Aug 1998 SW204: Software And Files Design (Question 3) Suggested Solutions

3.	(a) What is the basic purpose of a compiler?		[2]
	The basic purpose of a compiler is to translate source code (1 mark) into an equivalent machine code representation (1 mark). Other sensible answers should also receive credit.
	(b) Describe four differences which exist between a compiler and an interpreter.		[4]
	One mark should be awarded for each correct difference named (up to a maximum of four marks). Examples include the following: A compiler converts a whole program in one go, whereas an interpreter converts a program instruction by instruction. A compiler can retain object code, whereas an interpreter cannot. Code only needs to be translated once by a compiler, whereas translation is needed by an interpreter every time the program is executed. Code optimisation is possible with a compiler, whereas it is virtually impossible with an interpreter. Other sensible answers should also receive credit.
	(c) Describe the five functions performed by a compiler when source code is submitted for compilation.		[10]
	One mark should be awarded for identifying each stage, and a further mark should be awarded for a satisfactory elaboration (up to a maximum of ten marks). Lexical analysis (1 mark). This involves the ‘tidying up’ of source code, which involves checking for valid ‘words’ and standardising formats (1 mark). Syntax analysis (1 mark). This involves determining the structure of the source program and interpreting its meaning (1 mark). Code generation (1 mark). This involves translating each source statement into its corresponding object form (1 mark). Code optimisation (1 mark). This involves manipulating either the source code or the object code to produce a more efficient object program (1 mark). Error handling (1 mark). This involves detecting, and determining the causes of, errors (1 mark). Other sensible answers should also receive credit.
	(d) Draw a diagram to illustrate the behaviour of each of the following types of loader:
		(i) Compile-and-go loaders.	[2]
		Two marks should be awarded for a correct diagram; only one mark should be awarded if flowchart conventions are not followed; otherwise, no marks should be awarded.
		(ii) Absolute loaders.	[2]
		Two marks should be awarded for a correct diagram; only one mark should be awarded if flowchart conventions are not followed; otherwise, no marks should be awarded