Input/output devices and secondary storage – A-level Computer Science exam questions with mark scheme

1Core3 marks

A supermarket checkout uses a hand-held laser barcode reader.

Describe how the barcode reader reads the barcode on a product. [3]

A laser / light (beam) is shone across the barcode (allow: LED light; ignore 'it scans the barcode')(1)
Dark bars absorb the light and light (white) spaces reflect it (accept: different amounts of light are reflected by bars and spaces)(1)
Reflected light is detected by a sensor / photodiode / light-sensitive cell which converts it to an electrical signal / pattern of bits that is decoded into the digits/product code (ignore: 'sent to the computer' alone)(1)
Answer: Light shone on barcode; bars absorb, spaces reflect; sensor detects reflected light and converts the pattern to a digital code.
Examiner tip: Say which parts reflect and which absorb; 'the scanner reads the lines' gets no credit.

2Core4 marks

Describe how a digital camera captures a colour image and stores it as binary data. [4]

Light (from the scene) is focused by the lens onto an image sensor (accept: CCD / CMOS sensor)(1)
The sensor is made of a grid/array of light-sensitive cells (photosites), each corresponding to a pixel(1)
Each cell produces an electrical charge/voltage proportional to the intensity of the light falling on it(1)
The analogue values are converted to digital/binary by an analogue-to-digital converter (ADC); colour is recorded using red, green and blue filters over the cells (accept: colour filter array / Bayer filter); either point for this mark(1)
Answer: Lens focuses light onto CCD/CMOS array of photosites; each produces charge proportional to light intensity; RGB filters give colour; ADC converts to binary for each pixel.
Examiner tip: The ADC step is often missed: the sensor output is analogue until it is digitised.

3Standard5 marks

An office has a laser printer.

(a) Describe how a laser printer transfers an image onto a sheet of paper. [4]

(b) State one reason why a laser printer is more suitable than an inkjet printer for printing several hundred pages of text each day. [1]

(a) A rotating drum is given an (electrostatic) charge (accept positive or negative)(1)
(a) A laser beam scans the drum and changes/removes the charge on the areas that form the image (accept: laser 'draws' the image as a pattern of charge)(1)
(a) Charged toner (powder) is attracted to/sticks to the areas of the drum that form the image; the paper is given a charge so the toner is transferred from the drum onto the paper(1)
(a) The paper passes through a fuser (heated rollers) which melts/bonds the toner onto the paper (accept: heat and pressure; ignore 'ink is dried')(1)
(b) Any one: faster printing (more pages per minute); lower cost per page; toner cartridge prints many more pages before replacement; output does not smudge (allow: higher-capacity paper trays); do not accept 'better quality' alone(1)
Answer: (a) Drum charged; laser alters charge to form image; charged toner sticks to image areas then transfers to charged paper; fuser melts toner on. (b) e.g. faster / cheaper per page.
Examiner tip: Do not call toner 'ink', and remember the fuser stage that fixes the toner to the paper.

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4Standard5 marks

A school library fits a passive RFID tag inside each book so that students can borrow books at a self-service desk.

(a) Explain how the self-service desk reads the data stored on a passive RFID tag. [3]

(b) Give two advantages of using RFID tags rather than barcodes in this library. [2]

(a) The reader transmits radio waves / an electromagnetic field(1)
(a) This induces a current in the tag's antenna/coil which powers the tag's chip (accept: the tag has no battery and takes its energy from the reader's signal)(1)
(a) The tag transmits its data (e.g. unique ID) back to the reader using radio waves, which the reader decodes(1)
(b) Any two from: no line of sight is needed / the tag can be read inside the book cover; several books can be read at the same time; faster to process loans; tags can be rewritable so data can be updated(1)
(b) Second distinct advantage from the list above (also accept: tag can also be used by a security gate to detect unborrowed books; harder to damage than a printed label). Do not accept 'cheaper'(1)
Answer: (a) Reader sends radio waves; induced current powers tag; tag replies with its data by radio. (b) No line of sight needed; many books read at once.
Examiner tip: A passive tag has no power source: say where its energy comes from.

5Core5 marks

The diagram shows a simplified view from above of one platter of a magnetic hard disk drive.

(a) Identify the parts labelled A, B and C. [3]

(b) Describe how a bit is written to the platter. [2]

(a) A: track (accept: a circular track / ring of data)(1)
(a) B: sector (accept: block; reject: segment)(1)
(a) C: read/write head (D is the actuator arm; not required)(1)
(b) The actuator arm moves the read/write head to the correct track and waits for the correct sector to rotate under it (accept: platter spins)(1)
(b) The head magnetises a small region of the platter's surface in one of two directions/polarities, to represent 0 or 1(1)
Answer: (a) A track, B sector, C read/write head. (b) Head moved to track as platter spins; head magnetises region in one of two polarities to represent 0 or 1.
Examiner tip: In (b) state that the two magnetic directions represent the two binary values.

6Standard4 marks

A hard disk drive spins at 7200 revolutions per minute and has an average seek time of 8.5 ms.

(a) Calculate the average rotational latency of the drive in milliseconds. Show your working. [2]

(b) The average access time is the sum of the average seek time and the average rotational latency. A solid-state drive (SSD) has an average access time of 0.1 ms.

Calculate how many times faster the SSD's average access time is than the hard disk's. Give your answer to the nearest whole number. [2]

(a) One revolution takes 60 / 7200 s = 8.33 ms (accept 0.00833 s)(1)
(a) Average latency is half a revolution = 4.17 ms (accept 4.2 ms; allow ecf from time for one revolution)(1)
(b) Average access time = 8.5 + 4.17 = 12.67 ms (accept 12.7 ms; allow ecf from (a))(1)
(b) 12.67 / 0.1 = 127 times (accept 126.7 rounded to 127; allow ecf from their access time, correctly rounded to the nearest whole number)(1)
Answer: (a) 4.17 ms (b) 127 times faster
Examiner tip: Average rotational latency is half a revolution, not a full one.

7Standard4 marks

A software company distributes its programs on read-only optical discs (CD-ROMs).

Describe how data is read from the disc. [4]

The disc spins and a laser (beam) is shone onto the surface along a single spiral track(1)
The surface has pits and lands (accept: bumps and flat areas)(1)
Light reflected from the surface is detected by a sensor/photodiode; a transition between a pit and a land changes how the light is reflected (accept: pit scatters light / less light returned)(1)
A transition between pit and land represents a 1 and no transition represents a 0 (accept: pits and lands represent the binary data, max 3 marks if pit = 1 and land = 0 is stated)(1)
Answer: Spinning disc, laser follows spiral track of pits and lands; reflected light detected; change at pit/land boundary = 1, no change = 0.
Examiner tip: Saying 'pits are 1 and lands are 0' is a common misconception; it is the transition that encodes a 1.

8Hard5 marks

A solid-state drive (SSD) uses NAND flash memory and a controller. Data is stored in pages of 4 KiB, grouped into blocks of 256 pages.

(a) Explain how a NAND flash memory cell stores a single bit without a power supply. [2]

(b) A program modifies one page of data in a block in which every page already holds valid data. Explain why this write takes longer than writing the same page to an empty block, and state the amount of data, in MiB, that may have to be rewritten. [3]

(a) Each cell is a floating-gate transistor; electrons/charge are trapped on the floating gate (surrounded by an insulating layer)(1)
(a) The presence or absence of trapped charge (affects whether the transistor conducts so) represents 0 or 1, and the charge stays when power is removed(1)
(b) A page cannot be overwritten in place; flash must be erased a whole block at a time before it is rewritten(1)
(b) So the controller must copy the block's valid pages (to cache / another block), erase the block, then write the old pages and the modified page back (accept: read-erase-modify-write cycle / write to a new block then erase old one)(1)
(b) 256 × 4 KiB = 1024 KiB = 1 MiB(1)
Answer: (a) Charge trapped on floating gate of transistor represents the bit; insulated so retained without power. (b) Whole block must be erased before rewriting so other pages copied out and back; 1 MiB.
Examiner tip: Pages are written individually but erased only as whole blocks: make this distinction explicit.

9Hard6 marks

A business is choosing storage media. The table shows typical figures.

Medium	Capacity	Price	Average access time
Hard disk drive (HDD)	4 TB	£80	12 ms
Solid-state drive (SSD)	1 TB	£70	0.1 ms
Recordable Blu-ray disc	25 GB	£1	150 ms

(a) Calculate the cost per GB of the HDD and of the SSD. Use 1 TB = 1000 GB. [2]

(b) The business records video from 12 security cameras. Each camera produces 2 Mbit of data per second, continuously. Footage is kept for 30 days. Calculate how much storage is needed, in TB, and state how many of the HDDs in the table must be bought. Use 1 Mbit = 10⁶ bits. [3]

(a) HDD: £80 / 4000 = £0.02 per GB (accept 2p per GB)(1)
(a) SSD: £70 / 1000 = £0.07 per GB (accept 7p per GB)(1)
(b) Total bits = 12 × 2 × 10⁶ × 30 × 24 × 3600 (= 6.2208 × 10¹³ bits); dividing by 8 for bytes (method mark for both steps)(1)
(b) = 7.776 × 10¹² bytes = 7.78 TB (accept 7.8 TB)(1)
(b) 2 HDDs (ecf: next whole number above their total ÷ 4 TB)(1)
(c) Any one idea, explained: very low access time / fast transfer so the OS boots and programs load quickly; OR no moving parts so it is robust if the laptop is moved or dropped; accept low power use / less heat (longer battery life). Do not accept 'it is fast' with no link to the OS/laptop(1)
Answer: (a) HDD £0.02/GB, SSD £0.07/GB (b) 7.78 TB, so 2 HDDs (c) fast access and no moving parts (robust, portable).
Examiner tip: Remember to divide by 8 to convert bits to bytes before converting to TB.

10Stretch6 marks

A distribution warehouse currently tracks pallets of goods by scanning a printed barcode label on each pallet with a hand-held laser scanner as it enters and leaves. The manager is considering replacing the barcode labels with RFID tags and installing fixed RFID readers at the loading-bay doors.

Discuss whether the warehouse should make this change. In your answer you should refer to the principles of operation and the characteristics of both technologies. [6]

Barcode: needs line of sight; laser light reflected off bars/spaces and detected; only one label scanned at a time, by a worker(1)
Barcode labels can be damaged/dirty/obscured and then fail to scan; labels are very cheap to print(1)
Barcode stores only a fixed code (product ID) that cannot be changed(1)
RFID: reader sends radio waves; passive tag is powered by induction and transmits its ID by radio; active tags have their own battery and longer range(1)
RFID needs no line of sight, so pallets can be read automatically as they pass through the doors, even if wrapped, and many tags read at once: faster, fewer staff, fewer missed scans(1)
RFID tags can be read/write so status/destination can be updated; can give real-time location tracking(1)
Drawbacks of RFID: tags and readers cost more (cost per pallet; installation); metal/liquids can interfere with radio signals; tags may be read unintentionally from nearby pallets (range/collisions); security/privacy of unencrypted tags(1)
Conclusion e.g. RFID worthwhile for high-volume warehouse where speed and accuracy matter / tags reusable on returnable pallets; barcodes better if budget limited or pallets leave permanently(1)

Level	Marks	What it looks like
Level 3	5–6	A balanced, well-structured discussion that covers technical differences between RFID and barcodes and links them clearly to the warehouse scenario, considering both benefits and drawbacks, and reaches a justified conclusion.
Level 2	3–4	Several valid points about RFID and/or barcodes with some explanation and some application to the warehouse; may be one-sided or lack a clear conclusion.
Level 1	1–2	A few relevant points made, mostly as statements, with little explanation or reference to the warehouse.

Answer: Balanced discussion: RFID removes line-of-sight need, reads many pallets automatically and can be updated, but costs more and suffers interference; justified conclusion.
Examiner tip: Top-band answers explain HOW each technology works and link each feature to the warehouse, then commit to a conclusion.

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