1. Introduction of
Compiler
UNIT I & II

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COMPILERS
 Compilers are basically a Translators
 A compiler is a program takes a program written in a source
language and translates it into an equivalent program in a target
language.

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Analysis of Source Program
 Analysis consists of three phases:
 Linear Analysis (Lexical Analysis or Scanning)

Group the source string into Tokens
 Hierarchical Analysis (Syntax Analysis or Parsing)

Tokens are grouped into nested collections
 Semantic Analysis

Ensure the correctness of the source program

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Phases of Compiler

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Lexical Analysis
 Linear Analysis (Lexical Analysis or Scanning)
 Scan the complete source code
 Source program is broken up into group of strings called tokens
Example: position := initial+rate*60

The identifier position

The Assignment Symbol

The identifier initial

The plus sign

The identifier rate

The multiplication sign

The constant 60

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Syntax Analysis
 Hierarchical Analysis (Parsing)
 Grouping the tokens into grammatical Phrases
 Grammatical Phrases are represented by Parse Tree or Syntax
Tree

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Semantic Analysis
 Type checking
 checks the source program for semantic errors
 collects the type information

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Intermediate Code Generation
 Three Address Code
 Sequence of instructions each of which has atmost three
operands
 Properties:
 Easy to produce
 Easy to translate into target program
Example
t1 := inttoreal(60)
t2 := id3*t1
t3 := id2+t2
id1 := t3

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Code Optimization
 Improve the intermediate code (Improve the running time of the
target program)
Example
t1 := id3*60.0
id1 := id2+t1

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Code Generation
 Intermediate code instruction are translated into sequence of
machine instruction
 Example
MOVF id3,R2
MULF #60.0,R2
MOVF id2,R1
ADDF R2,R1
MOVF R1,id1

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Symbol Table Management
 A data structure containing a record for each identifier
 Stores the information about the attributes of all identifiers
 Attributes : name, type, size etc.
Error Detection and Handling
 It detects and handle the errors (Syntax error, Semantic errors)
 Errors are reported in the form of messages

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Cousins of Compiler
 Preprocessors
 Macro processing
 File inclusion
 Assemblers
 Loaders and Link-Editors

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Grouping of Phases
 Front end
 Primarily depends on the source language and independent
on the target language
 It includes lexical analysis, Syntax analysis and semantic
analysis
 Back end
 Primarily depends on the target language and independent
on the source language
 It includes code generation and code optimization

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Compiler Construction Tools
 Parser Generator
 Scanner Generator
 Syntax directed translation Engine
 Automatic code generator
 Data flow engine

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Lexical Analysis
 Input is scanned completely to identify the tokens
 Tokens (Logical unit)
 Identifier, Keywords, operators etc.

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Specification of Tokens
 Strings and Languages

Finite sequence of Symbols is called Strings

Set of strings over some alphabet is called Language
 Operation on Languages

Concatenation:
 L1L2 = { s1s2| s1∈ L1 and s2∈ L2}

Union
 L1 ∪ L2 = { s| s∈ L1 or s∈ L2}

Kleene Closure
 L* =

Positive Closure
 L+ =
 Regular Expressions

∞
=1i
i
L

∞
=0i
i
L

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Regular Expression
 Notation for representing Tokens
 Ex: Identifiers in Pascal
letter → A | B | ... | Z | a | b | ... | z
digit → 0 | 1 | ... | 9
id → letter (letter | digit ) *

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Recognition of Tokens
 Finite Automata
 DFA –
 NFA –
10 2
a b
start
a
b
10 2
ba
b
b a

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Questions
Part -A
 What are the issues of Lexical analyzer?
 Define compiler.
 What is preprocessor?
 What is finite automata?
 Draw the transition diagram to represent relational operators.
 Define Lexeme, pattern.
Part -B
 Explain the various phases of compiler.
 Explain the compiler construction tools.
 Explain input buffering in detail
 Write and explain about specification of tokens
 Construct NFA from the regular expression.