13. Program Construction

Question 1

What are the 4 stages of program construction?

Answer 1

1. Lexical analysis
2. Syntax analysis
3. Semantic analysis
4. Code generation and optimisation

Question 2

What happens during lexical analysis?

Answer 2

• comments, unnecessary spaces and line breaks are removed
• keywords, constants and identifiers are replaced by tokens (usually hex tokens)
• these tokens allocate a machine code value to each identifier to identify them in the computer’s memory

Question 3

What is an identifier table?

Answer 3

Identifies all of the variables/ constants in the program and their data types

Question 3

What is a reserved word table?

Answer 3

Identifies all of the words native to the programming language being used e.g. input, output

Question 4

What happens during syntax analysis?

Answer 4

• tokens are checked to see if they match the spelling and grammar expected by analysing each token to determine whether they are using the correct syntax
• if syntax errors are found, error messages are produced

Question 5

What happens during semantic analysis?

Answer 5

• variables are checked to ensure they have been properly declared and used, and that they are the correct data type
• operations are checked to ensure they are legal for the type of variable they are being used on

Question 6

What happens during code generation and optimisation?

Answer 6

• constructs in high level languages are replaced by machine code
• the code is optimised so that it is more efficient

Question 7

What are the aims of code optimisation?

Answer 7

• achieve the required output of the program
• increase the speed of the program
• decrease demand on resources
• not delay the overall compilation process

Question 8

What is a translator?

Answer 8

A program that converts source code (high level languages) into machine/object code.

Question 9

What is a compiler?

Answer 9

• takes the source code and translates it all in one go
• once converted it can be run at any time, but the entire source code file has to be successfully compiled first
• compilers can also optimise code

adv:
- programs run quickly as its already been completely translated
- supplied as an executable file, therefore source code cannot be easily modified

disadv:
- code is translated as a whole, so enough memory must be available to store all the code at once
- cannot spot errors
- must be recompiled every time source code is changed
- object code produced is specific to processor’s architecture

Question 10

What is an interpreter?

Answer 10

• translates source code one instruction at a time
• the resulting machine code is executed immediately
• this process is called interpretation

adv:
- instructions are executed as soon as they are translated
- requires less memory as instructions do not need to be stored for later use
- errors can be spotted quickly

disadv:
- interpreters run slower
- program has to be translated every time it is ran
- interpreters do not produce an executable file, so when being distributed the source code must be provided, which could be modified without permission
- interpreters do not optimise code

Question 11

What is an assembler?

Answer 11

A program that converts assembly language into machine code.