Cambridge IGCSE · 0478

Computer Science

Computer systems — Paper 1

Name: ____________________Date: October 14, 2026

1.
Convert the denary number 45 to 8-bit binary. Explain why an 8-bit unsigned integer cannot store the value 300.
[4 marks] · no calculator
Marking points
- Converts 45 to 00101101.
- States that 8 bits provide 256 distinct unsigned values.
- States the representable range is 0 to 255.
- Concludes that 300 exceeds the maximum and would cause overflow or require more bits.
Examiner tip: For an unsigned n-bit integer, the maximum is 2^n − 1.
2.
A school allows students to connect personal devices to its wireless network. Explain two security measures the school should use and how each reduces risk.
[4 marks] · no calculator
Marking points
- Identifies a valid measure such as strong authentication, network segmentation, encryption or firewall filtering.
- Explains how the first measure blocks, limits or protects against a specific risk.
- Identifies a second distinct valid measure.
- Explains how the second measure reduces a different or related risk.
Examiner tip: Name the threat each control addresses; listing two controls without effects is incomplete.
3.
A smart doorbell should sound an alert if EITHER the button is pressed OR the motion sensor detects movement. Identify the logic gate that matches this behaviour, explain why, and state its output when the button input is 0 and the sensor input is 1.
[3 marks] · no calculator
Marking points
- Identifies an OR gate.
- Explains that an OR gate outputs 1 if at least one input is 1.
- States the output is 1 for inputs 0 and 1.
Examiner tip: An OR gate only outputs 0 when every input is 0; any single 1 input is enough to make the output 1.
4.
A photographer needs to store very large video files and keep an off-site backup accessible from any location with an internet connection. Recommend a suitable storage solution and justify your choice.
[4 marks] · no calculator
Marking points
- Recommends cloud storage.
- Explains that cloud storage can be accessed remotely from any device with an internet connection.
- Explains that this makes it suitable for an off-site backup, protecting the files if local storage is lost or damaged.
- States a limitation, such as needing a reliable internet connection or an ongoing subscription cost.
Examiner tip: Cloud storage trades a dependency on internet access for remote availability and off-site safety.
5.
Convert the hexadecimal value 2F to denary.
[3 marks]
Marking points
- States that 2F means (2 × 16) + F.
- States that hexadecimal F equals denary 15.
- Calculates 32 + 15 = 47.
Examiner tip: Each hexadecimal digit represents a power of 16, just as each denary digit represents a power of 10.
6.
Explain the role of the Program Counter (PC) in the fetch-execute cycle.
[2 marks] · no calculator
Marking points
- States that the Program Counter holds the memory address of the next instruction to be fetched.
- Explains that it is incremented (or updated) after each fetch, so the CPU knows where to fetch the next instruction from.
Examiner tip: The Program Counter tracks position in a program's instructions, not the instruction's content itself.
7.
A company wants to email a large image file. Explain the difference between lossy and lossless compression, and state which is more appropriate if image quality must be preserved exactly.
[3 marks] · no calculator
Marking points
- Explains that lossy compression permanently removes some data to achieve a smaller file size, which can reduce quality.
- Explains that lossless compression reduces file size without permanently losing any data, so the original can be fully recovered.
- States that lossless compression is more appropriate when quality must be preserved exactly.
Examiner tip: Lossy compression trades some quality for a smaller file; lossless compression keeps every bit of the original data.
8.
A sound is sampled at a sample rate of 44,100 Hz for 10 seconds, using 16-bit samples and a single (mono) channel. Calculate the uncompressed file size in bits.
[3 marks]
Marking points
- States the formula: file size = sample rate × duration × bit depth × number of channels.
- Substitutes the given values: 44,100 × 10 × 16 × 1.
- Calculates the answer as 7,056,000 bits.
Examiner tip: Multiply all four factors together; forgetting the number of channels is a common error for stereo recordings.
9.
Add the two 8-bit binary numbers 00001101 and 00000110. Give your answer in binary.
[3 marks]
Marking points
- Converts or recognises 00001101 as 13 and 00000110 as 6.
- Adds 13 + 6 = 19.
- Converts 19 back to binary as 00010011.
Examiner tip: You can add binary digits directly using column addition and carrying, or convert to denary, add, then convert back — both give the same result.
10.
Explain how a character such as the letter 'A' is represented inside a computer, referring to a character encoding system such as ASCII.
[2 marks] · no calculator
Marking points
- Explains that each character is assigned a unique denary (and therefore binary) code number by the encoding system.
- Explains that the computer stores and processes the binary code number, not the character's visual shape directly.
Examiner tip: ASCII assigns each character a number from 0-127; Unicode extends this to represent many more characters from different languages.
11.
Distinguish between RAM and ROM in a computer system.
[2 marks] · no calculator
Marking points
- Explains that RAM is volatile (its contents are lost when power is off) and can be both read from and written to while the computer runs.
- Explains that ROM is non-volatile (its contents are kept without power) and is normally read-only, often storing start-up instructions.
Examiner tip: RAM temporarily holds data and programs currently in use; ROM permanently stores instructions needed before the operating system has even loaded.
12.
Explain the purpose of cache memory in a computer system.
[2 marks] · no calculator
Marking points
- Explains that cache is a small amount of very fast memory, located closer to the processor than main memory (RAM).
- Explains that it stores frequently used instructions or data, so the processor can access them more quickly than fetching from slower RAM each time.
Examiner tip: Cache sits between the processor and RAM in the memory hierarchy, trading a very small storage size for much greater speed.
13.
Explain one advantage of a solid-state drive (SSD) over a traditional hard disk drive (HDD) for storing a laptop's operating system.
[2 marks] · no calculator
Marking points
- States an advantage such as faster read/write speeds, since an SSD has no moving parts.
- Explains that this means the operating system and programs load more quickly than from an HDD.
Examiner tip: An SSD's lack of moving mechanical parts also makes it more resistant to physical shock than an HDD, in addition to being faster.
14.
Explain one function that an operating system performs to manage a computer's hardware resources.
[2 marks] · no calculator
Marking points
- Identifies a valid function, such as memory management, processor (task) scheduling, or managing input/output devices.
- Explains briefly what this function does, such as memory management allocating RAM space to different running programs.
Examiner tip: The operating system acts as a manager between application software and the computer's physical hardware resources.
15.
Explain the purpose of utility software, using anti-virus software as an example.
[2 marks] · no calculator
Marking points
- Explains that utility software performs a specific maintenance or support task for the computer system, rather than being an application for the end user's main tasks.
- Applies this to anti-virus software, which scans for, detects and removes malicious software to keep the system secure.
Examiner tip: Utility software (anti-virus, disk defragmenters, backup tools) supports and maintains the system, unlike application software used directly for a user's tasks.
16.
Distinguish between a high-level programming language and a low-level programming language.
[2 marks] · no calculator
Marking points
- Explains that a high-level language uses instructions closer to human language, making it easier to read and write, but it must be translated before it can run.
- Explains that a low-level language (such as assembly language) is closer to the processor's own instructions, giving more direct hardware control but making it harder to read and write.
Examiner tip: High-level languages prioritise ease of use for programmers; low-level languages prioritise direct, efficient control of the hardware.
17.
Distinguish between a compiler and an interpreter as methods of translating high-level program code.
[2 marks] · no calculator
Marking points
- Explains that a compiler translates the entire program into machine code in one go before it is run, producing a standalone executable file.
- Explains that an interpreter translates and executes the program line by line, without producing a separate stored executable file.
Examiner tip: A compiled program runs faster since translation is already done; an interpreted program is often easier to debug since errors are reported as each line runs.
18.
Complete the truth table logic for an AND gate: state the output when input A = 1 and input B = 0, and the output when input A = 1 and input B = 1.
[2 marks] · no calculator
Marking points
- States that the output is 0 when A = 1 and B = 0.
- States that the output is 1 when A = 1 and B = 1.
Examiner tip: An AND gate only outputs 1 when every one of its inputs is 1; any 0 input forces the output to 0.
19.
Explain what a NOT gate does to a single binary input signal.
[1 mark] · no calculator
Marking points
- Explains that a NOT gate reverses (inverts) its single input: a 1 input becomes a 0 output, and a 0 input becomes a 1 output.
Examiner tip: A NOT gate is the only basic logic gate with just one input, since its entire job is to invert that single signal.
20.
A logic circuit combines an AND gate feeding into a NOT gate: the AND gate takes inputs A and B, and its output becomes the single input to the NOT gate. Calculate the final output when A = 1 and B = 1.
[2 marks] · no calculator
Marking points
- Calculates the AND gate's output as 1, since both A and B are 1.
- Calculates the NOT gate's output by inverting this 1, giving a final output of 0.
Examiner tip: Work through a combined logic circuit one gate at a time, using the output of the first gate as the input to the next.
21.
Explain how a digital image is made up of pixels, and how resolution affects image quality.
[2 marks] · no calculator
Marking points
- Explains that a digital image is made up of a grid of small individual picture elements (pixels), each storing a colour value.
- Explains that a higher resolution means more pixels are used to represent the same image, generally producing a sharper, more detailed picture (but also a larger file size).
Examiner tip: Resolution describes the number of pixels in an image; more pixels generally improve clarity at the cost of a larger file size.
22.
An image uses a colour depth of 4 bits per pixel. Calculate the total number of different colours that can be represented by each pixel.
[2 marks]
Marking points
- States that the number of colours = 2 raised to the power of the number of bits.
- Calculates 2^4 = 16 colours.
Examiner tip: Each extra bit of colour depth doubles the number of colours that can be represented.
23.
Explain what is meant by an embedded system, using the computer inside a washing machine as an example.
[2 marks] · no calculator
Marking points
- Explains that an embedded system is a computer system built into a larger device to perform a specific, dedicated function, rather than general-purpose computing.
- Applies this to the example: the washing machine's embedded computer only controls wash cycles and settings, unlike a general-purpose computer that can run many different programs.
Examiner tip: An embedded system is designed to do one job well; a general-purpose computer is designed to run many different kinds of software.
24.
Using 8-bit two's complement, convert the denary value −5 to binary.
[3 marks]
Marking points
- Writes the binary representation of positive 5 as 00000101.
- Inverts all the bits to get 11111010.
- Adds 1 to get the final two's complement answer: 11111011.
Examiner tip: Two's complement for a negative number: write the positive binary value, invert every bit, then add 1.