System Software: Components and Language Processing

UNIT-1

Introduction to

System

Programming



 Introduction: Components of System

Software, Language Processing

Activities, Fundamentals of Language

Processing



Assemblers: Elements of Assembly

Language Programming, A simple

Assembly Scheme, Pass structure of

Assemblers, Design of Two Pass

Assembler, Single pass assembler



Macro Processor: Macro Definition and

call, Macro Expansion, Nested Macro

Calls and definition,Advanced Macro

Facilities, Design of Macro Processor



Introduction:



Components of System Software,



 Language Processing Activities,



Fundamentals of Language Processing

Unit-1

Basic



Software



Types of software



Application software-

Word, Excel

 etc.



System Software- 

Compiler, Assembler,O.S.,

Linker 

etc.

Unit-1

Sytem Programming

Application Software



Develop using Programming Lang. and Tools



Application software 

interact

 with systems

software;



systems software then directs computer hardware

to perform 

the necessary tasks

Unit-1

Sytem Programming

System Software



Needed for program development.



Controls operations of computer hardware



Definition: 

System Software Consist of a 

variety

of program

 that support the operation of a

computer.

Unit-1

Sytem Programming

Language processing Model

Source Program

C,C++,java etc

Target Assembly Program

Relocatable M/c Code

Lib+ relocatable obj file

Executable M/c Code

Components of System Software,

Example:



Assembler



Compiler



Macro processor



Loader



Linker



Interpreter



Operating System



Debugger



Text editor

Unit-1

Sytem Programming

Programming Languages: Terminology



Language translator



Systems software that converts a programmer’s

source code into its equivalent in machine language



Source code



High-level program code written by the programmer



Object code



Another name for machine language code

Assembler



A Translator



Assembler make 

two passes 

over the assembly file



First Pass

: reads each line and 

records labels 

in a

symbol table.



Second Pass 

: use info in symbol table to produce

actual machine code for each line.

Unit-1

Assembler…



Assembly lang. a 

symbolic representation 

of machine

language.



 uses a 

mnemonic

 to represent each low-level machine

instruction or operation.



 Assemblers with different 

syntax

 for a particular 

CPU

 or

instruction set architecture

.



Example

:- An instruction 

to add memory data to

     a register

x86

-family processor:  

add eax,[ebx],

    whereas this would be written 

addl (%ebx),%eax

in the 

AT&T syntax

 used by the 

GNU Assembler

.

Unit-1

Compiler



Once the target program is generated, the user

can execute the program.

Unit-1

Compiler…



Early compilers were written in assembly language.



A compiler verifies code syntax,



generates efficient object code,



performs run-time organization,



and 

formats the output 

according to 

assembler



and 

linker

 conventions

Unit-1

Macro Processor



It allows the programmer to write 

shorthand version 

of a

program .



Macro allows a 

sequence of source language 

code to be

defined once and then referred to by name each time it is to

be referred.



Each time this name Occurs in a program, the sequence of

codes is substituted at that point.

Unit-1

Copy code -- Example

Source

STRG 

MACRO

STA

DATA1

STB

DATA2

STX

DATA3

MEND

.

STRG

.

STRG

.

.

Expanded source

.

.

.

STA

DATA1

STB

DATA2

STX

DATA3

.

STA

DATA1

STB

DATA2

STX

DATA3

.

{

{

Unit-1

Loader and Linker



A program which 

accepts object program 

and

prepares them for 

execution.



4 main functions:

i.

Allocation

of space in main memory for the

programs.

ii.

Linking 

of object modules with each other.

iii.

Adjust all address dependent locations.

iv.

Physically 

loading

 the machine instructions and

data into the main memory.

Unit-1

Interpreter



A 

Interpreter

 reads the source code 

one

instruction

 or 

line

 at a  this line

into

machine

code 

or 

some intermediate form

 and 

executes

it.

Statement

execution

Fig.: Interpreter

Unit-1



Operating System- 

Interface between users and

the hardware of a computer system.



Debugger-  

helps to detect error in the program.



Text editor- 

used to create and modify the

program.

Unit-1

Language Processing Activities



It 

Involves

translation

 of a program written in a high

level language 

into

 machine code



It can be divided into two groups:

1.

Program generation activities

2.

Program execution activities

Unit-1

Language Processing Activities..

Unit-1

1.Program generation activities



The program generator is a software which 

accepts

 the

specification of program 

to be generated 

And

generates

a program in 

target

 programming lang.

Unit-1

Sytem Programming

1.Program generation activities…

Sytem Programming

Fig.: Specification and Execution gap

•

 Execution gap is 

bridged

 by the 

compiler 

or

 interpreter

.

•

The program generator 

bridges

 the gap between application

domain & Programming lang. (PL)domain

Unit-1

1.Program generation activities…

•

Example- A Screen handling Program.

•

Employee name:

•

Address:

•

Married:

•

Age                                                Sex

Sytem Programming

Unit-1

2.Program Execution activities



2 Models for Program Execution:

    1.Translation

    2.Interpretion



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.

Unit-1

1.Program Translation

Sytem Programming

Fig.: Program Translation Model

Unit-1

Program Interpretation

Sytem Programming

Unit-1

Fundamental of Language

Processing



Language Processing=Analysis of SP + Synthesis of TP

System Programming

Unit-1

Fig.: Phases of language of language processing

Sp=source prog.

TP=target program

Analysis Phase



Three components:

 1. 

Lexical rules -

the formation of valid lexical units in the

source language.

Example:

percent_profit = (profit * 100) / cost_price;

                     identifies =, * and / operators,

                      100 as constant,

                       and the remaining strings as identifiers.

          2. Syntax rules 

the formation of valid statements in the source

language.

Example :

 percent_profit as the 

left hand side 

and (profit * 100) /

cost_price as the expression on the 

right hand side.

System Programming

Unit-1

Analysis Phase…

3. Semantic rules -

associate meaning with valid

statements of the language.

  example:  

percent_profit = (profit * 100) / cost_price;

 Semantic analysis :

 assignment

 of profit X 100 / cost_price to percent_profit

Sytem Programming

Unit-1

Synthesis Phase



 It performs two main activities:

1.

      Creation of data structures in the target program

(

memory allocation

)

2.

      Generation of target code (

code generation

)

 Example

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

Sytem Programming

Unit-1



Assemblers:



Elements of Assembly Language Programming,



A simple Assembly Scheme,



 Pass structure of Assemblers,



Design of Two Pass Assembler,



Single pass assembler

Unit-1

Assembler



A Translator



Assembler make 

two passes 

over the assembly file



First Pass

: reads each line and 

records labels 

in a

symbol table.



Second Pass 

: use info in symbol table to produce

actual machine code for each line.

Unit-1

Assembler…



The translated program contains 3 kinds of entities:

1.

Relative entities- 

address of instructions and

variables.

2.

Absolute entities- 

operation code , numeric and

string constant and fixed addresses.

3.

The object program- 

addresses are relative

       symbols are defined externally

Sytem Programming

Unit-1

Assembler…



Assembly lang. a symbolic representation of machine

language.



 uses a 

mnemonic

 to represent each low-level machine

instruction or operation.



 Assemblers with different 

syntax

 for a particular 

CPU

 or

instruction set architecture

.



Example

:- An instruction 

to add memory data to

     a register

x86

-family processor:  

add eax,[ebx],

    whereas this would be written 

addl (%ebx),%eax

in the 

AT&T syntax

 used by the 

GNU Assembler

.

Unit-1

Assembler

Elements

 of assembly language programs:

A.

Basic features

B.

Statement format

C.

Operation code

Sytem Programming

Unit-1

A.Basic features

Sytem Programming

•

  Assembly lang. Provides 3 basic features:

1. Mnemonic Operation Codes(Opcodes)

   Ex: MOVER or MOVEM

2. Symbolic Operand:

Ex: DS – Declare as storage

     DC – Declare as Constant

3. Data Declaration:

Ex: X DC ‘-10.5’

Unit-1

B.Statement Format



Statement Format:

[Label]  <opcode>  <operand1> [ <operand2>..]

Label-Optional

Opcode-it contain 

symbolic

 operation code

Operand- Operand can also be a CPU register: AREG,

BREG,CREG.

Example-

LOOP :      MOVER AREG, ‘=5’

Unit-1

Machine Instruction Format

Sytem Programming

Fig.: Machine Instruction format for assembly lang.

•

 Sign is not part of the instruction

•

Reg.operand: AREG, BREG,CREG,DREG.

•

 Memory operand: Refers memory word using symbolic name

Unit-1

Machine supports 11 Different Operations

Unit-1

First

 operand is always a 

CPU register

Second

 operand is always a 

memory operand

Machine supports 11 Different Operations

Unit-1

Assembly language statements



 3 types of Statement in Assembly Language

1.

Imperative

2.

Declaration

3.

Assembler directives

Unit-1

1.Imperative  Statement



Executable statement



Indicates an 

action to be taken.



translates into a machine instruction.



class IS



Ex.:

1.

MOVER BREG, X

2.

STOP

3.

READ X

4.

PRINT Y

5.

BC NE,L1

Unit-1

2.Declaration statements



Reserves memory for variables



Initial value of a variable can also be specified.

[

label

]

   DS   

<

const specifying size of memory to  be reserved

>

[

label

]

   DC  

<

Initial value of variable

>

DS  



 declare storage,

DC 



 declare constant

Class

-DL

>

Unit-1

2.Declaration statements…

X

   DS

1

Y      DS     5

Unit-1

2.Declaration Statements…

ONE

DC

   ‘1’

Unit-1

MOT for declaration statements

Unit-1

3.Assembler Directive



Instructs the assembler 

to perform 

certain actions

during assembly of a program.



A 

directive

 is a 

direction for the assembler



A 

directive

 is also known as 

pseudo instruction



machine code 

machine code 

is 

is 

not generated 

not generated 

for AD.

for AD.

Unit-1

3.Assembler Directive…



START  <Constant>



It indicates that the 

first word 

of the m/c code

should be placed in the memory word with the

address 

<CONSTANT>

Unit-1



END  [<

OPERAND SPECIFICATION

>]



Optional, indicates address of the instruction

where the 

address of program should begin

.



By default, execution begins from the first

instruction.



It indicates the end of the source program.



Class

:AD

Unit-1

3.Assembler Directive…

Advanced Assembler Directives



These directive include:

1.

ORIGIN

2.

Symbol

3.

LTORG

ORIGIN



Useful when m/c code is not stored in consecutive memory

location.



ORIGIN <

address specification>

Operand

 or 

constant

 or 

expression

 containing an operand

 and a constant.



Sets LC to the address given by 

<address specification>



LC 

processing in a relative 

rather than absolute manner

ORGIN…..Example

EQU

Syntax:

<

symbol

>    EQU   <

address specification

>

Where,

<

address specification

> :

can be 

operand specification

 or a

constant

<

symbol

>: EQU 

Associates

 symbol 

with the 

<

address

specification

>

Ex.     

BACK  EQU  LOOP

The symbol 

BACK

 is set to the address of 

LOOP

LTORG



Permits a programmer to specify where literal should be placed.



If the LTORG statement not present, literal are present at the

END statement



At every LTORG Statement, 

memory is allocated 

to the literal

of the current pool of literals.



The 

pool contains all literal 

used in the program since the start

of the program or since the last LTORG statement.

Sytem Programming

MOT for Assemblers Directives

Unit-1

C.Mnemonic Opcode Table(MOT)…

Unit-1

Literal and Constant



A literal is an immediate operand



A literal is an operand with constant value.

   In the c-statement

   int z=5;

   x=y+5;

The constant value is ‘5’ known as literal.



Literal can not be change during program

execution



They are specified using immediate addressing.

Sytem Programming

Unit-1

Literal and Constant…



Literal in assembly language:



Assembly instruction for 8086 with immediate

operand



MOV AX  15  (8086 instruction)



But Hypothetical machine does 

not support

immediate operand.

Sytem Programming

Unit-1

Literal and Constant…



Handling a literal by our machine is as follows:

Sytem Programming

Unit-1

Forward Reference



It is reference to the entity which precedes its

declaration.



Examples

      x=y+5

     int x,y;



The compiler will 

not be able to generate the m/c

code

 for the statement “x=y+5” 

until

 it has been

declaration of two variables x & y

Sytem Programming

Unit-1

Forward Reference…



Sytem Programming

Unit-1

Types of Assembler



3 Types of Assemblers

1.

Load and Go-Assembler

2.

One-pass Assembler

3.

Two-pass Assembler

Sytem Programming

Unit-1

1.Load and Go-Assembler



Simplest form of assembler



It produces machine language as output which

are 

loaded directly 

in main memory and executed



The ability to 

design code 

and test the different

program components in parallel

Sytem Programming

Unit-1

2.One Pass Assembler



Normally , it does 

not allow 

forward referencing.



An assembler 

cannot generate m/c code 

for an

assembly instruction with FR.



Machine code is generated ,after the 

address of

variable 

used in the instruction is known.



Symbol table is used to record the address of the

variables.

Unit-1

One Pass Assembler with

 Forward Reference



FR can be tackled using  technique 

Backpatching

1. 

Operand field 

of instruction containing FR is left 

blank

 initially.

   2. A

 table 

of instruction containing forward reference is made

separately.

(<

Instruction address

>

 ,

<

Symbol making a forward reference

>)

This table can be used o fill-up address in incomplete instruction.

   An example of backpatching:

      Assign addresses to all statements in the program

Sytem Programming

Mnemonic Opcode Table(MOT)

Unit-1

Mnemonic Opcode Table(MOT)…

Unit-1

START    100

M

O

V

E

R

A

R

E

G

,

X

…

…

…

…

.

1

0

0

L

1

A

D

D

B

R

E

G

,

O

N

E

…

…

…

…

.

1

0

1

C

O

M

P

B

R

E

G

,

T

E

N

…

…

…

…

1

0

2

B

C

E

Q

,

L

A

S

T

1

0

3

A

D

D

A

R

E

G

,

O

N

E

1

0

4

B

C

A

N

Y

,

L

1

…

…

…

…

.

1

0

5

L

A

S

T

S

T

O

P

…

…

…

…

…

…

…

…

…

…

…

1

0

6

X

D

C

‘

5

’

…

…

…

…

…

…

…

1

0

7

O

N

E

D

C

‘

1

’

…

…

…

…

…

…

…

…

1

0

8

T

E

N

D

C

’

1

0

…

…

…

…

…

…

…

.

.

1

0

9

END

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Sytem Programming

START    100

M

O

V

E

R

A

R

E

G

,

X

…

…

…

1

0

0

0

4

1

-

-

-

-

-

L

1

A

D

D

B

R

E

G

,

O

N

E

…

…

…

1

0

1

0

1

2

-

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-

-

-

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O

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P

B

R

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,

T

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…

…

1

0

2

0

6

2

-

-

-

-

-

-

B

C

E

Q

,

L

A

S

T

1

0

3

0

7

1

-

-

-

-

-

A

D

D

A

R

E

G

,

O

N

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1

0

4

0

1

1

-

-

-

-

-

B

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A

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Y

,

L

1

…

…

…

…

.

1

0

5

0

7

7

-

-

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-

-

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L

A

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T

S

T

O

P

…

…

…

…

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6

0

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…

…

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…

…

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‘

1

0

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…

…

…

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0

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END

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Sytem Programming

   START    100

   MOVER AREG, X………  

100      04

1

107

L1  ADD BREG, ONE ……… 

101      01

2         

108

COMP BREG, TEN……     

102      06         2         

109

     BC EQ, LAST

103      07          1         

106

     ADD AREG,ONE

104      01          1         

108

     BC ANY, L1………….     

105      07          7         

101

LAST STOP……………………

106       00          00     000

X

DC

‘5’………………   

107       00

00

005

ONE   DC     ‘1’………………   

108       00

00

001

TEN    DC    ‘10’

…………   

109       00

00

010

END

Machine Code Meaning

    100                 04                 1                   

107

Start 100 – is assembler directive,100 LC(location count) 0f first instruction

STOP –M/c opcode-00,absense of register,

108  X    DS     1   

: Memory is reserved but no code is generated.

3.Two Pass Assembler



Two passes(stages),

Pass I

1. 

Separate

 the symbols, mnemonic op-code and operand

fields.

      2. Determine the 

storage requirement 

for every assembly

language statement and update the location counter.

      3. 

Build

 the symbol table.

Pass II

Generate machine code.

Sytem Programming

Design of Two Pass Assembler



First pass has to fix address of variables ,



the second pass can generate the machine code.

Sytem Programming

PASS I

PASS II

Mnemonic    Opcode       length

Source

Program

Assembly

Target

Program

Symbol

Table

Fig.: Data Structures for the Assembler

Working of pass I



Data structures required:



MOT-use to search the opcode



Symbol table-use to search the symbol



Literal table



Pool table :  starting literal number of each pool.

Mnemonic Opcode Table(MOT)

Unit-1

Mnemonic Opcode Table(MOT)…

Unit-1

Mnemonic Opcode Table(MOT)…

Unit-1

Class Field Indicate

Sytem Programming

Symbol  Table



It contains:

1.

Name of variable or a label

2.

Its address

3.

Its size in number of words

4.

Example-

Sytem Programming

 Index

Literal Table



It contains:

1.

Value of the  literal

2.

Address of the memory location associated with

 the literal.

Example-

Sytem Programming

Index

POOL Table



This table contains the literal number of the 

starting

literal 

of each literal pool.

Sytem Programming

Index

Intermediate Code



Is equivalent representation of source program.



Pass-I of the assembler involve 

scanning of the source file.



Every opcode is searched in MOT



Every operand is searched in symbol table.



It helps in avoiding:

 1. Scanning of source file in PASS-II

 2. Searching MOT and ST in PASS-II

Sytem Programming

Format of Intermediate Code



Each Mnemonic opcode field is represented as:

(  

Statement class

     ,    

Machine code  

)

IS

MOT entry of opcode

 AD

 DL

Ex.:  MOVER  



 (IS,04)

LTORG  



  (AD, 05)

         START



(AD,01)

DC  



  (DL,02)

Operand



(   Operand Class,    

reference

  )

C: constant

for a symbol or

S:  symbol

literal , reference

L:  literal(varible)

field contains the

RG: register

index of the operand’s

CC:condition code

entry in the symbol

table or literal table

START    200

MOVER AREG, ‘=5’………….  

200

MOVEM AREG, X    …………

201

L1

MOVER BREG, ‘=2’………….

202

ORIGIN L1+3

LTORG                      …………

205

NEXT

ADD AREG, ‘= 1’       …………

206

SUB BREG,’=2’           …………

207

BC LT, BACK              ………….

208

LTORG

210, 211

BACK  EQU  L1

           ………… 

202

             ORIGIN NEXT+5         ………….211

MULT AREG,’=4’         …………

212

STOP……………………………

213

X

DS

‘5’……………………….

214

END 

215

202+3=205

Index

Index

 Index

START    200

………..

(AD,01)    (C,200)

MOVER AREG, ‘=5’…..

200 ….(IS,04)     (RG,01)    (L,0)

MOVEM AREG, X……

201….(IS,05)     (RG,01)    (S,0)

 L1

MOVER BREG, ‘=2’…..

202 ….(IS,04)     (RG,02)    (L,1)

ORIGIN L1+3

(AD,03)    (C,205)

LTORG 

205……(DL,02)

     (C,5)

206

……(DL,02)     (C,2)

NEXT

ADD AREG, ‘= 1’…….

207 ….(IS,01)     (RG,01)    (L,2)

SUB BREG,’=2’  ………

208 ….(IS,02)     (RG,02)    (L,3)

BC LT, BACK………….   

209 ….(IS,07)     (CC,02)    (S,3)

LTORG

………..

210,

……(DL,02) (C,1)

211

……(DL,02)

     (C,2)

BACK EQU L1

………..

202 

………..

(AD,01)    (C,202)

ORIGIN NEXT+5

………..

(AD,01)    (C,212)

MULT AREG,’=4’……..

212 ….(IS,03)     (RG,03)    (L,4)

STOP……………………

213 ….(IS,00)

X

DS

‘5’…………….

214 

……(DL,01)

     (C,1)

END 

         -------(AD,02)

215 ………(DL,02) (C,4)

A

s

s

e

m

b

l

y

P

r

o

g

r

a

m

L

C

I

n

t

e

r

m

e

d

i

a

t

e

C

o

d

e

       START    100

        MOVER AREG, ‘=5’

        ADD CREG, ‘= 1’

        A  DS ‘3’

L1

   MOVER AREG, B

        ADD AREG,C

       MOVEM AREG,D

LTORG

  EQU A+1

L2  PRINT D

      ORIGIN A-1

      SUB AREG,’=1’

     MULT CREG,B

      C DC ‘5’

      ORIGIN  L2+1

      STOP

      B DC 19

      END

  START    100

MOVER AREG, 

‘=5’

     ADD CREG, 

‘= 1’

A

DS

‘3’

  L1: MOVER AREG, B

        ADD AREG,C

        MOVEM AREG,D

LTORG

       (AD,01)  (C,100)

100

(IS,04) (RG,01) (L,0)

101

(IS,01) (RG,03) (L,1)

102

(DL,01) (C,3)

105

(IS,04) (RG,01) (S,2)

106

(IS,01) (RG,01) (S,3)

107

(IS,05) (RG,01) (S,4)

108 

(DL,02)(C,5)

109  

(DL,02)(C,1)

      D  

EQU A+1

  L2 PRINT D

      ORIGIN A-1

      SUB AREG,’=1’

      MULT CREG,B

C DC ‘5’

    ORIGIN  L2+1

     STOP

B

DC   19

     END

     (AD,04) (C,103)

110 

(IS,10) (S,4)

     (AD,03) (C,101)

101

(IS,02) (RG,01) (L,2)

102

(IS,03) (RG,03) (S,2)

103

(DL,02)(C,5)

    (AD,03) (C,111)

111

(IS,00)

112

(DL,02) (C,19)

(AD,02)

113

(DL,02) (C,1)

 (AD,01)  (C,100)

100 

(IS,04) (RG,01) (L,0)

101 

(IS,01) (RG,03) (L,1)

102 

(DL,01) (C,3)

105 

(IS,04) (RG,01) (S,2)

106 

(IS,01) (RG,01) (S,3)

107

(IS,05) (RG,01) (S,4)

108

109

100)

04

  01

108

101)  01    03

109

102)

105)    04

   01

112

106)    01

   01

103

107)    05

   01

103

108)    00

   00

005

109)    00

   00

001

(AD,04) (C,103)

110

(IS,10) (S,4)

(AD,03) (C,101

)

101

(IS,02)(RG,01) (L,2)

102

(IS,03) (RG,03) (S,2)

103

(DL,02)(C,5)

(AD,03) (C,111)

111

(IS,00)

112

(DL,02) (C,19)

(AD,02)

113

(DL,02) (C,1)

110)

10

00

103

101)

02

01

113

102)

03

03

112

103)

00

00

005

111)

00

00

000

112)

00

00

019

113)

00

00

001

Sytem Programming



Macro Processor:



Macro Definition and call,



Macro Expansion,



Nested Macro Calls and definition,



Advanced Macro Facilities,



Design of Macro Processor

Macro Processor



Macro allows a sequence of source language code to be

defined once and then referred to by name each time it is to

be referred.



Each time this name Occurs in a program, the sequence of

codes is substituted at that point.



It allows the programmer to write shorthand version of a program .

Unit-1