6 stages of high level code completion

Stages of complation for high level code

• First off there are 6 stages to Compilation
• 1. Lexical Analysis
• 2. Symbol Table Construction
• 3. Syntax Analysis
• 4. Semantic Analysis
• 5. Code Generation
• 6. Optimisation

1. Lexical Analysis

• �Comments and unneeded spaces are removed.
• �Keywords, constants and identifiers are replaced by ‘tokens’.
• �Examples of keywords that would be replaced:
• �if
• �while
• �for

Symbol Table Construction

• �A symbol table is created to hold variable addresses, labels and subroutines.
• Examples

Syntax Analysis

• Tokens are checked to see of they match the rules (or syntax) for the language used.
• Examples:
• �does every open bracket have a close bracket?
• �Does an if command end with a colon?
• �If syntax errors are found, error messages are produced.

Semantic Analysis

• Variables are checked to see if they are properly declared and used.
• Variables are checked to ensure they are of the correct data type.
• Operators are checked to ensure they are legal.
• Examples:
• �5/3 can not be stored as an integer
• �"cabbage"+3 can not be evaluated

Syntax Error

• If any of these steps detects a problem, a syntax error is produced.
• A syntax error is an error that prevents the code compiling – it is caused by something in the code that is not valid for that programming language
• Examples:
• �Missing semicolon
• �Mismatched brackets or quotes
• �An invalid operator (=> instead of >=)
• �Comma used instead of dot
• Task: During which stage of compilation would each of the above syntax errors be identified

Code Generation

• �Provided there have been no syntax errors produced, the machine code is now generated
• �Complex algorithms are used to create machine code instructions that perform the same task as the high level code.

Optimisation

• �The generated code is scanned for repetitions or known inefficiencies that can be replaced with loops or more efficient code
• �This step is a big factor in the quality and expense of a compiler. A good compiler will produce better, more efficient machine code than a poor one.