📘 1. Structure of a Compiler

✅ Definition

The structure of a compiler refers to how a compiler is organized into different parts (phases), where each phase performs a specific task in translating a program from high-level language → machine code.

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📊 Overall Structure (Diagram)

Source Program
      ↓
Lexical Analysis
      ↓
Syntax Analysis
      ↓
Semantic Analysis
      ↓
Intermediate Code Generation
      ↓
Code Optimization
      ↓
Code Generation
      ↓
Target (Machine Code)

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🔗 Supporting Components

Two important components work with all phases:

• Symbol Table
• Error Handler

🔹 Symbol Table

• Stores information about variables, functions, identifiers

• Example:

  Variable Name | Type | Scope
  x             | int  | global
  

🔹 Error Handler

• Detects and reports errors in each phase
• Helps in debugging

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🧠 2. Phases of a Compiler (Step-by-Step)

There are 6 main phases:

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🔍 1. Lexical Analysis (Scanner)

✅ Definition

Converts source code into tokens (smallest units).

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📌 Example

int x = 10;

👉 Tokens:

int | x | = | 10 | ;

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🔧 Tasks

• Remove spaces and comments
• Identify keywords, identifiers, operators

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⚠️ Errors

• Invalid characters
• Unknown symbols

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⭐ Key Point (Exam)

👉 “Lexical analyzer converts characters → tokens”

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🔍 2. Syntax Analysis (Parser)

✅ Definition

Checks whether tokens follow grammar rules of the language.

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📌 Example

Correct:

int x = 10;

Incorrect:

int = x 10;

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🌳 Syntax Tree Diagram

    =
   / \
  x  10

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🔧 Tasks

• Build parse tree / syntax tree
• Check structure

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⚠️ Errors

• Missing semicolon
• Wrong structure

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⭐ Key Point

👉 “Parser checks grammar using CFG (Context-Free Grammar)”

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🔍 3. Semantic Analysis

✅ Definition

Checks the meaning of the program.

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📌 Example

int x;
x = "Hello";   ❌ Error

• Type mismatch

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🔧 Tasks

• Type checking
• Variable declaration check
• Scope resolution

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⚠️ Errors

• Undeclared variables
• Type mismatch

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⭐ Key Point

👉 “Ensures program is logically correct”

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🔍 4. Intermediate Code Generation

✅ Definition

Generates machine-independent intermediate code.

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📌 Example

x = a + b

👉 Intermediate Code:

t1 = a + b
x = t1

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🔧 Tasks

• Convert to simple instructions
• Easier for optimization

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⭐ Key Point

👉 “Creates intermediate representation (IR)”

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🔍 5. Code Optimization

✅ Definition

Improves code to make it faster and efficient.

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📌 Example

Before:

x = a + 0

After:

x = a

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🔧 Tasks

• Remove unnecessary code
• Reduce execution time

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⭐ Key Point

👉 “Optimization improves performance without changing output”

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🔍 6. Code Generation

✅ Definition

Converts intermediate code into machine code.

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📌 Example

ADD R1, R2
MOV R1, x

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🔧 Tasks

• Assign registers
• Generate final instructions

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⭐ Key Point

👉 “Final phase produces executable code”

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🧪 3. Example (Full Flow)

Input Code:

int x = a + b;

Output at Each Phase:

Phase	Output
Lexical	int, x, =, a, +, b
Syntax	Parse tree
Semantic	Type checked
Intermediate	t1 = a + b
Optimization	t1 = a + b
Code Generation	Machine instructions

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🎯 4. Important Exam Concepts

👉 Frequently asked:

• List and explain all compiler phases
• Draw compiler structure diagram
• Role of symbol table
• Difference between syntax and semantic analysis
• Examples of each phase

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📝 5. Short Notes (Quick Revision)

• Compiler has 6 phases
• Each phase transforms code step-by-step
• Symbol table + error handler support all phases
• Output: efficient machine code

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📊 6. Final Summary Table

Phase	Input	Output	Main Function	Errors
Lexical Analysis	Source code	Tokens	Break code into tokens	Invalid symbols
Syntax Analysis	Tokens	Parse tree	Check grammar	Syntax errors
Semantic Analysis	Parse tree	Annotated tree	Check meaning	Type errors
Intermediate Code	Annotated tree	IR code	Generate intermediate form	—
Code Optimization	IR code	Optimized IR	Improve efficiency	—
Code Generation	Optimized IR	Machine code	Final output	—

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✅ Final Conclusion

• A compiler works in multiple phases, each with a specific role

• Understanding each phase helps in:

  • Debugging programs
  • Designing compilers
  • Exam preparation

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