Revision Notes

Computer Systems revision is difficult because the exam rarely presents topics in the neat order of the lessons. A paper may move from a two's complement conversion to a graphics comparison, then into a fetch-execute explanation and a security question. The skill today is switching method quickly. Before answering, decide what kind of question you have: a mechanical conversion, a comparison, a process explanation, or an impact explanation. That choice controls the shape of your answer.

Data representation questions

Binary and two's complement questions reward careful working. For positive integers, use place values and pad with leading zeroes where the question asks for a fixed number of bits. For negative two's complement, convert the positive size, invert the bits, then add one. When converting from two's complement to denary, check the leftmost bit first. If it is 0, use ordinary place values. If it is 1, treat the leftmost place value as negative or invert-and-add-one to find the magnitude. Always check that the final answer fits the number of bits given.

Floating-point questions usually assess the trade-off between range and precision. The mantissa stores the significant digits, so more mantissa bits gives a more precise value. The exponent stores how far the binary point moves, so more exponent bits gives a larger range of possible values. If the total number of bits is fixed, giving more bits to one part means taking bits away from the other. This is why a change can improve range but reduce precision, or improve precision but reduce range.

Text and graphics questions

ASCII and Unicode questions often ask for advantages or disadvantages. Unicode can represent a far wider range of characters, including characters from many writing systems and symbols beyond standard English text. The trade-off is that Unicode may require more storage per character, depending on the encoding used. Bitmap and vector graphics are another common comparison. Bitmap graphics store individual pixels and are well suited to photographs and pixel-level editing, but they can have large file sizes and lose quality when scaled up. Vector graphics store objects and their properties, so they scale cleanly and are often smaller for simple drawings, but they are not suitable for realistic photographs.

Computer structure and performance

Fetch-execute questions need ordered steps. A strong answer mentions the address of the next instruction, the address bus, a read signal, the data bus, the instruction register, decoding by the control unit, and execution by the processor. You do not always need every register name for every mark, but order matters. Performance questions need both the factor and the effect. For example, saying cache alone is not enough; say that a larger cache can store more frequently used instructions and data close to the processor, reducing slower RAM access and improving performance.

Environmental, legal and security questions

Environmental impact answers need a system plus a clear effect. Intelligent heating can use sensors to heat only occupied rooms, reducing wasted energy. Traffic management can use sensors or cameras to adjust signals and reduce congestion, which can reduce idling and emissions. Security answers need precise vocabulary. The Computer Misuse Act covers unauthorised access, unauthorised access with intent to commit further crime, unauthorised modification, and making or supplying tools for misuse. Tracking cookies create privacy risks because they can build a profile of browsing behaviour. DoS attacks attempt to prevent legitimate users from accessing a system, often through bandwidth consumption, resource starvation or DNS-related attacks.