Assembler Design

Role of Assembler

Source

ProgramAssembler

Object

Code

Loader

Executable Code

Linker

Elements of Assembly Language Format of Statement

[label] <opcode> <operand>[,..<operand>..]

Label – symbolic name associated with memory word.

Opcode – m/c instruction or assembler directive or declaration.

Operand – First operand (Register)

Second operand(Memory Word)

<symbolic name> [+<displacement>] [<Index Register>]

AREA

AREA + 5

AREA(4)

Types of Assembly Statements IMPERATIVE STATEMENTS DECLARATION STATEMENTS ASSEMBLER DIRECTIVES

Imperative Statements Memory Reference/ Transfer Control Arithmetic Logical Flow Control

Imperative StatementsSTOP 00 READ 09

ADD 01 PRINT 10

SUB 02

MULT 03

MOVER 04

MOVEM 05

COMP 06

BC 07

DIV 08

EXAMPLES : MOVER BREG , ONE MOVEM BREG, TERM

NEXT MULT CREG,TERM

COMP CREG, N BC LE, NEXT

DECLARATION STATEMENTS DS – Reserves storage space DC – Declares constant & reserves memory Formats

DS: [label] DS <constant>

e.g. A DS 1

A DS 10

DC: [label] DC ‘<value>’

e.g. T DC ‘5’

Literals vs. DC vs. Immediate OperandsLiterals Constants Immediate

OperandsADD AREG, ‘=5’ ADD AREG,FIVE

……..……..FIVE DC ‘5’

ADD AREG, 5

Value can’t be changed

Value can be changed

No architectural provision is needed

Architectural provision is needed

Assembler Directive Statements Instructs Assembler No m/c equivalent is generated START, END, ORIGIN, LTORG, EQU

START

Syntax: START <constant>

e.g. START 200

END

Syntax: END [<operand spec>]

e.g. END

END 205

ADVANCED ASSEMBLER DIRECTIVES

ORIGIN

Syntax: ORIGIN < address spec>

<address spec> - constant or operand

Set the value of LC to address

EQU

Syntax: <symbol> EQU <Address Spec>

LTORG Assigns memory locations to various literals. Literal Pool BY default assembler places literals after the

END statement.

???? START 100 L1 MOVER AREG, =’6’ BC LE, NEXT ORIGIN L2 C DC ‘5’ N DS 1 END L2

Language Processor Analysis Phase Synthesis Phase

ASSEMBLER: General Design Specification Phases Data Structures Translation Schemes

General Design Specifications

Specify the Problem & identify information

Specify data structures Define the format of the data structures

required Specify the algorithm to be used

Translation Scheme Single Pass Translation Two Pass Translation

Single Pass Assembler Two Pass Assembler

Two Pass Assembler

PASS 1 Isolate label, mnemonic opcodes, operands Uses OPTAB Symbol Table Literal Table Location Counter Processing Construct IC

PASS II Uses OPTAB, SYMTAB, LITTAB Generate machine instructions of the

statements.

Pass 1 Pass 2IR Object programSource program

OPTAB

SYMTABLC

LITTAB

Data Structures Used Symbol Table (SYMTAB) Literal Table (LITTAB) Operation Code Table (OPTAB,POT,MOT) Location Counter (LC) Pool table (POOLTAB)

OPTABMnemonic

OpcodeClass Mnemonic Info

MOVER IS (04,1)

DS DL R#7

START AD R#1

Symbol Table (SYMTAB)SYMBOL LOCATION LENGTH

A 101 3

L1 104 1

Literal Table ( LITTAB)

Literal Address

1 =‘6’

2 =‘1’

POOLTAB

LITERAL NO.

#1

#2

FLOW CHART & ALGORITHM

PASS - I