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Free A-Level Computer Science guide
Start with the job the system must perform. Processor architecture, devices and storage are useful choices only when their characteristics match that job.
Content owner: Michael Print · Written for A-Level learners · Checked against official specifications
RISC emphasises a relatively regular set of simpler instructions, commonly with explicit load/store operations. CISC provides richer instructions, potentially doing more work per instruction. Actual performance depends on the implementation and program.
GPUs suit large amounts of similar independent work. Device and storage choices should consider accuracy, latency, capacity, portability, durability, access pattern and cost.
OCR H446 · 1.1.2(a–c), 1.1.3(a–d)
Useful foundations
By the end, you should be able to
These are tendencies rather than a guarantee that one instruction always takes one cycle, one type always uses less energy, or every RISC program is larger. Contemporary CISC implementations can internally decode instructions into simpler operations.
Compatibility with an existing instruction set can be decisive: changing hardware is expensive if required software no longer runs. Compare a stated task and implementation, not brand stereotypes.
The worked table compares a possible operation; these descriptions are tendencies, not performance guarantees.
| Approach | Possible instruction sequence | Trade-off |
|---|---|---|
| RISC load/store style | Load operands; add registers; store result | Several regular instructions |
| CISC style | Instruction may combine memory access and addition | Fewer instructions may require more complex execution |
A graphics processing unit contains many execution resources designed for high throughput across similar operations. Rendering many pixels is one use; image processing, matrix calculations, scientific simulation and suitable machine-learning calculations are other uses. Thousands of independent values can be processed with the same operation.
A task dominated by unpredictable branches or serial dependencies may fit a CPU better.
A multicore processor contains several CPU cores on a chip. A parallel system uses several processing resources simultaneously on divided work; it may use multicore CPUs, GPUs or multiple computers. Splitting data is only useful when dependencies permit it.
Data transfer, synchronisation and combining results have costs. More processors also do not multiply RAM capacity unless the system actually provides it.
Input devices capture a measurement or an instruction: a barcode reader identifies a labelled item, a keyboard supports free text, a microphone captures sound and a temperature sensor supplies repeated measurements. Choose according to speed, accuracy, contact requirements, lighting, noise and user needs.
A barcode reader is faster than typing many existing labels but cannot invent missing labels or reliably identify an unlabelled item.
Output devices communicate or act: a display presents changing information, a printer provides a physical record, a speaker gives an audible warning and an actuator changes the physical system.
A warehouse alarm might combine sound and a warning light because noise or hearing differences can make sound insufficient. A temperature-control system needs a heater actuator rather than only a screen showing that it is cold.
Magnetic disks store patterns magnetically on rotating platters. They suit high-capacity bulk storage, with mechanical movement affecting random access and shock resistance. Magnetic tape is well suited to large sequential backups/archives but locating one arbitrary file can require substantial winding.
Both preserve data without continuous power.
Flash storage stores data electronically and has no mechanical seek. SSDs suit responsive operating-system and application storage; removable flash suits portable transfers. Flash has finite write endurance and its controller manages wear, so no moving parts does not mean infinite life.
Optical media use laser-readable marks; they can distribute fixed media or provide an offline copy, but require a compatible drive and usually offer lower capacities/access rates than current bulk disk systems. Exact costs and capacities change; compare the stated figures in a question.
| Need | Plausible choice | Reason and limitation |
|---|---|---|
| Interactive workstation | SSD | Fast random access; cost/endurance still matter |
| Large sequential offline archive | Tape | Capacity and sequential transfer; slow arbitrary retrieval |
| Physical read-only distribution | Optical disc | Portable fixed copy; compatible reader required |
RAM is the writable working memory holding active program instructions/data. Conventional main RAM is volatile: its contents disappear when power is lost. ROM is non-volatile memory intended to hold relatively fixed instructions such as firmware.
Some modern firmware stores can be updated using flash; non-volatile does not mean that every implementation is physically impossible to rewrite.
Virtual storage presents a logical storage service without requiring the user to manage its physical disks. Cloud storage is an example: a provider manages remote hardware while the user accesses files through a service.
Benefits include remote access, sharing and managed capacity; limitations include connectivity, provider dependency, access control and recurring cost. A synchronised local copy can allow offline work if configured.
Virtual storage is different from virtual memory, where an OS gives processes an address-space abstraction and may keep some pages on secondary storage.
Worked example
Worked example
4 original questions total 14 marks. Attempt each before opening the independently written indicative marking guidance.
Give two RISC/CISC comparisons, with a limitation on making a performance claim.
Indicative answer
A laboratory applies the same filter independently to millions of images. Explain a GPU benefit and a condition that could reduce it.
Indicative answer
Choose input, output and two storage types for a remote weather station sending hourly summaries. Justify the choices.
Indicative answer
Distinguish RAM and ROM, then explain why a cloud file synchronisation service is not automatically a backup.
Indicative answer
This guide addresses OCR H446 1.1.2(a–c), 1.1.3(a–d). Check your examination year and the complete specification for the assessment scope.
These are independently written explanations and practice questions. CompSciTutoring.co.uk is not affiliated with or endorsed by an examination board. The marking guidance is indicative; always check the syllabus for your examination year.
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