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Free A-Level Computer Science guide
Choose software by what the user needs to achieve. Then distinguish the program's purpose, its licence and the way its source is translated: these are separate questions.
Content owner: Michael Print · Written for A-Level learners · Checked against official specifications
Applications carry out user tasks, such as analysing measurements or producing a report. Utilities support maintenance/management, such as backup, compression or malware scanning. Select features and constraints for the scenario.
A compiler translates source into target code; an interpreter executes through a runtime translation/evaluation process; an assembler translates assembly mnemonics into machine instructions. The compilation pipeline checks and transforms source before producing target code; linking and loading make the resulting program ready to execute.
OCR H446 · 1.2.2(a–f)
Useful foundations
By the end, you should be able to
A spreadsheet is useful for modest tabular data, formulas and quick scenario analysis. A database is better when related data, constraints, concurrent updates and structured queries dominate. Word processing supports a structured written report; presentation software supports a visual talk; graphics/CAD tools address image/design tasks.
A bespoke application may meet unusual workflows precisely but needs development, testing, support and maintenance.
State the required feature and its consequence: a database's referential integrity protects linked records; a spreadsheet's formulas support recalculation. Check compatibility, accessibility, training, licensing and support. An answer that a spreadsheet is easy to use supplies no reason tied to the task.
A utility such as backup software supports the system rather than directly authoring the user's report; an OS may include utilities without making every utility part of its kernel.
An open-source licence makes source available with rights to use, inspect, modify and redistribute subject to its terms. Merely letting someone read source does not necessarily grant those rights. Closed-source software ordinarily keeps source/control with its owner and grants more limited use through a licence.
Either model can charge money, receive support and contain defects.
Open source can permit audit/customisation and reduce dependence on a single supplier, but maintenance still needs expertise and licence compliance. Closed source can offer a supported integrated product, while restricting modification and creating vendor dependency.
Availability of skilled support, required features and the actual licence matter more than an assumption that one model is automatically secure, free of cost or legally unrestricted.
A compiler translates a source program to another representation, commonly object/machine code or intermediate code. Compilation costs occur before that target is run; a native executable can run without recompiling its source. A different platform may require a new target build.
An interpreter executes source or intermediate instructions through a runtime, which can aid interactive development and portability but has runtime overhead. Implementations may combine compilation, interpretation and just-in-time compilation.
An assembler maps assembly-language mnemonics, labels and operands to the target machine's instruction encodings and resolves relevant addresses. Assembly instructions closely reflect the machine architecture. High-level statements may expand into many instructions, so an assembler is not simply a high-level compiler with a different name.
Runtime errors, such as division by zero for an input, can occur even after valid translation.
Semantic checking asks whether grammatically valid structures make sense: are names declared and types/operations compatible? It is useful depth alongside OCR’s four named stages, rather than a replacement for them.
Optimisation must preserve the specified observable behaviour. Changing a result to make code faster is invalid. A compiler may optimise at several points; the diagram is a teaching sequence rather than one universal physical order.
Lexical analysis → tokens
Group characters into identifiers, keywords, numbers and operators. Discard insignificant whitespace/comments and record identifiers in a symbol table. A malformed token is a lexical error.
Syntax analysis → structure
Check token order against the grammar and construct a representation such as a syntax tree. A missing operand or closing bracket is a structural error.
Code generation → target
Turn a checked representation into target instructions, intermediate code or object code.
Optimisation → lower cost
Preserve behaviour while reducing costs; for example, calculate a constant expression once or remove unreachable instructions.
A library supplies reusable routines/types so developers can use tested functionality rather than reimplement it. APIs define how to call it. A linker combines separately compiled modules and required library code, resolving references between them and relocating addresses as needed.
A missing implementation can produce an unresolved-reference link error even when individual source files compiled.
Static linking includes needed library code in the executable; dynamic linking uses shared libraries at load/run time. Dynamic sharing can reduce duplication and permit updates, but compatible libraries must be available.
A loader places the executable and necessary data into memory, establishes required mappings/addresses and begins execution. The OS participates in loading and protection. Linking resolves a program's pieces; loading prepares an executable to run.
Worked example
total = price * (2 + 3) becomes tokens: identifier total; =; identifier price; *; (; integer 2; +; integer 3; ).(2 + 3) with 5. Target code loads price, multiplies by 5 and stores total. It must preserve the language's numerical semantics.| Example | Likely stage | Reason |
|---|---|---|
| Unrecognised character in a token | Lexical | Cannot form valid token |
| price * ) | Syntax | Missing operand/invalid structure |
| Undeclared price | Semantic | Name has no valid declaration |
| Missing compiled library routine | Linking | Reference cannot be resolved |
Worked example
4 original questions total 14 marks. Attempt each before opening the independently written indicative marking guidance.
Distinguish compiler, interpreter and assembler.
Indicative answer
For result = count * (4 + 2), describe lexical analysis, syntax analysis, optimisation and code generation.
Indicative answer
Two source modules compile, but one calls a library routine absent from the final build. Identify the failure and distinguish the loader's role.
Indicative answer
A charity has a developer but needs reliable support for customised records software. Evaluate open and closed source.
Indicative answer
This guide addresses OCR H446 1.2.2(a–f). 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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