Chapter 4 Assemblers

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Chapter 4

Assemblers


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Assembly Language

An Assembly language is a machinedependent,low level programminglangauges which is specific to a certain

computer system (or a family of computersystems).


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Basic Features :

1. Mnemonic operation codes Usage of this eliminates the need to memorize numeric

operation codes

enables the assembler to provide helpful daigonistics ,forexample indication of misspelt operation codes.

2. Symbolic operands Symbolic names can be associated with data or instructions.

Can be used as operands

Assembler performs memory bindings to these names.

3. Data declarations Data can be declared in variety of notations, including the

decimal notation.


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has the followingsyntax:

[+ ] [)]

AREA : memory word with which the nameAREA is associated.

AREA+5 : 5 words away from the word namedwith AREA.5 is the displacement offset fromAREA.

AREA(4) : indexing with index register 4 (theoperand address is obtained by adding thecontents of index register 4 to the address ofAREA.

AREA+5(4) : combination of the previous twospecifications


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Mnemonic Operation code

Mnemonic_operation_codes.doc
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Branch Condition Instruction

Condition can be tested by a branch oncondition (BC) instruction.

Assembly statement corresponding to ithas the format :

BC ,

Transfers control to the memory word with the address

if the current value of condition codematches .

is a character string GT,EQ.


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Assembly Language Statements

Three kinds of statements :

Imperative statements

Declaration statements

Assembler directives


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Imperative statements

It indicates an action to be performedduring the execution of the assembledprogram.


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Declaration Statements

The syntax of declaration statements is asfollows :

[Label] DS ; declare storage

[Label] DC value; declare constant


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Declaration Statements Examples

A DS 1

Reserves a memory area of 1 word and assocaitesthe name A with it. G DS 200

Reserves a block of 200 memory words. Name G is associated with the first word of the

block and can be accessed through offsets fromG,e.g G+5 is the sixth word of the memory block. ONE DC 1

Associates the name ONE with a memory wordcontaining the value 1.

Constants can be declared in different forms : Decimal Binary hexadecimal


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Use of Constants

DC statement does not really implementconstants , it merely initializes memory word togiven values.

These values are not protected by theassembler ;they may be changed by moving anew value into the word.

AP (Assembly Program) can use constants (like

in a HLL) in two ways : Immediate operands literals


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Immediate operands

It should be supported by the architectureof the target machine.

ADD AREG, 5

Translated into an instruction with twooperands AREG and 5 as immediate

operand.

Supported by Intel 8086.

t


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tera A literal is an operand with the syntax =


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Design specification of anassembler

Four step approach to develop a designspecification for an assembler :

1. Identify the information necessary to perform the

task.2. Determine a suitable data structure to record the

information.

3. Determine the processing necessary to obtain

and maintain the information.4. Determine the processing necessary to perform

the task.


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Two phases of the assemblers Synthesis Phase and Analysis Phase

Synthesis Phase

MOVER BREG,ONE

Following information is needed to synthesizethe machine instruction corresponding to thisstatement:

Address of the memory word with which nameONE is associated.

Machine operation code corresponding to themnemonic MOVER.


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Two data structures during theSynthesis phase:

Symbol table Each entry of the symbol table has two primary

fields Name and address

This table is built by the analysis phase Mnemonic Table

Has two primary fields Mnemonic and opcode.

The synthesis phase uses these tables to obtain themachine address with which a name is associated andthe machine opcode corresponding to a mnemonic.

Tables have to be searched with the symbol name andthe mnemonic as keys.


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Analysis Phase Primary function

Build the symbol table. It must determine the address with which the

symbolic names in a program are associated.

It is possible to determine the address of thefirst instruction in the program, others may beinferred.


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Memory Allocation

Fixing the address of all program elementspreceding it is called Memory Allocation.

Location Counter (LC) To implement memory allocation LC data

structure is used. It always made to contain the address of the next

memory word in the target program.

It is initialized to the constant specified in the

START statement. LC Processing

Process involved in maintaining the location counter isknown as LC Processing.


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Tasks performed by the analysisand synthesis phase are as follows

Analysis Phase1. Isolate the label, mnemonic opcode and

operand fields of a statement.

2. If the label is present enter the pair (symbol,) in a new entry of the symboltable.

3. Check validity of the mnemonic opcode

through a look-up in the Mnemonics table.4. Perform LC processing i.e. update the value

contained in LC by considering the opcodeand operands of the statement.


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Synthesis Phase

1. Obtain the machine opcodecorresponding to the mnemonic from theMnemonics table.

2. Obtain address of a memory operandfrom the Symbol table.

3. Synthesize a machine instruction or themachine form of a constant , as the casemay be.