — USB sends one bit at a time[1]
2. (i) Full duplex in modern versions — data is read and written in both directionsaccept half duplex for USB 1 and 2 with justification[1]
3. (ii) Serial and half duplex — one party speaks at a time[1]
4. (iii) Parallel, because the distance is very short, and full duplexinside the machine skew is not a problem[1]

(i) serial full duplex (ii) serial half duplex (iii) parallel

1. The distances between rooms are far too great for parallel transmission[1]
2. Skew and crosstalk would corrupt the data, and serial also needs fewer wires so the cabling is cheaper[1]
3. USB is a universal standard, so one type of connector serves many different devices[1]
4. The connector cannot be inserted the wrong way round, and devices are detected automatically with drivers loaded — plug and play[1]
5. It also supplies power as well as data, so small peripherals need no separate supplyany three advantages[1]
6. The error-checking value in the trailer fails, so the receiver requests that packet again — only the one packet is resent, not the whole filethe point is that just one packet is affected[1]

(a) skew and crosstalk over distance (b) universal, one-way connector, plug and play, supplies power (c) that packet alone is requested again

One bit of each byte is set so that the total number of 1s is even. The receiver counts the 1s in the byte it received; if the total is odd, at least one bit has been corrupted in transmission.

If two bits flip in the same byte, the parity is restored and the error is not detected. It also cannot identify which bit is wrong, so the data must be resent rather than repaired.

An extra digit calculated from the others and appended to a code such as an ISBN, barcode or account number, so that a data entry error can be detected immediately when the code is typed in.

1. Each byte carries a parity bit, so a corrupted byte fails its own row check[1]
2. An extra parity byte is sent whose bits give the parity of each column across the block[1]
3. A single flipped bit causes exactly one row and exactly one column to fail their checks[1]
4. The bit at the intersection of that row and column is the faulty one, so it can simply be flipped back — no resend is neededthe intersection is the key idea[1]

The failing row and failing column intersect at the corrupted bit, which is then flipped back.

1. The receiver checks each packet using an error-detection method such as a checksum[1]
2. It sends back a positive acknowledgement if the packet is intact, or a negative one if it is corrupted, prompting a resend[1]
3. The sender starts a timer when it transmits each packet[1]
4. If no acknowledgement arrives before the timeout, the packet is assumed lost and resent — which covers a packet that never arrived, since the receiver had nothing to checkthe timeout is the mark most often missed[1]

Acknowledgements handle corrupted packets; a timeout handles ones that never arrive.

1. Attenuation — the signal weakens over the distance until a bit is misread[1]
2. Interference from nearby electrical equipment, which a hospital has a great deal ofaccept skew if parallel[1]
3. A checksum on each packet, since it detects most corruption across the whole block rather than one byte at a timeaccept parity block with justification[1]
4. Patient data must be exactly correct — a single wrong digit in a dosage could cause harm — so a strong check is justified even at the cost of extra datathe justification must fit the context[1]
5. Detection only tells you the data is wrong; it does not deliver the correct data[1]
6. ARQ must be added so that any packet failing the check, or failing to arrive at all, is automatically requested again until it is received intact[1]

(a) attenuation and interference (b) a checksum, because the data must be exact (c) detection must be paired with ARQ so faulty packets are resent

It scrambles the data using a key so that it becomes meaningless to anyone who intercepts it. Only someone with the correct key can decrypt it back into readable form.

No. The data can still be intercepted, copied or deleted exactly as before. What encryption prevents is the interceptor understanding what they have taken, because without the key the ciphertext is unreadable.

Each extra bit doubles the number of possible keys, so a brute force attack — trying every key in turn — takes far longer. A key short enough to be tried exhaustively offers no real protection.

1. Symmetric uses the same key to encrypt and decrypt[1]
2. Advantage: it is much faster, so it suits large volumes of data[1]
3. Asymmetric uses a public key to encrypt and a matching private key to decrypt[1]
4. Advantage: no secret key needs to be exchanged, which removes the problem of getting the key to the recipient safelythe key distribution problem is the point[1]

Symmetric: one key, fast. Asymmetric: a public/private pair, no shared secret needed.

1. A public algorithm can be examined by cryptographers worldwide[1]
2. so weaknesses are found and corrected, rather than remaining hidden until an attacker discovers one[1]
3. Security rests entirely on the key, which remains secret — knowing the method does not read the message[1]
4. A secret algorithm has been checked by very few people, and once leaked offers no protection at all — "security through obscurity" is not security[1]

Public scrutiny finds weaknesses; secrecy rests on the key alone, not on the method.

1. The data is encrypted in the browser before it is sent, so anything intercepted on the way is ciphertext[1]
2. Only the shop's server holds the key needed to decrypt it, so an interceptor sees nothing usable[1]
3. Asymmetric encryption is used first, to exchange a symmetric key safely without any prior shared secret[1]
4. Then symmetric encryption is used for the actual data, because it is far faster for the volume involvedthe combination is the mark[1]
5. Encryption does not protect the data once it has been decrypted at the destination[1]
6. If the shop's database is later breached, or if the customer was tricked into sending details to a fake site in the first place, encryption offers no protection — it secured the journey, not the endpointsaccept phishing as the example[1]

(a) encrypted before sending, decryptable only by the server (b) asymmetric to exchange the key, symmetric for speed (c) it protects the journey, not the endpoints — a breached database or a fake site defeats it

RAM is volatile, fast and comparatively small; it holds the programs and data currently in use and loses everything when the power goes. Secondary storage such as an SSD is non-volatile, slower and much larger; it keeps data permanently.

The control unit, which fetches and decodes instructions and sends control signals to the rest of the system; and the arithmetic and logic unit, which performs calculations and logical comparisons.

It holds the address of the next instruction to be fetched. It increments during the fetch stage so that the following cycle picks up the next instruction rather than repeating the current one.

1. Fetch: the address in the program counter is placed on the address bus and the instruction is brought back along the data bus into the instruction registermention of a bus expected[1]
2. Decode: the control unit interprets the instruction to determine the operation and its operands[1]
3. Execute: the ALU carries out the operation[1]
4. Store: the result is written back to a register or to memory, and the cycle repeatsthe repetition should be stated[1]

Fetch, decode, execute, store — repeated for every instruction.

1. The address bus carries the memory address being accessed[1]
2. The data bus carries the actual data or instruction[1]
3. The control bus carries signals such as read and write[1]
4. The address bus is unidirectional — addresses only ever travel from the CPU to memory, whereas data travels both waysthe reason is required[1]

Address = where (one-way), data = what (two-way), control = read or write.

1. Laptop B[1]
2. Four cores can execute four instruction streams at once, so it handles several programs far better than one faster core[1]
3. 16 GB of RAM means far less swapping to disk, and the SSD has no moving parts so it reads much faster than a hard diskeither point accepted[1]
4. Laptop A has 1 TB of storage against 512 GB, so it holds roughly twice as many filescapacity is the only advantage here[1]
5. Clock speed measures cycles per second on ONE core, so it says nothing about how many cores there are[1]
6. It also ignores cache size and the amount of RAM, both of which decide how often the CPU has to wait for data[1]

(a) Laptop B — more cores, far more RAM, and an SSD (b) twice the storage capacity (c) clock speed ignores cores, cache and RAM

A touchscreen. It displays information, which is output, and senses where the user touches, which is input — both functions in one device.

A sensor produces a continuously varying analogue reading, while a microprocessor can only work with digital values. The ADC converts the analogue signal into a number the processor can use.

A moisture sensor in the soil. It measures the water content directly, which is the quantity the system needs to control, so watering can be triggered when the soil dries below a preset level.

1. An inkjet sprays ink droplets; it is cheap to buy and gives good colour quality, but is slow with a high cost per page[1]
2. A laser uses toner and a drum; it is faster with a much lower cost per page, though dearer to buy[1]
3. Recommend the laser printer[1]
4. 2000 pages a month is high volume, so the lower cost per page and greater speed outweigh the higher purchase price, and invoices need no photographic colour qualitythe justification must use the figures given[1]

Laser — high volume makes speed and cost per page decisive, and invoices need no photo quality.

1. A light or laser is shone at the barcode and the reflection is measured — dark bars reflect less light than the light spaces[1]
2. The pattern of reflections is converted into a number, which is looked up in the product database[1]
3. Advantage: it is much faster than typing a long code[1]
4. Advantage: it is far more accurate — no mistyping — and a check digit in the code catches any misread[1]

Reflected light is read as a pattern and converted to a number; faster and more accurate than typing.

1. A proximity or infrared sensor to detect that a vehicle is present at the barrier[1]
2. A barcode or card reader to read the ticket presented by the driveraccept a camera reading the number plate[1]
3. An actuator — a motor — raises and lowers the barrier arm[1]
4. Sensors only measure; they produce readings and make no decisions[1]
5. A microprocessor must compare the ticket against stored valid tickets and check that a vehicle is actually present[1]
6. Only then does it send a signal to the actuator, so the processor is what turns two readings into an action[1]

(a) a proximity sensor and a ticket reader (b) a motor as actuator (c) sensors only measure — the processor compares and decides

RAM is volatile and loses its contents when the power is removed; ROM is non-volatile and retains them. RAM can be written to freely and holds the programs currently running; ROM is not normally written to and holds the start-up instructions.

RAM is volatile, so everything in it is lost when the machine is switched off, and it is comparatively small and expensive. Secondary storage retains data permanently without power and provides far greater capacity at a much lower cost.

It has no moving parts, so it accesses data much faster, uses less power, makes no noise and is far more resistant to damage from being dropped or vibrated.

1. Virtual memory is an area of secondary storage used as though it were additional RAM[1]
2. It is used when the programs being run need more memory than the physical RAM provides[1]
3. Pages of memory not currently in use are written out to disk, freeing real RAM for active data[1]
4. When a page is needed again it is read back and another is written out — a process called paging or swapping[1]

Disk space used as extra RAM, with unused pages swapped out and read back when needed.

1. RAM has filled, so the operating system is using virtual memory and paging to diskthe constant disk activity is the clue[1]
2. Disk access is thousands of times slower than RAM, so every switch between programs now waits on the disk[1]
3. Installing more RAM is the better remedy: it removes the need to page at all[1]
4. Fitting a faster SSD helps but only shortens each swap — it reduces the symptom while leaving the cause in placethe distinction is the mark[1]

RAM is full and the OS is paging. More RAM removes the cause; a faster disk only shortens each delay.

1. Monitoring machine: solid state, for speed and because it has no moving parts to fail[1]
2. Archive: magnetic hard disks, for the very large capacity at low cost per gigabyte[1]
3. Between sites: optical or solid state portable media, as it must be carried and survive handlingeither accepted with justification[1]
4. If it began paging, the machine would slow unpredictably as it waited on disk access[1]
5. In a monitoring system a delayed response could be dangerous, so the timing must stay predictable — which means never depending on virtual memorythe safety point is the mark[1]
6. ROM holds the start-up instructions that run before any disk is read, so the machine can find and load the operating system — without it nothing could boot at all[1]

(a) SSD, magnetic archive, portable media (b) paging would make response times unpredictable, which is unsafe here (c) ROM boots the machine before any disk is available

A LAN covers a small geographical area such as one building and its hardware is usually owned by the organisation. A WAN covers a large area and typically uses third-party communication links.

An IP address identifies a device on a network and can change when the device moves to a different network. A MAC address is fixed in the hardware and uniquely identifies the network adapter itself.

The data is divided into equal-sized blocks. Each packet carries a header with the source and destination addresses and its sequence number, and packets may travel by different routes and are reassembled in order at the destination.

1. A LAN[1]
2. The computers are all within a single building and the school owns the cabling[1]
3. A switch connects devices within the LAN[1]
4. It forwards each frame only to the port of the intended recipient, using MAC addresses[1]
5. A router connects the LAN to another network, usually the internet[1]
6. It forwards packets between networks using IP addresses[1]
7. Advantage: files and printers can be shared[1]
8. Advantage: software and backups can be managed centrally[1]

1. The browser sends the domain name to a DNS server[1]
2. The server looks up the matching IP address[1]
3. The IP address is returned and the browser requests the page from that server[1]
4. HTTPS encrypts the data in transit, HTTP sends it in plain text[1]
5. So HTTPS protects information such as passwords from being read if intercepted[1]
6. To block access to unsuitable or unauthorised sites[1]
7. To inspect incoming traffic and block unauthorised access to the network[1]

1. Faster and more reliable, with less interference[1]
2. Devices can be moved freely and no cabling is needed[1]
3. Use WPA encryption on the wireless network[1]
4. Data sent over the air is scrambled[1]
5. So anyone intercepting the signal cannot read it without the keyaccept MAC filtering or a strong passphrase, explained[1]

System software manages and maintains the computer itself — for example Windows, an operating system. Application software performs tasks for the user — for example a word processor. The test is who the software serves: the machine, or the person using it.

Any two of: process management (scheduling which program uses the CPU), memory management (allocating RAM and keeping programs separate), file management, device management through drivers, and providing a user interface.

Utility software is system software that maintains, analyses or protects the computer rather than doing the user's own work. Examples: antivirus, backup software, disk defragmenter, file compression.

1. A compiler translates the entire program at once, producing an executable file; an interpreter translates and runs one line at a time[1]
2. A compiler reports all errors together at the end of compilation; an interpreter halts at the first error it reaches[1]
3. A compiler is preferable for finished software being distributed, since it runs faster and the source code need not be shipped[1]
4. An interpreter is preferable while learning or testing, because a change can be run immediately with no compilation step[1]

Whole program vs line by line; all errors vs first error; compiled for distribution, interpreted for development.

1. Proprietary: professional technical support is guaranteed, with someone responsible for fixing faults[1]
2. Proprietary: releases are formally tested, and compatibility with other schools and offices is more predictable[1]
3. Open source: no licence cost, which for a school with many machines is a substantial saving[1]
4. Open source: the source code can be inspected and modified, and the school is not locked into one supplier[1]

Proprietary: support and tested compatibility. Open source: no cost and freedom to modify.

1. Applications cannot access hardware directly — they make requests to the operating system, which carries them out[1]
2. Without an OS there is nothing to load the application into memory, allocate it CPU time, or give it access to files[1]
3. A device driver is software that translates general operating system requests into the specific commands one particular device understands[1]
4. So the OS can say "print this page" without knowing anything about that printer model; installing a new printer means installing its driver[1]
5. A CLI uses far fewer system resources than a graphical desktop, leaving more memory and CPU for the server's actual work[1]
6. Commands can be written into scripts and repeated automatically, and a CLI works well over a slow remote connectioneither reason accepted[1]

(a) applications reach hardware only through the OS (b) a driver translates OS requests for one specific device (c) lighter on resources and scriptable

The internet is the global infrastructure of interconnected networks that carries data. The world wide web is a collection of pages and resources accessed over that infrastructure — one of several services using it, alongside email and file transfer.

It translates a domain name that people can remember into the IP address the network needs to route data. This also allows a site to move to a different server without its address changing for visitors.

Keeping a user logged in as they move between pages of a site, and remembering preferences such as language, currency or the contents of a shopping basket.

1. The browser extracts the domain name and requests its IP address from a DNS server[1]
2. The DNS server returns the IP address, querying other DNS servers if necessary[1]
3. The browser sends an HTTP request to that address and the server responds with the HTML[1]
4. The browser requests the further resources the page references, then parses the HTML, applies the CSS and renders the page[1]

DNS lookup → HTTP request → server response → further resource requests → parse and render.

1. The data is encrypted in the browser before it is transmitted[1]
2. so anyone intercepting it on the network sees only ciphertext and cannot read the card details[1]
3. A digital certificate issued by a trusted authority supplies the public key and confirms the site's identity[1]
4. The padlock does not guarantee the site is honest — only that the connection is encrypted and the site is genuinely the one named on the certificatea fraudulent site can hold a valid certificate[1]

Encryption in transit plus a certificate proving identity; the padlock says nothing about the site's honesty.

1. A session cookie is held in memory and deleted when the browser closes — used to keep the customer logged in while moving between pages[1]
2. A persistent cookie is stored on disk and survives until it expires — used to remember preferences or a basket between visits[1]
3. A cookie set by an advertiser present on many different sites can be read on each of them[1]
4. so a record of the user's browsing can be assembled across sites without their being aware of itthe cross-site point is the concern[1]
5. A cookie is a small text file, not a program, so it cannot be executed[1]
6. It can only be read by the site that set it, and cannot access other files on the machine — the genuine risk is to privacy, not to the computer[1]

(a) session in memory until the browser closes; persistent on disk until expiry (b) cross-site tracking builds a browsing profile (c) it is data, not a program

Cloud computing is the delivery of computing resources — storage, processing power, software — over the internet, rented from a provider rather than owned. One advantage is scalability: capacity can be increased for a busy period and reduced afterwards, so you pay only for what you use.

It depends entirely on internet connectivity — an outage stops all work. And the data is stored on hardware the business does not control, raising security and legal questions about who can access it and in which country it is held.

The Internet of Things is the network of everyday physical objects fitted with sensors and network connections, which collect and exchange data automatically. Example: a smart thermostat that reports the temperature and can be adjusted remotely.

1. IoT devices are cheap and produced in huge numbers, and many ship with default passwords that users never change[1]
2. They often receive no security updates after release, so a known vulnerability remains exploitable for the life of the devicethis is the core of the problem[1]
3. Measure: change default credentials immediately and apply any firmware updates the manufacturer provides[1]
4. Measure: place IoT devices on a separate network from computers holding important data, so a compromised device cannot reach themnetwork segmentation[1]

Default passwords and absent updates; fix with changed credentials and a separate network.

1. Records are grouped into blocks, and each block stores a cryptographic hash of the previous block[1]
2. Changing an old record changes its hash, which invalidates every block after it, so the alteration is immediately detectable[1]
3. The ledger is copied across many computers, so a single altered copy disagrees with the rest and is rejecteddistribution is essential to the argument[1]
4. An ordinary database is better when one trusted organisation owns the data — it is far faster, cheaper and uses vastly less energy[1]

Chained hashes plus many copies make tampering detectable; a normal database is better when there is a trusted owner.

1. Records become accessible from any authorised device in the hospital, and from other sites if a patient is transferred[1]
2. The provider handles backup, redundancy and hardware maintenance, so the hospital needs no server room or specialist staff[1]
3. Risk: a loss of internet connectivity would make patient records unavailable, which in a hospital could be dangerous[1]
4. Risk: highly sensitive personal data is stored on a third party's hardware, possibly in another country with different privacy laws[1]
5. Measure: encrypt all records both in transit and at rest, so intercepted or stolen data is unreadable[1]
6. And keep a local cached or offline copy of critical records so care can continue during an outageaccept a contract specifying the country of storage[1]

(a) access anywhere, and no maintenance burden (b) outage risk and loss of control over sensitive data (c) encryption plus a local copy for outages

A virus attaches itself to a host file and requires a user to run that file in order to spread. A worm is self-replicating and spreads across a network by itself, needing no user action at all.

An attacker tries every possible password in turn until one works. The most effective measure is to lock the account after a small number of failed attempts, which makes exhaustive guessing impossible however fast the attacker is.

It examines data entering or leaving a network and blocks anything not permitted by its rules, preventing unauthorised access and filtering out unwanted traffic.

1. Phishing sends a fraudulent message imitating a trusted organisation, containing a link to a fake site the victim must click[1]
2. Defence: check the sender and the true destination of any link, and never log in through a link — type the address yourself[1]
3. Pharming uses malicious code to alter DNS settings so that typing the correct address still leads to a fake siteno link is clicked[1]
4. Defence: anti-malware to prevent the DNS settings being altered, and checking the HTTPS certificate matches the site you intendedtyping the address does not help here[1]

Phishing needs a click; pharming redirects a correctly typed address. Different attacks, different defences.

1. Firewall — filters incoming and outgoing traffic, preventing unauthorised access from outside[1]
2. Anti-malware with automatic updates — detects and removes viruses, worms and spyware, and updates close known vulnerabilities[1]
3. Access levels — each user sees only what they need, so a compromised student account cannot reach staff records[1]
4. Two-factor authentication or strong password policy with lockout — protects against brute force and stolen passwordsaccept staff training against social engineering[1]

Firewall, anti-malware with updates, access levels, and strong authentication — each matched to its threat.

1. The website incident is a DDoS attack — the server is flooded with requests from many machines until it cannot respond to genuine users[1]
2. The password incident is social engineering — the attacker manipulated a person into revealing the password, with no technical attack at all[1]
3. A firewall filters network traffic against a set of rules[1]
4. The employee volunteered the password over the telephone, which never touched the network — no traffic rule could have detected itthe reason is the mark[1]
5. Against DDoS: traffic filtering at the firewall, rate limiting, or a service that absorbs and distributes the load[1]
6. Against social engineering: staff training and a strict procedure that credentials are never given out over the phone, with any such request verified by calling back on a known number[1]

(a) DDoS and social engineering (b) the password never crossed the network (c) traffic filtering, and training with a call-back verification policy

A monitoring system measures a quantity and reports or records it, taking no action. A control system also compares the reading with a preset value and acts through an actuator to change the quantity.

Sensors produce analogue readings that vary continuously, while a microprocessor can only work with digital values. The analogue-to-digital converter translates between the two.

They work continuously without fatigue, giving consistent quality on repetitive tasks, and they can operate in conditions hazardous to people such as paint spraying or welding.

1. A light sensor continuously measures the ambient brightness[1]
2. An ADC converts the analogue reading into a digital value for the processor[1]
3. The microprocessor compares the value with a stored preset brightness levelthe comparison is essential[1]
4. If the reading falls below the preset it signals the actuator to switch the lamps on; the loop then repeats so they switch off again at dawn[1]

Light sensor, ADC, microprocessor comparing with a preset, and an actuator switching the lamps.

1. The action taken by the actuator changes the physical quantity being measured[1]
2. The sensor then measures that changed quantity, so the system responds to its own effect — a closed loop[1]
3. Without feedback the system would act blindly, for example heating for a fixed time regardless of conditions[1]
4. It would then overshoot on a mild day and undershoot on a cold one; feedback lets it stop at exactly the right moment whatever the conditionsthe example is what earns this mark[1]

The action changes what is measured, so the system responds to the actual state rather than an assumption.

1. Advantage: consistent quality with no fatigue, and the ability to work continuously without breaks[1]
2. Advantage: they can work safely in conditions hazardous to people, such as welding fumes or heavy lifting[1]
3. Disadvantage: high initial purchase and installation cost, and skilled staff needed to program and maintain them[1]
4. Disadvantage: existing workers lose their jobs, and retraining rarely reaches the same people[1]
5. Sensors let the robot detect the position and presence of components, so it responds to what is actually there rather than assumingwithout them a misplaced part would be handled as though correctly positioned[1]
6. An unfamiliar situation — a component of the wrong shape, or an obstruction — would be handled badly, because the robot follows a program covering only anticipated cases, whereas a person would notice something was wrong and stop[1]

(a) consistency and safety against cost and job losses (b) so it responds to actual positions rather than assumptions (c) anything unanticipated — a person notices and stops

Artificial intelligence is the broad field of making machines perform tasks that would ordinarily require human intelligence. Machine learning is a subset of AI in which the system learns patterns from data rather than being given explicit rules. All machine learning is AI; not all AI is machine learning.

Learning from data in which each example is labelled with the correct answer. The model learns to reproduce those labels on new data. Example: training on photographs already tagged "cat" or "dog" so the system can classify unseen photographs.

Because a model can score perfectly on data it has effectively memorised, which says nothing about how it will behave on new examples. Only unseen data measures whether it has learned the general pattern — the property that actually matters in use.

1. Supervised: trained on labelled data — for example spam detection from emails already marked spam or not[1]
2. Unsupervised: trained on unlabelled data and finds structure itself — for example grouping customers into segments[1]
3. Reinforcement: learns by acting and receiving rewards or penalties — for example a program learning to play a game[1]
4. The distinguishing factor is what the training data contains: answers, no answers, or a reward signalthe unifying statement earns the fourth mark[1]

Labelled data, unlabelled data, and reward feedback respectively.

1. The model learns the patterns in the historical decisions, including any that were unfair[1]
2. If certain groups were refused disproportionately in the past, the model reproduces that pattern while appearing objectivethe appearance of objectivity is part of the harm[1]
3. Reduce it by examining the training data for representativeness, and testing outcomes separately for each group[1]
4. And by keeping a human decision-maker for consequential decisions, with a documented route to appealaccept requiring the model to be explainable[1]

The model learns historical unfairness. Audit the data, test outcomes per group, and keep human oversight.

1. Historical records of past students — attendance, assessment marks, homework completion — each labelled with whether that student ultimately passed or failedlabelled data, so this is supervised learning[1]
2. The data is split into a training set and a test set; the model learns from the first and is measured on the second[1]
3. Unfairness: if the historical data reflects a group that was under-supported in the past, the model predicts failure for that group and the prediction becomes self-fulfilling[1]
4. Or: a feature that correlates with background rather than ability — such as distance travelled to school — drives the predictioneither example accepted[1]
5. A prediction is a statistical likelihood about a group, not a fact about the individual student in front of the teacher[1]
6. The model cannot explain its reasoning, and being labelled "likely to fail" could itself affect how a student is treated and how they see themselves[1]

(a) labelled historical records, split into training and test sets (b) it reproduces past under-support as a prediction (c) a prediction is about likelihood, not about the individual, and cannot justify itself

A finite sequence of unambiguous instructions that solves a problem or completes a task, with clearly defined inputs and outputs.

Abstraction is removing the details that are not relevant to the problem being solved, so that the essential structure is easier to work with. A metro map shows the order of the stations and the connections between lines, but omits real distances and street layout — which makes it more useful for planning a journey, not less.

A flowchart is graphical, using standard shapes and arrows, and shows the flow of control visually. Pseudocode is textual, resembles program code and is quicker to write and to translate into a real language. Flowcharts become unwieldy for long algorithms, where pseudocode stays readable.

1. INPUT mark[1]
2. Validation: WHILE mark < 0 OR mark > 100 → OUTPUT "Invalid" and INPUT mark againa loop, not a single if — the user may enter a bad value twice[1]
3. IF mark >= 40 THEN OUTPUT "Pass"the >= is required; > alone fails a mark of exactly 40[1]
4. ELSE OUTPUT "Fail" ENDIF[1]

Input with a validation loop, then IF mark >= 40 output Pass ELSE output Fail.

1. Columns for n, i, total, and outputone column per variable[1]
2. After pass 1: i = 2, total = 1; after pass 2: i = 3, total = 3[1]
3. After pass 3: i = 4, total = 6; after pass 4: i = 5, total = 10[1]
4. The loop ends because 5 > 4; output is 10the algorithm sums 1 to n[1]

Output 10 — the algorithm computes 1 + 2 + 3 + 4.

1. Break the system into separate sub-problems, such as: record one student's attendance; store a day's records; calculate one student's monthly totalat least two named sub-problems[1]
2. And: produce the report; handle login. Each is small enough to be designed and tested on its ownthe reason for decomposing is part of the answer[1]
3. Abstraction: store only what the task needs — a student's roll number and present/absent flag — and ignore irrelevant details such as their address or hobbies[1]
4. The unnecessary detail would make the system larger and slower without improving the report[1]
5. Boundary data tests the values at the edges of what is acceptable, which is where most errors occur[1]
6. Example: a month with 31 days, or a class with exactly one student, or an attendance percentage of exactly 75% if that is the pass thresholdany sensible boundary value[1]

(a) split into recording, storing, calculating and reporting (b) store only roll number and present/absent (c) edges are where errors hide — e.g. exactly 75% attendance

The data must already be sorted. The algorithm discards half the list at each step by comparing with the middle item, which is only valid if the order is known.

A measure of how the number of operations grows as the size of the input grows, written in big-O notation. It describes the trend, not the exact running time on a particular machine.

About 10, because 2¹⁰ = 1024. Each comparison halves the remaining list, so the maximum is log₂ n rounded up.

1. After pass 1: 3, 5, 1, 8[1]
2. After pass 2: 3, 1, 5, 8[1]
3. After pass 3: 1, 3, 5, 8[1]
4. Three passes; a fourth confirms no swaps[1]
5. A flag records whether any swap was made; if a whole pass makes none, the list is sorted[1]
6. O(n²)[1]
7. Doubling the data quadruples the work, so it becomes impractically slow[1]

1. Initialises low = 1 and high = 5000[1]
2. Loops while low <= high[1]
3. Calculates mid = (low + high) DIV 2[1]
4. Compares and returns mid if the name matches[1]
5. Otherwise sets high = mid − 1 or low = mid + 1 correctly[1]
6. Returns a value such as −1 or FALSE once low > high[1]
7. Binary search: at most 13 comparisons, since 2¹³ = 8192[1]
8. Linear search: up to 5000, so binary search is dramatically faster[1]

1. Merge sort or quicksort (average case)[1]
2. The log n factor grows very slowly compared with the extra factor of n[1]
3. So the gap between the two widens rapidly as n increases[1]
4. Doubling n slightly more than doubles n log n[1]
5. So roughly 4.2 to 4.5 seconds, a little over doubleaccept "just over 4 seconds" with reasoning[1]

A variable is a named location in memory holding a value. x = 10 creates the name x and assigns the integer value 10 to it. Assigning again later replaces the value rather than creating a second variable.

Because input() always returns a string, even when the user types digits. Without conversion, + would join the text instead of adding — "5" + "3" gives "53" — and comparisons would compare text rather than numeric value.

= is the assignment operator, storing a value in a variable. == is the comparison operator, testing whether two values are equal and producing True or False.

1. length = float(input("Enter length: "))float rather than int allows decimal measurements[1]
2. width = float(input("Enter width: "))[1]
3. area = length * widtha meaningful variable name is expected[1]
4. print("The area is", area)output must be labelled, not a bare number[1]

Read both values as floats, multiply, and print with a label.

1. (i) 3 — integer division discards the remaindernot 3.5[1]
2. (ii) 1 — the remainder when 7 is divided by 2[1]
3. (iii) 3.5 — / always produces a floatnote the decimal point[1]
4. (iv) 8 — ** is the power operator2 cubed[1]

(i) 3 (ii) 1 (iii) 3.5 (iv) 8

1. Error 1: the inputs are strings, so they are joined rather than addedint() or float() is missing[1]
2. Error 2: operator precedence — a + b / 2 divides b by 2 first, so brackets are needed: (a + b) / 2this error survives even after the type is fixed[1]
3. Corrected input lines using float(input(…))[1]
4. avg = (a + b) / 2 followed by a labelled print[1]
5. Test with normal data, such as 60 and 80, and check the answer is 70a value you can verify by hand[1]
6. Test with boundary data such as 0 and 100, and with erroneous data such as text, to see how it behavesall three kinds of test data[1]

(a) inputs not converted, and missing brackets (b) float(input(…)) and (a+b)/2 (c) normal, boundary and erroneous data

Sequence — statements written one after another, needing no keyword. Selection — if. Iteration — for or while.

When the number of repetitions is not known in advance and depends on a condition evaluated during the loop — for example repeating until the user enters valid input or types "quit". A for loop is used when the count is fixed beforehand.

Five times, with i taking the values 2, 3, 4, 5 and 6. The upper bound 7 is where the sequence stops and is not itself used.

1. for n in range(1, 21): — the upper bound must be 21 to include 20the off-by-one is examined here[1]
2. if n % 3 == 0:the remainder test for divisibility[1]
3. print(n) correctly indented inside the ifindentation is part of the mark[1]
4. Output: 3, 6, 9, 12, 15, 18[1]

A for loop over range(1, 21) with an if n % 3 == 0 test inside.

1. Nothing inside the loop changes count[1]
2. So the condition count > 0 is true on every pass and never becomes false — an infinite loopnaming it as infinite is expected[1]
3. Add count = count - 1 inside the loopaccept count -= 1[1]
4. It must be indented inside the loop; placed outside it would run only once, after the loopthe indentation point is a mark[1]

The loop variable is never decremented. Add count = count - 1 indented inside the loop.

1. A for loop, because the number of marks is known in advance to be tenthe reason must be given[1]
2. Initialise total = 0 and passes = 0 before the loopaccumulators must start outside the loop[1]
3. for i in range(10): then read a mark and add it to total[1]
4. if mark >= 50: passes = passes + 1, correctly indented inside the loop[1]
5. After the loop: print(passes) and print(total / 10)[1]
6. The prints must be outside the loop, because inside they would run on every pass and print ten partial results instead of one final answerthe reason is the mark[1]

(a) for, since ten is known (b) accumulators before, loop reads and tests, prints after (c) outside, or they print ten partial results

Advantage: any element can be reached directly from its index in constant time. Disadvantage: inserting at the front requires every later element to be shifted.

A stack is last-in-first-out: items are added and removed at the same end. A queue is first-in-first-out: items are added at one end and removed from the other.

On a push the pointer is incremented and the new item is written at that position. On a pop the item at the pointer is read and the pointer is decremented. The pointer must be checked against the array bounds first.

1. Jobs must be printed in the order they were submitted[1]
2. A queue removes items in the same order they were added, which is first-in-first-out[1]
3. In a linear queue the front pointer moves forward and the space behind it is wasted[1]
4. A circular queue wraps the rear pointer round to the start of the array[1]
5. So the freed space is reused and the array does not appear full while space remains[1]
6. That the queue is not full[1]

1. 50 at the root with 30 on its left and 70 on its right[1]
2. 20 and 40 as the children of 30[1]
3. 60 as the left child of 70[1]
4. Visits left subtree, then node, then right subtree[1]
5. 20, 30, 40, 50, 60, 70[1]
6. The values in ascending order[1]
7. It can therefore be used to sort the data, or to output it in order without re-sorting[1]

(b) 20, 30, 40, 50, 60, 70

1. Traverse from the head until the node after which Chand belongs is found, here Bilal[1]
2. Create the new node and set its pointer to whatever Bilal was pointing at, that is Danish[1]
3. Set Bilal's pointer to the new nodethis order matters or the rest of the list is lost[1]
4. The nodes are not stored in consecutive memory locations[1]
5. Order is held by the pointers, so changing two pointers is enough[1]

Variables are held in RAM, which is volatile — their contents are lost the moment the program ends or the power fails. A file is stored on secondary storage, so the data persists and can be read by the same program on another day or by a different program entirely.

It returns NULL. Using a NULL file pointer in any subsequent read or write causes the program to crash, so the check turns an unexplained failure into a clear message such as "cannot open file".

"w" erases the entire contents of an existing file the moment it is opened, then writes from the beginning. "a" preserves the contents and adds new data at the end. Both create the file if it does not exist.

1. FILE *fp; int n; fp = fopen("data.txt", "r");[1]
2. if (fp == NULL) { printf("Cannot open file\n"); return 1; }the NULL check is a mark[1]
3. while (fscanf(fp, "%d", &n) == 1) { printf("%d\n", n); }testing the return value, not feof[1]
4. fclose(fp);[1]

Open, check NULL, loop while fscanf returns 1, close.

1. The file has been opened in "w" mode instead of "a"[1]
2. "w" truncates the file to zero length as soon as it is opened, destroying every existing recordthe truncation happens on opening[1]
3. The new record is then written into an empty file, which is why only it remains[1]
4. Correct it by opening with "a", which preserves the contents and appends at the end[1]

The file was opened with "w", which truncates it. Use "a" to append.

1. Opens the file in "r" mode and checks the pointer is not NULL[1]
2. while (fscanf(fp, "%s %d", name, &marks) == 2) {two items requested, two expected back[1]
3. if (marks >= 50) count++; } then prints the count after the loopcount initialised before the loop[1]
4. fclose flushes the output buffer, so data still held in memory is actually written to disk[1]
5. It also releases the file handle, a limited operating-system resource that would otherwise run out[1]
6. feof becomes true only after a read has already failed, so the loop body runs one extra time and the previous values are processed twicetesting the read's return value avoids this[1]

(a) open, check, loop on fscanf == 2 counting passes (b) flushes the buffer and releases the handle (c) feof is set after a failed read, so the last record is processed twice

A primary key is a field, or combination of fields, that uniquely identifies each record in a table. It must be unique across all records and must never be empty. It should also never change.

A field in one table that holds the primary key value of another table, creating the link between them. For example, ClassID stored in the STUDENT table refers to the primary key of the CLASS table.

Names are not unique — two students may share one — and they can change, which would break every foreign key referring to that record. A primary key should be a meaningless, stable identifier such as a roll number.

1. Data integrity — validation rules and constraints reject invalid data[1]
2. Security — access permissions per user, per table, or per field[1]
3. Concurrency control — simultaneous updates by different users cannot corrupt the data[1]
4. Backup and recovery, and a query language for answering unanticipated questionseither earns the mark[1]

Integrity, security, concurrency control, backup and recovery, and a query language.

1. Relationship identified as one to many, from instructor to student[1]
2. INSTRUCTOR(InstructorID, Name, Phone)primary key underlined or stated[1]
3. STUDENT(StudentID, Name, Address, InstructorID)[1]
4. InstructorID in STUDENT is a foreign key referring to INSTRUCTOR — the key goes on the "many" sideplacing it on the wrong side is the standard error[1]

Two tables, with InstructorID as a foreign key in STUDENT.

1. Many to many — a student takes many courses and a course has many students[1]
2. A field holds a single value, so neither table can hold a list of the other; a third table is requiredthe reason, not just the assertion[1]
3. STUDENT(StudentID, Name) and COURSE(CourseID, Title)[1]
4. ENROLMENT(StudentID, CourseID, Grade) with a composite primary key, both fields also being foreign keysaccept a separate EnrolmentID[1]
5. The grade belongs in ENROLMENT[1]
6. Because it is a property of that particular student's enrolment in that particular course — not of the student, who has many grades, nor of the course, which awards manythe reasoning is the mark[1]

(a) many to many — a field cannot hold a list (b) STUDENT, COURSE and an ENROLMENT linking table (c) the grade goes in ENROLMENT, since it belongs to the relationship

A query is a saved question that selects, filters, sorts or calculates from one or more tables. It stores no data — only the question — and produces its results afresh from the tables each time it runs, so the answer is always current.

AND requires a record to satisfy every condition, so the result is smaller than either condition alone. OR requires only one, so the result is larger. In the Access design grid, criteria on the same row are ANDed and criteria on different rows are ORed.

All records whose value in that field begins with A. The asterisk is a wildcard standing for any sequence of characters, so Like "*A*" would instead find values containing A anywhere.

1. SELECT Name, Marksonly the requested columns[1]
2. FROM STUDENT[1]
3. WHERE Marks > 50no quotation marks — Marks is numeric[1]
4. ORDER BY Marks DESCDESC is required for highest first[1]

SELECT Name, Marks FROM STUDENT WHERE Marks > 50 ORDER BY Marks DESC

1. A Null field is empty — the value is unknown or was never entered[1]
2. A field containing 0 holds a known value, namely zeroa real mark of nothing, not a missing one[1]
3. Criterion Is Null finds the empty ones; =0 finds the zeros[1]
4. The distinction matters because an average ignores nulls but includes zeros, giving two different answers from the same columnthe consequence is the fourth mark[1]

Null is unknown, 0 is a known value. Is Null and =0 respectively; averages treat them differently.

1. SELECT Title, DateDue[1]
2. FROM BOOK INNER JOIN LOAN ON BOOK.ISBN = LOAN.ISBNjoined on the shared key[1]
3. WHERE DateDue < #1/3/2026#dates enclosed in hash symbols[1]
4. Title is stored only in BOOK and DateDue only in LOAN, so neither table alone can produce both columnsa direct consequence of normalisation[1]
5. Use a totals query: group by MemberID[1]
6. and apply Count to LoanID, giving one row per member with their number of loansaccept SELECT MemberID, Count(LoanID) FROM LOAN GROUP BY MemberID[1]

(a) a join on ISBN with a date criterion (b) the two columns live in different tables (c) a totals query grouping by MemberID with Count

Inputs 00 → 1, 01 → 1, 10 → 1, 11 → 0. NAND is the inverse of AND, so it outputs 0 only when both inputs are 1.

(A · B)′ = A′ + B′ and (A + B)′ = A′ · B′. Complementing a whole expression swaps AND for OR and complements each variable.

Because any logic function whatsoever can be built using copies of just one of them. NOT, AND, OR and every larger circuit can be constructed from NAND alone, or from NOR alone, which is why chip manufacturers can make a whole processor from one repeated gate design.

1. Four rows for two inputs, with columns for A′ and B′[1]
2. A=0,B=0 → 0; A=0,B=1 → 1A′·B is 1 in the second row[1]
3. A=1,B=0 → 1; A=1,B=1 → 0[1]
4. Output is 1 exactly when the inputs differ, so this is an XOR gatethe identification is the point of the question[1]

The table matches XOR: output 1 when A and B differ.

1. Expand: A·A + A·B′ + B·A + B·B′distributive law[1]
2. A·A = A (idempotent) and B·B′ = 0 (complement)[1]
3. X = A + A·B′ + A·Bthe zero term disappears[1]
4. By absorption A + A·anything = A, so X = Anaming the law is required by the question[1]

X = A

1. 0+0 → Sum 0, Carry 0; 0+1 → Sum 1, Carry 0[1]
2. 1+0 → Sum 1, Carry 0; 1+1 → Sum 0, Carry 1the last row is the whole point — 1+1 in binary is 10[1]
3. Sum is produced by an XOR gateoutput 1 when the inputs differ[1]
4. Carry is produced by an AND gateoutput 1 only when both are 1[1]
5. A half adder has only two inputs, so it has nowhere to accept the carry produced by the previous column[1]
6. A full adder is needed instead, taking three inputs — A, B and carry-innaming the full adder is expected[1]

(b) XOR for Sum, AND for Carry (c) it has no carry-in input, so a full adder is required