Unit-1: Lexical Analysis (INTRODUCTION TO COMPILING)

Unit-1: Lexical Analysis (INTRODUCTION TO COMPILING) in Lexical Analysis uploaded at P.R.Engineering college

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UNIT – 1
INTRODUCTION TO COMPILING
1.1 INTRODUCTION 
TRANSLATORS:
A translator is one kind of program that takes one form of program as the input and converts 
it into another form. The input program is called source language and the output program is 
called target language.
Source Translators Target
Language (Software Language
Programs)
Fig 1.1: Translators
Some example translators are Assemblers, Compilers and Interpreters 
ASSEMBLERS:
It is a translator for an assembly language of a computer. It is the lowest level programming 
language. So it is machine dependent.
Assembly Assembler Machine
Language Language
 (object code)
Fig 1.2: Assembler
COMPILERS:
A compiler is a program that reads a program in one language, the source language and 
translates into an equivalent program in another language, the target language. The 
translation process should also report the presence of errors in the source program.
Source Compiler Target
Program Program
Error
Messages
Fig 1.3: Compiler
INTERPRETER:
It reads a source program written in high level programming language line by line basis 
rather than producing object code. It needs more memory during execution. Some of the 
difference between compilers and interpreters are listed below. Lexical Analysis unitI
Compiler Design Page 2
INTERPRETER COMPILER
Demerit:
 The source program gets interpreted 
every time it is to be executed, and every time 
the source program is analyzed. Hence, 
interpretation is less efficient than compiler.
Merit:
 In the process of compilation the program 
is analyzed only once and then the code is 
generated. Hence compiler is efficient than 
interpreter.
The interpreter s do not produce object code. The compilers produce object code.
Merit:
 The interpreters can be made portal 
because they do not produce object code.
Demerits:
 The compiler has to present on the host 
machine when particular needs to be compiled.
Merits:
 Interpreters are simpler and give us 
improved debugging environment. 
Demerits:
 The compiler is a complex program and 
it requires large amount of memory.
Table1.1: Difference between interpreter and compiler
LOADER:
Loader loads the binary code in the memory ready for execution. It transfers the control 
to the first instruction. It is responsible for locating program in main memory every time it is 
being executed.
There are various loading schemes.
1. Assemble and go loader: The assembler simply places the code into memory and the 
loader executes a single instruction that transfers control to the starting instruction of the 
assembled program.
2. Absolute loader: Object code must be loaded into absolute addresses in the memory to 
run.
3. Relocating loader: This loader modifies the actual instruction of the program during 
the process of loading a program.
LINKER:
It is the process of combining various pieces of code and data together to form a single 
executable file.
Two types of linking are 
Static Linking: All references are resolved during loading at linkage time.
Dynamic Linking: References are resolved during run time. It take advantage of the full 
capabilities of virtual memory.
Loader is a program which performs two functions: loading and link editing. Loading is a 
process in which the re-locatable machine code is read and the re-locatable addresses are altered. 
Then that code with altered instructions and data is placed the memory at proper location. The 
job of link editor is to make a single program from several files of re-locatable machine code. If 
code in on file refers the location in another file then such a reference is called external 
reference. Lexical Analysis unitI
Compiler Design Page 3
SOFTWARE TOOLS :
Many software tools that manipulate source programs first perform some kind of 
analysis. Some examples of such tools include:
1) Structure editor:
 Takes as input a sequence of commands to build a source program.
 Structure editor not only performs the text-creation and modification functions of an 
ordinary text editor, but it also analyzes the program text, putting an appropriate
hierarchical structure on the source program.
 For example, it can supply key words automatically - while …. do and begin….. end.
2) Pretty printers :
 A pretty printer analyzes a program and prints it in such a way that the structure of the
program becomes clearly visible.
 For example, comments may appear in a special font.
3) Static checkers :
 A static checker reads a program, analyzes it, and attempts to discover potential bugs 
without running the program.
 For example, a static checker may detect that parts of the source program can never be
executed.
4) Interpreters :
 Translates from high level language ( BASIC, FORTRAN, etc..) into machine language.
 An interpreter might build a syntax tree and then carry out the operations at the nodes as
it walks the tree.
 Interpreters are frequently used to execute command language since each operator
executed in a command language is usually an invocation of a complex routine such as an
editor or complier.
The following are similar to conventional compilers
Text formatters.
It takes input as a stream of characters, most of which is text to be type set, but some of 
which includes commands to indicate paragraphs, figures or mathematical structures like 
subscripts and superscripts.
Silicon Compiler.
It has a source language that is similar or identical to a conventional programming 
language. The variable of the language represent logical signals (0 or 1) or group of signals in a 
switching circuit. The output is a circuit design in an appropriate language.
Query interpreters.
It translates a predicate containing relational and Boolean operators into commands to 
search a database or records satisfying that predicate. Lexical Analysis unitI
Compiler Design Page 4
1.2 ANALYSIS OF THE SOURCE PROGRAM
The source program is analyzed by the compiler to check whether the program is up to the 
standard of the corresponding programming language or not. A compiler is composed of 
several components called phases, each performing one specific task. A complete 
compilation procedure can be divided into six phases and these phases can be regrouped into 
two parts.
They are,
1. Analysis part
2. Synthesis Part
The analysis part breaks up the source program into constant piece and creates an 
intermediate representation of the source program. Analysis can be done in three phases.
 Lexical Analysis
 Syntax Analysis (or) Hierarchical analysis
 Semantic Analysis
The synthesis part constructs the desired target program from the intermediate 
representation. Synthesis consists of three phases:
 Intermediate code generation
 Code Optimization
 Code generator.
Linear analysis (Lexical analysis or Scanning):
The lexical analysis phase reads the characters in the source program and grouped into 
them as tokens. Tokens are sequence of characters having a collective meaning.
Example:position: = initial + rate * 60.
This statement is grouped into 7 tokens.
Identifiers: position, initial, rate.
Assignment symbol: =
Operators: +, *
Constant: 60
Blanks: eliminated.
Hierarchical analysis (Syntax analysis or Parsing):
It involves grouping the tokens of the source program into grammatical phrases that are 
used by the complier to synthesize output.
Example : position : = initial + rate * 60
:=
Position +
Initial *
Rate 60
Fig 1.4 Syntax Tree (or) Parse Tree Lexical Analysis unitI
Compiler Design Page 5
Semantic analysis:
In this phase checks the source program for semantic errors and gathers type information for 
subsequent code generation phase. An important component of semantic analysis is type 
checking.
Example : int to real conversion.
:=
Position +
Initial *
Rate Int to real
Num(60)
Fig 1.5.Semantic tree
1.2.1 PHASES OF COMPILER
The compiler has a number of phases plus symbol table manager and an error handler. 
The first three phases, forms the bulk of the analysis portion of a compiler. Symbol table 
management and error handling, are shown interacting with the six phases.
Phase: A phase is a logically organized operation that takes input as one representation 
of source program and produces output as another representation without changing its meaning.
Pass: Portion of one or more phases are combined into a module called PASS. A pass 
reads the source program of output of previous pass and makes the transformations of current 
pass and finally writes the output of the current pass into an intermediate file. This intermediate 
file can be used by the subsequent passes. 
Symbol table management (or) Book keeping:
An essential function of a compiler is to record the identifiers used in the source program 
and collect information about various attributes of each identifier. A symbol table is a data 
structure containing a record for each identifier, with fields for the attributes of the identifier.
The data structure allows us to find the record for each identifier quickly and to store or retrieve 
data from that record quickly. When an identifier in the source program is detected by the lexical 
analyzer, the identifier is entered into the symbol table.
For example: The symbol table entries for the statement Position=initial + rate * 60 are
Symbol table
Lexeme Token Attribute
Position Identifier Pointer Lexical Analysis unitI
Compiler Design Page 6
= Operator -
Initial Identifier Pointer
+ Operator -
Rate Identifier Pointer
* Operator -
60 Constant 60
Table 1.2: Symbol Table
Fig 1.6: Phases of Compiler (or) Logical structure of a compiler
The Analysis phases:
As translation progresses, the compiler’s internal representation of the source program 
changes. Consider the statement,
position := initial + rate * 10
The lexical analysis phase reads the characters in the source program and groups them into a 
stream of tokens in which each token represents a logically cohesive sequence of characters, 
such as an identifier, a keyword etc.
The character sequence forming a token is called the lexemefor the token. Certain tokens 
will be augmented by a ‘lexical value’. For example, for any identifier the lexical analyzer 
generates not only the token identifier but also enters the lexeme into the symbol table, if it is not 
already present there. The lexical points to the symbol table entry for this lexeme. The 
representation of the statement given above after the lexical analysis would be:
Language
Dependent 
front Analysis phase
Task
Dependent 
back 
Synthesis 
phase Lexical Analysis unitI
Compiler Design Page 7
id1: = id2 + id3 * 10
Syntax analysis imposes a hierarchical structure on the token stream, w

Title:
Unit-1: Lexical Analysis (INTRODUCTION TO COMPILING)

Subject:
Lexical Analysis

Institute:
P.R.Engineering college

Unit-1: Lexical Analysis (INTRODUCTION TO COMPILING)

Unit-1: Lexical Analysis (INTRODUCTION TO COMPILING) in Lexical Analysis uploaded at P.R.Engineering college

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