Chapter 2a Assembler-Introduction

Chapter 2: Assemblers

Mrs. Sunita M Dol (Aher),Assistant Professor,

Computer Science and Engineering Department,Walchand Institute of Technology, Solapur, Maharashtra

05/02/2023 Mrs. Sunita M Dol, CSE Dept 2

• Elements of Assembly Language Programming• A simple Assembly Scheme• Pass Structure of Assemblers• Design of a two pass Assembler• A Single Pass Assembler for IBM PC

2. Assemblers


• Elements of Assembly Language Programming• A simple Assembly Scheme• Pass Structure of Assemblers• Design of a two pass Assembler• A Single Pass Assembler for IBM PC

2. Assemblers


Assembly Language Programming

• An assembly language is a machine dependent, low level programming language which is specific to a certain computer system.

• Compared to machine language of computer, it provides three basic features:– Mnemonic operation codes

• Eliminates the need to memorize numeric operation code .

– Symbolic operands• Symbolic names can be used.

– Data declaration• Data can be declared in any form e.g. -5, 10.5 etc.


AssemblerAssembly Language Program

Machine Language

Figure: Assembler



• Statement Format

[Label] <Opcode> <operand spec> [<operand spec>….]1. Label:- Is optional.2. Opcode:- Symbolic opcode3. Operand Spec:- <operand spec> has the following syntax

<symbolic name> [+<displacement>][(<index register>)] e.g. AREA

AREA + 5 AREA (4) AREA + 5(4)



• A Simple Assembly Language– Each statement has two operands

• The first operand is always a register (AREG, BREG, CREG and DREG)

• The second operand refers to a memory word.



Instruction Opcode

Assembly Mnemonic

Remarks

00 STOP Stop Execution

01 ADD Op1 Op1+ Op2

02 SUB Op1 Op1 – Op2

03 MULT Op1 Op1* Op2

04 MOVER CPU Reg Memory operand05 MOVEM Memory CPU Reg06 COMP Sets Condition Code

07 BC Branch on Condition

08 DIV Op1 Op1/ Op209 READ Operand 2 input Value10 PRINT Output Operand2

Figure: Mnemonic Operation Codes



• Instruction Format


sign opcode register operand

memory operand

Figure: Instruction Format


• Assembly Language to Machine Language– Find address of variables and labels.– Replace Symbolic address by numeric address.– Replace Symbolic opcode by machine opcode.– Reserve storage for data.



START 101READ NMOVER BREG, ONEMOVEM BREG, TERM

AGAIN MULT BREG, TERMMOVER CREG, TERMADD CREG, ONEMOVEM CREG, TERMCOMP CREG, NBC LE, AGAINMOVEM BREG, RESULTPRINT RESULTSTOP

N DS 1RESULT DS 1ONE DC ‘1’TERM DS 1

END


Figure: Sample program to find n!


START 101READ N 101) + 09 0 113MOVER BREG, ONE 102) + 04 2 115MOVEM BREG, TERM 103) + 05 2 116

AGAIN MULT BREG, TERM 104) + 03 2 116MOVER CREG, TERM 105) + 04 3 116ADD CREG, ONE 106) + 01 3 115MOVEM CREG, TERM 107) + 05 3 116COMP CREG, N 108) + 06 3 116BC LE, AGAIN 109) + 07 2 104MOVEM BREG, RESULT 110) + 05 2 114PRINT RESULT 111) + 10 0 114STOP 112) + 00 0 000

N DS 1 113)RESULT DS 1 114)ONE DC ‘1’ 115) + 00 0 001TERM DS 1 116)

END



Variable Address

AGAIN 104

N 113

RESULT 114

ONE 115

TERM 116

Figure: After LC Processing



LC Opcode Register Address101 09 0 113

102 04 2 115

103 05 2 116

104 03 2 116

105 04 3 116

106 01 3 115

107 05 3 116

108 06 3 113

109 07 2 104

110 05 2 114

111 10 0 114

112 00 0 000

113

114

115 00 0 001

116

Assembly Language Programming• Machine Code

05/02/2023 15

START 101READ XREAD YMOVER AREG, XADD AREG, YMOVEM AREG, RESULTPRINT RESULTSTOP

X DS 1Y DS 1

RESULT DS 1END

Figure: Sample program to find X+Y


Mrs. Sunita M Dol, CSE Dept



X DS 1Y DS 1RESULT DS 1

END

LC101102103104105106107108109110

+ 09 0 108+ 09 0 109+ 04 1 108+ 01 1 109+ 05 0 110+ 10 0 110+ 00 0 000

Opcode Register Memory operand



Variable Address

X 108

Y 109

RESULT 110

Figure: After LC Processing



LC Opcode Register Address101 09 0 108

102 09 0 109

103 04 1 108

104 01 1 109

105 05 0 110

106 10 0 110

107 00 0 000

108

109

110

111

Assembly Language Programming• Machine Code


• Assembly Language Statement– Imperative Statement.

• Indicates an action to be taken during execution of a program.• Eg: MOV, ADD, MULT, etc.

– Declaration Statement.• To reserve memory for variable. [Label] DS <constant> eg: X DS 5 [Label] DC ‘<value>’ eg: X DC ‘3’

– Assembler Directives• Instructs the assembler to perform certain action during

assembly of a program.

START <constant>

END



• Literals and Constants– Literal cannot be changed during program execution– Literal is more safe and protected than a constant.– Literals appear as a part of the instruction.– e.g.

ADD AREG, =‘5’ => ADD AREG, FIVE--------------------

FIVE DC ‘5’



• Assembler Directives– START <constant> : The first word of the target program

by the assembler should be placed in the memory with the address <constant>e.g. START 100

– END [<operand spec>] : It indicates the end of source program. The optional <operand spec> indicates the address of instruction where the execution of the program should begin.



• Advantages of Assembly Language


START 101READ XREAD YMOVER AREG, XADD AREG, YDIV AREG, TWOMOVEM AREG, RESULTPRINT RESULTSTOP

X DS 1Y DS 1RESULT DS 1TWO DC ‘2’

END

LC101102103104105106107108109110111112

+ 09 0 109+ 09 0 110+ 04 1 109+ 01 1 110+ 08 2 112+ 05 0 111+ 10 0 111+ 00 0 000

+ 00 0 002

Opcode Register Memory operand


• Elements of Assembly Language Programming• A Simple Assembly Scheme• Pass Structure of Assemblers• Design of a two pass Assembler• A Single Pass Assembler for IBM PC

2. Assemblers


• Design Specification for an Assembler– Four step approach

• Identify the information necessary to perform a task.• Design a suitable data structure to record the information.• Determine the processing necessary to obtain and maintain the

information.• Determine the processing necessary to perform the task.

A Simple Assembly Scheme


• Synthesis Phase– Consider instruction MOVER AREG, X



X DS 1Y DS 1RESULT DS 1

END

LC101102103104105106107108109110

+ 09 0 108+ 09 0 109+ 04 1 108+ 01 1 109+ 05 0 110+ 10 0 110+ 00 0 000


• Synthesis Phase– we must have the following information to synthesize the

machine instruction corresponding to this statement:• Address of the memory word with which name X is

associated.• Machine operation code corresponding to the mnemonic

MOVER.– The first item of information depends on the source

program. Hence it must be available by the analysis phase. The second item of information does not depend on the source program



• Synthesis Phase– we consider the use of two data structures during the

synthesis phase:• Symbol table• Mnemonics table



• Analysis Phase– The primary function performed by the analysis phase is

the building of the symbol table. – For this purpose it must determine the address with

which the symbolic names in a program are associated. – To determine the address of any instruction, we must fix

the address of all program elements preceding it. This function is called memory allocation.

– To implement memory allocation a data structure called location counter is introduced.



• Analysis Phase– It is initialized to the constant specified in the START

statement.– The location counter is always made to contain the

address of the next memory word in the target program.




Analysis Phase

Synthesis Phase

Mnemonic Opcode lengthADD 01 1

SUB 02 1

Source Program

Target Program

Figure: Overview of Two Pass Assembler

Symbol AddressAGAIN 104

N 113

Symbol Table

Mnemonic Table

Data access

Control access


• Analysis Phase– Isolate the label, mnemonic opcode and operand fields

of a statement.– If a label is present, enter the pair(symbol, <LC counter>)

in a new entry of the symbol table– Check the validity of the mnemonic opcode through a

look-up in the mnemonic table.– Perform LC processing i.e. update the value contained in

LC by considering the opcode and operands of the statement.



• Synthesis phase– Obtain the machine opcode corresponding to the

mnemonic from mnemonics table.– Obtain address of a memory operand from the symbol

table.– Synthesize a machine instruction or the machine form of

a constant, as the case may be.



• Elements of Assembly Language Programming• A Simple Assembly Scheme• Pass Structure of Assemblers• Design of a two pass Assembler• A Single Pass Assembler for IBM PC

2. Assemblers


• Two pass translation– Two pass translation of an assembly language program

can handle forward references easily. – LC processing is performed in the first pass and symbols

defined in the program are entered into the symbol table. – The second pass synthesizes the target form using the

address information found in the symbol table.

Pass Structure of Assembler


• Two pass translation– The first pass constructs an intermediate representation

of the source program for use by the second pass .– This representation consists of two main components—

• data structures, e.g. the symbol table, and • a processed form of the source program



Pass I Pass II

Intermediate Code

Source Program

Target Program

Figure: Overview of Two Pass Assembler

Data Structures

Data access

Control access



• Single pass translation– LC processing and construction of the symbol table

proceed as in two pass translation. – The problem of forward references is tackled using a

process called backpatching. – The operand field of an instruction containing a forward

reference is left blank initially. – The address of the forward referenced symbol is put into

this field when its definition is encountered.



• Single pass translation– The need for inserting the operand’s address at a later

stage can be indicated by adding an entry to the table of incomplete instructions (TII).

– In TII, each entry is a pair (<instruction address>,<symbol>)



• Single pass translation– By the time the END statement is processed, the symbol

table would contain the addresses of all symbols defined in the source program and TII would contain information describing all forward references.

– The assembler can now process each entry in TII to complete the concerned instruction.




• Single Pass Translation– The problem of forward reference can be handled

using a technique called as back patching.– The need for inserting the second operand’s address

at a later stage can be indicated by adding an entry to the Table of Incomplete Instruction (TII)

– The entry in TII is a pair(<instruction address>, <symbol>)


• Single Pass Translation Example

START 100MOVER AREG, XADD BREG, ONEADD CREG, TENSTOP

X DC ‘5’ONE DC ‘1’TEN DC ‘10’

END





END

100 04 1 _ _ _101 01 2 _ _ _102 06 3 _ _ _103 00 0 000104 105 106

Instruction Address

Symbol Making a forward reference

100 X

101 ONE

102 TEN

Figure : TII (Table of Incomplete Instruction)


Symbol AddressX 104

ONE 105

TEN 105

Figure : Symbol Table






END

100 04 1 104101 01 2 105102 06 3 106103 00 0 000104 105 106



• Elements of Assembly Language Programming• A Simple Assembly Scheme• Pass Structure of Assemblers• Design of a Two Pass Assembler• A Single Pass Assembler for IBM PC

2. Assemblers


Design of a Two Pass Assembler

• Pass I:-1. Separate the symbol, mnemonic, opcode and

operand.2. Build Symbol Table.3. Perform LC Processing.4. Construct Intermediate Representation.

• Pass II:-1. Process IR to synthesize the target program.


• Advanced Assembler Directives– ORIGIN:

• The syntax isORIGIN <address specification>where address specification is an <operand spec> or <constant>

• This directives indicates that LC should be set to the address given by <address spec>.

• The ORIGIN is useful when the target program does not consist of consecutive memory word.



Design of a Two Pass Assembler1 START 2002 MOVER AREG, =’5’ 200) + 04 1 2113 MOVEM AREG, A 201) + 05 1 2174 LOOP MOVER AREG, A 202) + 04 1 2175 MOVER CREG, B 203) + 05 3 2186 ADD CREG, =’1’ 204) + 01 2 2127 …..12 BC ANY, NEXT 210) + 07 6 21413 LTORG

=’5’ 211) + 00 0 005=’1’ 212) + 00 0 001

14 …..15 NEXT SUB AREG, =’1’ 214) + 02 1 21916 BC LT, BACK 215) + 07 1 20217 LAST STOP 216) + 00 0 00018 ORIGIN LOOP+219 MULT CREG, B 204) + 03 3 21820 ORIGIN LAST+121 A DS 1 217)22 BACK EQU LOOP23 B DS 1 218)24 END25 =’1’ 219)

Figure : An assembly language illustrating ORIGIN

Statement number 18 sets LC to the value 204 since the symbol LOOP is associated with the address 202


• Advanced Assembler Directives– EQU:

• The syntax is<symbol> ORIGIN <address specification> where address specification is an <operand spec> or <constant>• The EQU statement defines the symbol to represent <address

spec>.• No LC processing is implied.




=’5’ 211) + 00 0 005=’1’ 212) + 00 0 001



Statement 22 introduces the symbol BACK to represent the operand LOOP.


• Advanced Assembler Directives– LTORG:

• The LTORG statement permits a programmer to specify where literals should be placed .

• By default, assembler places the literals after the END statement.

• At every LTORG statement, as also at the END statement, the assembler allocates memory to the literals of a literal pool.

• The pool contains all literals used in the program since start of the program or since the last LTORG statement.




=’5’ 211) + 00 0 005=’1’ 212) + 00 0 001



The literals =’5’ and =’1’ are added to the literal pool in the statement 2 and 6 respectively. The first LTORG statement allocates the addresses 211 and 212 to the values ‘5’ and ‘1’. A new literal pool is now started. The value ‘1’ is put into this pool in statement 15.


START 200MOVER AREG, =‘5’MOVEM AREG, X

L1 MOVER BREG, =‘2’ORIGIN L1+3LTORG

NEXT ADD AREG,=‘1’SUB BREG,=‘2’BC LT, BACKLTORG

BACK EQU L1ORIGIN NEXT+5MULT CREG,=‘4’STOP

X DS 1END

LC

200 +04 1 205201 +05 1 214202 +04 1 206

205 +00 0 005206 +00 0 002207 +01 1 210208 +02 2 211209 +07 1 202

210 +00 0 001211 +00 0 002

212 +03 3 215213 +00 0 000214215 +00 0 004



• Pass I uses the following data structures1. Machine Opcode table (OPTAB)2. Symbol Table (SYMTAB)3. Literal Table (LITTAB)4. Pool Table (POOLTAB)



• Pass-I Data Structures– OPTAB

• OPTAB contains the fields – Mnemonic opcode – Class : the class indicate whether the opcode corresponding to

an imperative statement (IS), a declaration statement (DL) or an assembler directives (AD).

– Mnemonic info: If an imperative statement, the mnemonic info field contains the pair (machine opcode, instruction length) else it contains the id of a routine to handle the declaration or directive statement.

• A SYMTAB entry contains the fields address and length.• A LITTAB entry contains the fields literal and address.



START 200MOVER AREG, =‘5’MOVEM AREG, X

L1 MOVER BREG, =‘2’ORIGIN L1+3LTORG

NEXT ADD AREG,=‘1’SUB BREG,=‘2’BC LT, BACKLTORG

BACK EQU L1ORIGIN NEXT+5MULT CREG,=‘4’STOP

X DS 1END

LC200 +04 1 205201 +05 1 214202 +04 1 206

205 +00 0 005206 +00 0 002207 +01 1 210208 +02 2 211209 +07 1 202

210 +00 0 001211 +00 0 002

212 +03 3 215213 +00 0 000214215 +00 0 004