Edexcel GCSE Computer Science (1CP2) Exam Questions

Award up to two marks for a linked description such as:

• Statements are written using words/phrases that look like English or maths (1) so the code is easier to read/understand/write (1)
• One statement is translated into several machine code instructions (1) so programs are shorter to write than in a low-level language (1)
• High-level languages provide built-in data structures such as arrays/lists/records (1) so a collection of data items can be stored and accessed under one name (1)

Do not accept responses that only give a benefit or drawback, such as 'it is easier to learn', with no characteristic.

Do not accept 'it is easier than a low-level language' without saying what the characteristic is.

Award up to two marks for a linked explanation such as:

• The compiler produces object code/machine code for the whole program (1) so there is no complete executable until every line has been translated (1)
• The compiler translates all the source code before producing the object code (1) so the program cannot start until translation has finished (1)
• The compiler checks the whole program for errors during translation (1) so all of the source code must be read before an executable can be produced (1)

For both marks, the expansion must follow/associate with the statement.

Allow 'in one go'/'all at once' for the translation point, but this does not earn the second mark unless it is linked to why the program cannot be executed until translation has finished.

A statement that the whole program is translated, with no linked reason about when the program can be executed, earns 1 mark only.

Do not accept 'a compiler is faster' or 'compiled code runs faster' on its own; this does not explain why translation must finish before execution.

Do not accept 'it translates and runs the program one line at a time', as this describes an interpreter.

Indicative content

Control of the hardware

• Low-level languages can read and write the contents of registers and memory addresses directly, so the firmware can take the values from the distance sensors without calling a library routine.
• Assembly language instructions can read from and write to specific input/output ports on the vacuum cleaner's processor, so the motors can be switched on and off directly.
• The programmer decides exactly which processor operations are carried out and in which order, which matters when the vacuum must reverse away from a step or an obstacle.
• A low-level language puts nothing between the program and the hardware, so the programmer can control the hardware of the vacuum cleaner precisely.

Efficiency of the code

• Assembly language code can be hand-optimised instruction by instruction, so the code that checks a sensor is as short as possible.
• The programmer can choose which registers hold values that are used again and again, which cuts down the number of memory accesses the firmware needs.
• A hand-written low-level program can be made compact, so it is more likely to fit in the small amount of memory the vacuum cleaner has.
• Because each instruction does one defined job, the time taken to react to a sensor reading is predictable, which suits firmware that must not overrun.

Link between the language and the processor

• Low-level languages are processor-specific, so the instructions used are those of the exact processor in the vacuum cleaner rather than general statements that need translating.
• Each assembly language instruction maps to one machine code instruction, so the programmer knows precisely which processor operation will be carried out by each line of code.
• An assembler turns each mnemonic into its machine code equivalent, so the machine code running on the vacuum cleaner's processor is exactly what the programmer wrote.
• Code written for a different processor would have to be rewritten, which is acceptable here because the vacuum cleaner always uses the same processor.

Levels

• Level 0 (0 marks): No rewardable content.
• Level 1 (1–2 marks): Basic, independent points are made, showing elements of understanding of key concepts/principles of computer science. (AO1) The discussion will contain basic information with little linkage between points made or application to the context. (AO2)
• Level 2 (3–4 marks): Demonstrates adequate understanding of key concepts/principles of computer science. (AO1) The discussion shows some linkages and lines of reasoning with some structure and application to the context. (AO2)
• Level 3 (5–6 marks): Demonstrates comprehensive understanding of key concepts/principles of computer science to support the discussion being presented. (AO1) The discussion is well developed, with sustained lines of reasoning that are coherent and logically structured, and which clearly apply to the context. (AO2)

Within Level 1, award 1 mark for a single basic point and 2 marks for two or more independent basic points; Level 2 requires the points to be linked to the vacuum cleaner; Level 3 requires both control of the hardware and efficiency of the code, and a few concise, linked sentences can reach Level 3, so extra length is not required.

Accept registers, memory addresses, input/output ports, machine code and mnemonics as ways of describing access to the hardware, and accept 'a low-level language is faster' as a basic point on its own. Do not award 'binary is a low-level language' (machine code is written in binary, but binary is not a language), or characteristics of high-level languages such as being easy to read or portable, as characteristics of a low-level language.

Indicative content

How each translator deals with a syntax error

• An interpreter stops as soon as it finds a syntax error and displays an error message.
• With an interpreter, the lines before the error have already been executed, so the student can see how far the program got before it stopped.
• A compiler reports errors at the end of translation, so the student gets a list of all the syntax errors in the program in one go.

How much of the program is translated at a time

• An interpreter translates one line at a time and executes each line immediately.
• A compiler translates the whole program in one go before any of it is run.
• A compiler produces object code, so after every change the whole program must be compiled again before the student can run it.
• An interpreter produces no object code, so the student does not have to wait for the whole program to be translated before it runs.

How this affects the student's testing

• An interpreter suits this student because the program can be run straight away after each small change.
• The interpreter stops at the first error found, so the student fixes that error and runs the program again, but may only find the next error on the next run.
• Reporting every error at the end of compilation means the student can fix several errors at once, but cannot run the program until the whole program compiles.
• The student should use an interpreter while testing, and compile the finished program once it works.

Levels

• Level 0 (0 marks): No rewardable content.
• Level 1 (1–2 marks): Basic, independent points are made, showing elements of understanding of key concepts/principles of computer science. (AO1) The discussion will contain basic information with little linkage between points made or application to the context. (AO2)
• Level 2 (3–4 marks): Demonstrates adequate understanding of key concepts/principles of computer science. (AO1) The discussion shows some linkages and lines of reasoning with some structure and application to the context. (AO2)
• Level 3 (5–6 marks): Demonstrates comprehensive understanding of key concepts/principles of computer science to support the discussion being presented. (AO1) The discussion is well developed, with sustained lines of reasoning that are coherent and logically structured, and which clearly apply to the context. (AO2)

Level 3 responses must relate the way each translator works to the student's testing, not just describe the two translators.

Do not accept answers about run-time or logic errors where no comparison of the translators is made.