Chap 1-dhamdhere system programming

 System Programming & Operating Systems
 D. M. Dhamdhere
 Chapters : 1 to 8
 Device Drivers

 System:
 Software :
 Computer software, or simply software, refers
to the non-tangible components
of computers, known as computer programs.
The term is used to contrast with computer
hardware, which denotes the physical
tangible components of computers.

 Software can be classified into
◦ System software:
◦ System software (or systems software) is computer software designed to
operate and control the computer hardware and to provide a platform for
running application software.
 System software is collection of software program that perform a variety
of functions like IO management, storage management,generation and
execution of programs etc.
 Operating Systems
 Compiler / Assembler (utility softwares)
 Device Drivers
◦ Application software:
 Application software is kind of software which is designed for fulfillment
specialized user requirement.
 MS Office
 Adobe Photoshop

 The system software work as middleware
between application software and hardware.
Application software
System Software
Hardware

 Language processors (Why?)
◦ Language processing activities arise due to the
differences between the manner in which a software
designer describes the ideas concerning the
behavior of software and the manner in which these
ideas are implemented in computer system.
◦ The designer expresses the ideas in terms related
to the application domain of the software.
◦ To implement these ideas, their description has to
be interpreted in terms related to the execution
domain.

 The term semantics to represent the rules of
meaning of a domain, and the term semantic
gap to represent difference between the
semantics of two domains.
Application
domain
Execution
domain
Semantic
gap

 The semantic gap has many consequences,
some of the important are
◦ Large development times
◦ Large development effort
◦ Poor quality software.
 these issues are tackled by software
engneering thru’ use of methodologies and
programming languages.

 s/w development team
 Programing language processor
Application
domain
PL
domain
Specification
gap
Execution
domain
Execution
gap

 Language processor: A language processor is
software which bridge a specification or
execution gap.
◦ A Language Translator
◦ De-translator
◦ Preprocessor
◦ Language migrator

C++
Program
C++
Preprocessor
C
Program
Errors
C++
Program
C++
Translator
Machine
language
program
Errors

Interpreter domain
 An interpreter is language processor which
bridges an execution gap without generating
a machine language program that means the
execution gap vanishes totally.
Application
domain
PL
domain
Execution
domain

 Three consequences of the semantic gap are
in fact the consequences of specification gap.
 A classical solution is to develop a PL such
that the PL domain is very close or identical
to the application domain.
 Such PLs can only used for specific
applications, they are problem oriented
languages.

 A procedure oriented language provides
general purpose facilities required in most
application domains.
Application
domain
Problem
oriented
language
domain
Specification
gap
Execution
domain
Execution
gap

 Fundamental activities divided into those that
bridge the specification gap and execution
gap.
◦ Program generation activities
◦ Program execution activities

 Program generation activities
◦ A program generation activity aims at automatic
generation of a program.
◦ A source language is a specification language of an
application domain and the target language is
procedure oriented PL.
◦ Program generator introduces a new domain
between the application and PL domain , call this
the program generator domain.
◦ Specification gap now between Application domain
and program generation domain, reduction in the
specification gap increases the reliability of the
generated program.

 This arrangement also reduces the testing
effort.
Program
specification
Program
generator
Program in
target PL
Errors

 Program Execution
◦ Two popular model
 Program Translation
 Program Interpretation

 The program translation model bridges the
execution gap by translating a sources
program into program in the machine or
assembly language of the computer system,
called target program.
Source
program
Translator
m/c Language
program
Target
program
Errors Data

 Characteristics of the program translation
model:
◦ A program must be translated before it can be
executed
◦ The translated program may be saved in a file. The
saved program may be executed repeatedly.
◦ A program must be retranslated following
modifications.

 Program interpretation: during interpretation
interpreter takes source program statement,
determines its meaning and performs actions
which implement it.
 The function of an interpreter is same as the
execution of machine language program by
CPU.

PC
Source
Program
+
Data
Interpreter
Errors
Memory
PC
Machine
language
Program
+
Data
CPU Memory
Program execution
Interpretation

 Characteristics
◦ The source program is retained in the source form
itself, no target program form exists,
◦ A statement is analyzed during its interpretation.
 Comparison
◦ In translator whole program is translated into target
and if modified the source program, whole source
program is translated irrespective to size of
modification.
◦ That not the in case of interpreter, interpretation is
slower than execution of m/c language program.

 Language Processing = Analysis of SP +
Synthesis of TP.
 Analysis phase of Language processing
 Lexical rules which govern the formation of
valid lexical units in the source language.
 Syntax rules which govern the formation of
the valid statements in the source language.
 Semantic rules which associate meaning with
the valid statements of the language.

 The synthesis phase is concerned with the
construction of target language statement
which have same meaning as a source
statement.
◦ Creation of data structures in the target
program(memory allocation)
◦ Generation of target code.(Code generation)
Language Processor
Analysis Phase Synthesis Phase
Source
Program
Target
Program
Errors Errors

percent_profit = (profit*100) / cost_price;
Lexical Analysis
Syntax Analysis
Semantic Analysis

 Forward references: for reducing execution
gap the language processor can performed
on a statement by statement basis.
 Analysis of source statement can be
immediately followed by synthesis of
equivalent target statements. But this may
not feasible due to :Forward reference
 “A forward reference of a program entity is a
reference to the entity which precedes its
definition in the program.”

 Language processor pass: “A language
processor pass is the processing of every
statement in a source program, or its
equivalent representation, to perform a
language processing function.”
 Intermediate representation(IR): “An
intermediate representation is a
representation of a source program which
reflects the effect of some, but not all,
analysis and synthesis tasks performed
during language processing.”

Language Processor
Front end Back end
Source
Program
Target
Program
Intermediate
representation
(IR)

 Semantic Action: “All the actions performed
by the front end, except lexical and syntax
analysis, are called semantic action.
◦ Checking semantic validity of constructs in SP
◦ Determining the meaning of SP
◦ Constructing an IR

 The Front End
◦ The front end performs lexical, syntax and semantic
analysis of the source program, each kind of
analysis involves the following functions:
 Determine validity of source statement from the view
point of the analysis.
 Determine the ‘content’ of a source statement
 For lexical, the lexical class to which each lexical unit
belongs.
 Syntax analysis it is syntactic structure of source program.
 Semantic analysis the content is the meaning of a
statement.
 Construct a suitable representation of source
statement for use by subsequent analysis
function/synthesis phase.

Scanning (Lexical
Analysis)
Parsing (Syntax
Analysis)
Semantic Analysis
Source
Program
IR
Symbol table
Constants
table
Etc..
IC

 Out put of front end produced two
components: (IR)
◦ Table of information
 The symbol table which contain information
concerning all identifier used in the source program.
◦ An intermediate code (IC) which is a description of
the source program.
 The IC is a sequence of IC units, each IC unit
representing the meaning of one action in SP. IC units
may contain references to the information in various
table.

 Lexical Analysis (Scanning):
◦ Lexical analysis identifies the lexical units in source
statement
◦ it then classifies the unit into different classes
◦ Ex. Id’s, Constant reserved id’s etc. and enters them into
different tables.
◦ This classification may be based on the nature of a
string or on the specification of the source language.
◦ Lexical analysis build descriptor called token, for each
lexical unit. It contain two fields class code and number
in class
 Class code: identifies the class to which a lexical unit
belongs.
 Number in class: entry number of lexical unit in the relevant
table.

 Syntax Analysis(Parsing)
◦ Syntax analysis process the string token built by
lexical analysis to determine the statement class
e.g. assignment statement, if statement, etc.
◦ Syntax Analysis builds an IC which represents the
structure of the statement.
◦ IC is passed to semantic analysis to determine the
meaning of the statement.

 Semantic analysis
◦ Semantic analysis of declaration statements differs from
the semantic analysis of imperative statements.
◦ The former results in addition of information to the
symbol table e.g. Type, length and dimensionality of
variables.
◦ The latter identifies the sequence of action necessary to
implement the meaning of a source statement.
◦ When semantic analysis determines the meaning of a
sub tree in the IC, it adds information to a table or adds
an action to sequence of the action.
◦ The analysis ends when the tree has been completely
processed. The update tables and the sequence of action
constitute the IR produced by the analysis phase.

Language Processor
Front end Back end
Source
Program
Target
Program
Intermediate
representation
(IR)
Front end
Scanning
Parsing
Semantic Analysis
LEX
YACC

Lex accepts an input specification which consist
of three components.
1. Definations
2. Rules
3. User Code
This components are seprated by %% symbol.

Defination Section
• It contains declaration of simple name
defination to simplify scanner specification
or in simple words it contains the variables
to hold regular expressions.
• For example, if you want to define D as a
numerical digit, you would write the
following: D [0-9]

Rules Section
• Once you have defined your terms, you can
write the rules section. It contains strings
and expressions to be matched by
the yylexsubroutine, and C commands to
execute when a match is made.
• This section is required, and it must be
preceded by the delimiter %%(double percent
signs), whether or not you have a definitions
section. The lex command does not
recognize your rules without this delimiter.

Defining Patterns in Lex
• X
match the character `x‘
• .
any character except newline.
• [xyz]
a "character class"; in this case, the pattern
matches either an `x', a `y', or a `z‘.
• r*
zero or more r's, where r is any regular expression
• r+
one or more r's

User Code Section
• This section can contain any C/C++ program
code that user want to execute.
• Yylex() function is used to flex compiler ,
which is embeded in this section so user
need to include this function.

Sample flex program
%{
#include <iostream>
%}
%%
[ t] ;
[0-9]+.[0-9]+ { cout << "Found a floating-point number:" << yytext << endl;
}
[0-9]+ { cout << "Found an integer:" << yytext << endl; }
[a-zA-Z0-9]+ { cout << "Found a string: " << yytext << endl; }
%%
main() {
// lex through the input:
yylex();
}

Each string specification in the input to yacc
resembles a grammar production.
The parser generated by yacc performs
reductions according to this grammar.
The action associated with a string specification
are executed when a reduction is made
according to specification.

Finite Automata
• A recognizer for a language is a program that takes
a string x as an input and answers "yes" if x is a
sentence of the language and "no" otherwise.
• One can compile any regular expression into a
recognizer by constructing a generalized transition
diagram called a finite automation.
• A finite automation can be deterministic means
that more than one transition out of a state may be
possible on a same input symbol.
• Both automata are capable of recognizing what
regular expression can denote.

Nondeterministic Finite Automata (NFA)
• A nondeterministic finite automation is a
mathematical model consists of :
1. a set of states S;
2. a set of input symbol, ∑, called the input symbols
alphabet.
3. a transition function move that maps state-symbol
pairs to sets of states.
4. a state so called the initial or the start state.
5. a set of states F called the accepting or final state

Deterministic Finite Automata (DFA)
• A deterministic finite automation is a special case
of a non-deterministic finite automation (NFA) in
which
1. no state has an -transition
2. for each state s and input symbol a, there is at
most one edge labeled a leaving s.
• A DFA has st most one transition from each state
on any input. It means that each entry on any input.
It means that each entry in the transition table is a
single state (as oppose to set of states in NFA).

Syntax Analysis
• During the first Scanning phase i.e Lexical Analysis
Phase of the compiler,symbol table is created by
the compiler which contain the list of leximes or
tokens.
• It is also Called as Hierarchical Analysis or Parsing.
• It Groups Tokens of source Program
into Grammatical Production
• In Syntax Analysis System Generates Parse Tree

Parse Tree Generation :
sum = num1 + num2

Explanation : Syntax Analysis
• We know , Addition operator plus (‘+’) operates on two
Operands
• Syntax analyzer will just check whether plus operator has two
operands or not . It does not checks the type of operands.
• Suppose One of the Operand is String and other is Integer
then it does not throw error as it only checks whether there
are two operands associated with ‘+’ or not .
• So this Phase is also called Hierarchical Analysis as it
generates Parse Tree Representation of the Tokens generated
by Lexical Analyzer

Semantic Analysis
• Syntax analyzer will just create parse tree.
Semantic Analyzer will check actual meaning
of the statement parsed in parse
tree. Semantic analysis can compare
information in one part of a parse tree to
that in another part (e.g., compare reference
to variable agrees with its declaration, or that
parameters to a function call match the
function definition).

Semantic Analysis is used for
the following -
1. Maintaining the Symbol Table for each block.
2. Check Source Program for Semantic Errors.
3. Collect Type Information for Code
Generation.
4. Reporting compile-time errors in the code
(except syntactic errors, which are caught by
syntactic analysis)
5. Generating the object code (e.g., assembler
or intermediate code)

Now In the Semantic Analysis
Compiler Will Check -
1. Data Type of First Operand
2. Data Type of Second Operand
3. Check Whether + is Binary or Unary.
4. Check for Number of Operands Supplied to
Operator Depending on Type of Operator
(Unary | Binary | Ternary)

Fundamentals of Lang. Specification
Terminal Symbol
Denotes character set with all symbols.
this all are metasymbols.
Differentiate from terminal symbol
String – finite sequence of symbols
Nonterminal symbol- name of syntax
category of symbol
denoted by single capital letter
eg. Noun,verb …

• Productions
A productions also called a rewriting rule, is a rule of of grammar.
A production has the form
A Nonterminal symbol = String of Ts and NTs
Example
<Noun Phrase> ::= <Article> <Noun>
<Article> ::= a | an | the
<Noun>::= boy | apple
• Distinguished symbol/start NT of grammer

Programming Language Grammars
• Grammar (G)
A grammar G of a language LG is a
Quadruple (, SNT, S, P) where
 = is the set of Ts
SNT= is the set of NTs
S = is the distinguished symbols /starting
symbol
P= is the set of productions

Derivation
A grammar G is used for two purpose
To generate valid strings of LG
To recognized valid strings of LG
The derivation operations helps to generate valid
strings.
Derivation -- Example
<Noun Phrase> ::= <Article> <Noun>
<Article> ::= a | an | the
<Noun>::= boy | apple
Suppose we want to derivate strings “the boy”
“” denote direct derivation.

<Noun Phrase> <Article> <Noun>
the <Noun>
the boy Leftmost Derivation
<Noun Phrase> <Article> <Noun>
<Article> boy
the boy Rightmost Derivation

<Sentence> <Noun Phrase> <Verb Phrase>
<Article> <Noun><Verb Phrase>
the <Noun><Verb Phrase>
the boy<Verb Phrase>
the boy<Verb><Noun Phrase>
the boy ate <Noun Phrase>
the boy ate <Article> <Noun>
the boy ate an <Noun>
the boy ate an apple

Reductions
The reductions operation helps to
recognize valid strings.

Parse tree
• A parse tree is used to depict syntactic structure of
a valid string as it emerges during a sequence of
derivations or reductions
Recursive Specification
• A grammar is in recursive specification, if NT being
defining in a production, itself occurs in a RHS string of
the production, e..g. X::=AXB
• The RHS alternative employing recursion is called
recursive rules.

Consider the grammar G

[..] denotes an optional specification

• Two types of recursive rules
• Left recursive rule  NT appears on the extreme
left in the recursive rule
• Right recursive rule  NT appears on the extreme
right in the recursive rule

Indirect recursion
Occurs when two or more NTs are defined in terms of
one another.
Such recursion is useful for specifying nested
constructs in a language

Grammars are classified as
• Type–0 (Phrase structure grammar)
α=β (strings of Ts and NTs)
-Permits arbitrary substitutions of strings
-No limitation on production rules: at least one
nonterminal on LHS.
-not relevant to specification of PLs.
Example:
Start = <S>
<S> ⇒<S> <S> <A><B> ⇒<B><A>
<S> ⇒<A> <B> <C> <B><A> ⇒<A><B>
<A> ⇒ a <A><C> ⇒<C><A>
<B> ⇒b <C><A> ⇒<A><C>
<C> ⇒ c <B><C> ⇒<C><B>
<S> ⇒ ε
Strings generated:
ε, abc, aabbcc, cabcab, acacacacacacbbbbbb, ...

Type–1 (Context sensitive
grammar)
αA β = α Πβ
-not relevant to specification of PLs.

Type–2 (Context free grammar)
• A = Π
• Limit production rules to have exactly one nonterminal
on LHS, but anything on RHS.
-suited for programming language specification
Example:
<PAL> ⇒ 0 <PAL> 0 Start = <PAL>
⇒ 1 <PAL> 1
⇒ 0
⇒ 1
⇒ ε
Strings generated:
ε, 1, 0, 101, 001100, 111010010111, …

Type–3 (regular grammar/
linear grammar)
A= tB|t or Bt|t
<id> = l|<id>l|<id>|d
-Limit production rules to have exactly one
nonterminal on LHS and at most one nonterminal
and terminal on RHS:
- restricted to the specification of lexical units
- nesting of construct or matching parenthesis can
not be specified
Example:
<A>⇒<B> 0 Start = <A>
<B> ⇒<A> 1
<A> ⇒ ε
Strings generated:
ε, 10, 1010, 101010, 10101010, …

Operator Grammar (OG)
An Operator grammar is a grammar none of whose
productions contain two or more consecutives NTs
in any RHS alternatives.
• Operator Grammar (OG)
An Operator grammar is a
grammar none of whose
productions contain two or
more consecutives NTs in any
RHS alternatives.

Ambiguity in Grammatic specification
• For a given string and grammar, two distinct parse
tree exists then grammar known as ambiguous
grammar.
• For example
• Two parse tree exist for string a+b*c

<exp> <exp>+<exp>
 <id>+<exp>
 a+<exp>
 a+<exp>*<exp>
 a+<id>*<exp>
 a+b*<exp>
 a+b*<id>
 a+b*c
<exp> <exp>*<exp>
<exp>+<exp>*<exp>
<id>+<exp>*<exp>
a+<exp>*<exp>
a+<id>*<exp>
a+b*<exp>
a+b*<id>
a+b*c

Eliminating ambiguity
• An ambiguous grammar should be rewritten to
eliminate ambiguity.
• The grammar must be rewritten such that
reduction of ‘*’ precedes the reduction of ‘+’ in
string a+b*c
• The normal method of achieving this is to use a
hierarchy of NTs in the grammar and to associate
the derivation or reduction of an operator with an
appropriate NT.

Programming Language Grammars
• Unambiguous Grammar
E:=E+T|T
T:=T*F|F
F:=P
P:=a|b|c

 “A Binding is the association of an attribute of a
program entity with a value”
 Binding time is the time at which a binding is
performed.
 Different Binding times:
◦ Language definition time of L
 The keywords of the programming language L are bounded
to their meanings. Example: main, for, while
◦ Language implementation time of L
 The time when language translator designed example the
size of type int could be bounded to 2 or 4 bytes, its
determined by the architecture of the target machine.
Binding

– Compilation time of P
• The binding of the attributes of variables is
performed. Example the int is bounded with a
variable var.
– Execution init time of Proc
• Memory addresses of local variables of procedure
are bound at every execution init time of proc.
– Execution time of Proc
• Value attribute binding may be done more then one
during the execution of the procedure or function.
Cont.

 The binding time of an attribute of program
entity determines the manner in which a
language processor can handle the use of the
entity.
 This affect execution efficiency of the target
program.
 Type of binding
◦ Static binding:
 Static binding is a binding performed before the execution
of program begins.
◦ Dynamic binding
 Dynamic binding is a binding performed after the execution
of program has begun
Importance of binding time

Lex program for symbol table
• %{
#include<stdio.h>
int flag=0,flag2=0,flag3=0,value;
char *id,*datatype;
%}
datatype int|float|double|char;
%%
{datatype} {flag=1;datatype=yytext;}
([(A-Za-z)]+[(_*)]*[0-9]*)*
{if(flag==1){flag2=1;id=yytext;}else{return
0;}}
[(=?)] {if(flag2==1){flag3=1;}}
([0-9])*
{if(flag3==1){createsymboltable(datatype,id,y
ytext);}}
%%

• createsymboltable(datatype,id,value)
{
printf("datatype=>%snid=>%snvalue=>%
s",datatype,id,value);
}
main()
{
yylex();
}

Chap 1-dhamdhere system programming

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Chap 1-dhamdhere system programming