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14.3 Compilation Phases

There are three stages:

1. Lexical analysis
2. Syntax and semantic analysis
3. Code generation

14.3.1 Lexical Analysis

1. Extra spaces removed

   Print       A$
   becomes...
   Print A$

2. Comments are removed

   REM My lovely program
Print A$
   becomes...
   Print A$

3. Simple error check
   E.g. Illegal variable names are flagged
4. Keywords, variables, constants and operators are converted to tokens.
   Tokens are unique codes that are stored in the symbol table.

14.3.2 Syntax And Semantic Analysis

This is the process of checking to see that the sequence of input characters is a valid sentence.

Examples

• The use of stacks to check that brakets are correctly paired
• Checking to see that string data is not being assigned to an integer variable
• By the use of meta-language such as Backus-Naur form

14.3.3 The Symbol Table

This table contains an entry for every keyword, variable, constant and operator (called identifiers)

Generating the Symbol Table

Example Program

Dim Radius As Single
Dim Area As Single
Const Pi = 3.1415926536 As Single

Input Radius

Area = Pi * Radius * Radius

Token	Item Name	Kind of Item	Data Type	Run Time Value
1
2	Input	Keyword
3	Pi	Constant	Single	3.1415926536
4	Radius	Variable	Single	?
5	=	Operator
6	Area	Variable	Single	?
7
8	*	Operator

Using this table, the program can be tokenised as the lexical string:
2 4
6 5 3 8 4 8 4

Note that the syntax analyser fills in the columns "Kind of Item" and "Data Type". The lexical analyser only adds the "Item Name" and "Run Time Value".

The most common way of organising the symbol table is to use a hash table. The identifier is hashed to a memory location.

14.3.4 Code Generation

In this final phase the machine code (or object code) is produced.

Code Optimisation

Some compilers attempt to make the object code run quickly by removing redundant instructions and by spotting better ways to produce the same effect as the source program.

Disadvantages

• compilation time is increased
• unwanted results may be produced

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