System Programming Overview

System Programming
Prof. D. P. Gandhmal
WIT, Solapur

Chapter 1: Language Processor
• Introduction
• Language Processing Activities
• Fundamentals of Language Processing
• Fundamentals of Language Specification
• Language Processing Development Tools

Introduction
• Why Language Processor?
– Difference between the manner in which software
designer describes the ideas (How the s/w should
be) and the manner in which these ideas are
implemented(CPU).

Semantic Gap
Application
Domain
Execution
Domain
Software
Designer
CPU

• Consequences of Semantic Gap
1. Large development times
2. Large development efforts
3. Poor quality of software
• Issues are tackled by Software Engineering
through use of Programming Language.

Application
Domain
Execution
Domain
PL
Domain
Specification Gap Execution Gap

Language Processors
• A Language Processor is a software which
bridges a specification or execution gap.
• Program to input to a LP is referred as a
Source Program and output as Target
Program.
• Languages in which they are written are called
as source language and target languages
respectively.

A Spectrum of Language Processor
• A language translator bridges an execution gap to
the machine language of a computer system.
• A detranslator bridges the same execution gap as
the language translator but in reverse direction.
• A Preprocessor is a language processor which
bridges an execution gap but is not a language
translator.
• A language migrator bridges the specification gap
between two PL’s.

C++
preprocessor
C++
program
C
program
Errors
C++
translator
Errors
C++
program
Machine
language
program

Interpreters
• An interpreter is a language processor which
bridges an execution gap without generating a
machine language program.
• Here execution gap vanishes totally.
Application
Domain
PL
Domain
Execution
Domain
Interpreter
Domain

Application
Domain
Problem Oriented Language
Execution
Domain
PL
Domain

Application
Domain
Procedure Oriented Language
Execution
Domain
PL
Domain

Language Processing Activities
• Divided into those that bridge the
specification gap and those that bridge the
execution gap.
1. Program generation activities.
2. Program execution activities.

Program Generation
• It is a software which accepts the specification
of a program to be generated and generates a
program in the target PL.
Program
generator
Program
specification
Program in
target PL
Errors

Application
Domain
Execution
Domain
PL
Domain
Specification Gap
Program
generator
domain

Example
• A screen handling Program
• Specification is given as below
Employee name : char : start(line=2,position=25)
end(line=2,position=80)
Married : char : start(line =10, position=25)
end(line=10,position=27)
default(‘Yes’)

Yes
Employee Name
Address
Married
Age Gender
Figure: Screen displayed by a screen handling program

Program Execution
• Two models of Program Execution
– Program translation
– Program interpretation

Program Translation
• Program translation model bridges the
execution gap by translating a program
written in PL i.e Source Program into machine
language i.e Target Program.
Translator m/c language program
Source
Program
Target
Program
Errors Data
Figure: Program translation model

Program Interpretation
• Interpreter reads the source program and
stores it in its memory.
• During interpretation it determines the
meaning of the statement.

• Instruction Execution Cycle
1. Fetch the instruction
2. Decode the instruction and determine the
operation to be performed.
3. Execute the instruction.

• Interpretation Cycle consists of—
1. Fetch the statement.
2. Analyze the statement and determine its
meaning.
3. Execute the meaning of the statement.

Source
program
+
Data
Interpreter Memory
PC
Machine
language
program
+
Data
CPU Memory
PC
Errors
Figure (a) : Interpretation Figure (b) : Program Execution

• Characteristics of interpretation
1. Source program is retained in the source
form itself i.e no target program.
2. A statement is analyzed during its
interpretation.

Fundamentals of Language Processing
• Lang Processing = Analysis of SP+ Synthesis of TP.
• Analysis consists of three steps
1. Lexical rule identifies the valid lexical units
2. Syntax rules identifies the valid statements
3. Semantic rules associate meaning with valid
statement.

Example:-
percent_profit := (profit * 100) / cost_price;
Lexical analysis identifies-------
:=, * and / as operators
100 as constant
Remaining strings as identifiers.
Syntax analysis identifies the statement as the
assignment statement.
Semantic analysis determines the meaning of
the statement as
profit x 100
cost_price
to percent_profit

• Synthesis Phase
1. Creation of DS
2. Generation of target code
Refered as memory allocation and code
generation, respectively.
MOVER AREG, PROFIT
MULT AREG, 100
DIV AREG, COST_PRICE
MOVEM AREG, PERCENT_PROFIT
-------------------
PERCENT_PROFIT DW 1
PROFIT DW 1
COST_PRICE DW 1

Analysis Phase and Synthesis phase is not
feasible due to
1. Forward References
2. Memory Requirement

Forward Reference
• It is a reference to the entity which precedes
its definition in the program.
percent_profit := (profit * 100) / cost_price;
……..
……..
long profit;

Language Processor Pass
• Pass I : performs analysis of SP and notes
relevant information.
• Pass II: performs synthesis of target program.
Pass I analyses SP and generates IR which is
given as input to Pass II to generate target
code.

Intermediate Representation (IR)
• An IR reflects the effect of some but not all,
analysis and synthesis tasks performed during
language processing.
Front End Back End
Intermediate
Representation (IR)
Source
Program
Target
Program

• Properties
1. Ease of use.
2. Processing efficiency.
3. Memory efficiency.

Toy Compiler
• Front End
Performs lexical, syntax and semantic analysis of
SP.
Analysis involves
1. Determine validity of source stmt.
2. Determine the content of source stmt.
3. Construct a suitable representation.

Output of Front End
1. Tables of information
2. Intermediate code (IC)
IC is a sequence of IC units, represents meaning
of one action in SP

Example
i: integer;
a,b: real;
a := b+i;
Symbol Table.
No. Symbol Type Length Address
1 i int
2 a real
3 b real
4 i* real
5 temp real

• Intermediate code
1. Convert (id, #1) to real, giving (id, #4)
2. Add (id, #4) to (id, #3) giving (id, #5)
3. Store (id, #5) in (Id, #2)

Lexical Analysis (Scanning)
• Identifies lexical units in a source statement.
• Classifies units into different classes e.g id’s,
constants, reserved id’s etc and enters them
into different tables
• Token contains
– Class code and number in class
Code #no Id #10e.g.

• Example
Statement a := b + i;
a := b + i ;
Id #2 Op #5 Id #3 Op #3 Id #1 Op #10

Syntax Analysis (Parsing)
• Determines the statement class such as
assignment statement, if stmt etc.
e.g.:- a , b : real; and a = b + I ;
real
a b
:=
a
+
b i

Semantic Analysis
• Determines the meaning of the SP
• Results in addition of info such as type, length
etc.
• Determines the meaning of the subtree in IC
and adds info to the IC tree.

Stmt a := b +i; proceeds as
1. Type is added to the IC tree
2. Rules of assignment indicate expression on
RHS should be evaluated first.
3. Rules of addition indicate i should be
converted before addition
i. Convert i to real giving i*;
ii. Add i* to b giving temp.
iii. Store temp in a.

:=
a, real +
b, real i, int
:=
a, real +
b, real i*, int
:=
a, real temp, real
(a) (b)
(c)

Scanning
Parsing
Semantic analysis
Symbol table
Constant table
Other table
Source Program
Lexical
errors
Syntax
errors
Semantic
errors
IC
IR
Figure: Front end of a Toy Compiler
tokens
Parse tree

Back End
1. Memory Allocation
Calculated form its type, length and dimentionality.
No. Symbol Type Length Address
1 i int 2000
2 a real 2001
3 b real 2002

• Code generation
1. Determine places where results should be
kept in registers/memory location.
2. Determine which instruction should be used
for type conversion operations.
3. Determine which addressing modes should
be used for accessing variables.

CONV_R AREG, I
ADD_R AREG, B
MOVEM AREG, A
Figure: Target Code

Memory allocation
Code generation
Symbol table
Constants table
Other tables
Target
program
IR
IC
Figure: Back End of the toy compiler

Fundamentals of Language
Specification
• Programming Language grammars.
alphabets
Words / Strings
Sentences/ Statements
Language

Terminal symbols, alphabet and strings
• The alphabet of L is represented by a greek
symbol Σ.
• Such as Σ = {a , b , ….z, 0, 1,…. 9}
• A string is a finite sequence of symbols.
α= axy

Productions
• Also called as rewriting rule
A nonterminal symbol ::= String of T’s and NT’s
e.g.:
<Noun Phrase> ::= <Article> <Noun>
<Article> ::= a| an | the
<Noun> ::= boy | apple

Grammar
• A grammar G of a language LG is a quadruple
(Σ, SNT, S, P) where
– Σ is the alphabet
– SNT is the set of NT’s
– S is the distinguished symbol
– P is the set of productions

Derivation
• Let production P1 of grammar G be of the
form
P1 : A ::= α
And let β be such that β = γAθ
β = γαθ

Example
<Sentence> :: = <Noun Phrase> <Verb Phrase>
<Noun Phrase> ::= <Article> <Noun>
<Verb Phrase> ::= <Verb> <Noun Phrase>
<Article> ::= a| an| the
<Noun> ::= boy | apple
<Verb> ::= ate

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

Binding and Binding Times
• Program entity pei in program P has a some
attributes. pei
kind
variable procedure Reserved id etc
type dimen Mem addr etc
size

Definition
• A binding is an association of an attribute of a
program entity with a value.
• Binding time is the time at which binding is
performed.
int a;

Different Binding Times
1. Language definition time of L
2. Language implementation time of L
3. Compilation time of P
4. Execution init time of proc
5. Execution time of proc

program bindings(input , output);
var
i : integer;
a,b : real;
procedure proc (x: real; j: integer);
var
info : array[1…10,1…5] of integer;
p : integer;
begin
new (p);
end;
begin
proc(a,i);
end

1. Binding of keywords with its meaning.
e.g.: program, procedure, begin and end.
2. Size of type int is bind to n bytes.
3. Binding of var to its type.
4. Memory addresses are allocated to the variables.
5. Values are binded to memory address.

Types of Binding
• Static Binding:
– Binding is performed before the execution of a
program begins.
• Dynamic Binding:
– Binding is performed after the execution of a
program has begun.

Language Processor Development
Tools (LPDT)
LPDT requires two inputs:
1. Specification of a grammar of language L
2. Specification of semantic actions
Two LPDT’s which are widely used
1. LEX (Lexical analyzer)
2. YACC (Parser Generator)

It contains translation rules of form
<string specification> { <semantic action>}
Front end
generator
Grammar
of L
Semantic
action
Scanning
Parsing
Semantic analysis
Source Program
IR
Front end

Scanner
Parser
LEX
YACC
Source Program in L
IR
Lexical
Specification
Syntax
Specification

LEX
• LEX consists of two components
– Specification of strings such as id’s, constants etc.
– Specification of semantic action

%{
//Symbols definition; 1st section
%}
%%
//translation rules; 2nd section
Specification Action
%%
//Routines; 3rd section

% {
letter [A-Za-z]
digit [0-9]
}%
%%
begin {return(BEGIN);}
end {return(END);}
{letter} ( {letter}|{digit})* {yylval = enter_id();
return(ID);}
{digit}+ {yylval=enter_num();
return(NUM);}
%%
enter_id()
{ /*enters id in symbol table*/ }
enter_num()
{/* enters number in constabts table */}

YACC
• String specification resembles grammar
production.
• YACC performs reductions according to the
grammar.

Example
%%
E : E+T {$$ = gencode(‘+’, $1, $3);}
| T {$$ = $1;}
T : T*V {$$ = gencode(‘*’, $1, $3);}
| V {$$ = $1;}
V : id {$$ = gendesc($1);}
%%
gencode (operator, operand_1, operand_2){ }
gendesc(symbol) { }

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