1. Assemblers

2. Define
A program that translates programs from assembly language to machine language.

3. Elements:
Mnemonic Operations Codes (Mnemonic opcodes):
 Eliminates:
Need to memorize numeric opcodes
Enables:
Helpful diagnostics
Symbolic Operands:
Symbolic Names
Associated:
Data or Instructions
Performs:
Memory Bindings
Data Declarations
Declared:
Variety of notations
Conversions:
-5 into ( )2

4. Statement Format
[ Label ] <Opcode> <operand spec> [,<operand spec>..]
[..]  Enclosed Specification is optional
Label  Symbolic name with memory
<operand spec>
<symbolic name> [+<displacement>] [<index register>]

5. Assembly Statements: Imperative Statement Declarative Statement
Assembler Statement

6. Imperative Statement:
Indicates: Action to be performed during execution of an
assembly statement
Translates: Into one Machine Instructions

7. Declarative Statement:
Syntax:
[label] DS <constant>
[label] DC `<values>’
DS: Reserves Area of Memory and associated with them
DC: Constructs Memory words containing constants

8. Example: DS
A DS
Reserves a memory area of 1 word and associates with name A.
G DS
Constructs memory words containing constructs. DC
One DC ‘1’
Associates: Name with a memory containing the value ‘1’
Conversions: Different forms Hexadecimal,binary,decimal

9. Constants Use:
Use: Initializes memory words to given values
Values:
Not protected by Assembler
It can be changed by new values into the memory word

10. Constants Type: An immediate Operands: Literals:
Can be used in an assembly statement only if the architecture of the target machine includes the necessary
features.
Literals:

11. Design Specification:
Identify : Necessary to perform task
Design : Suitable data structure to record the information
Determine :
 Processing necessary to obtain and maintain the information
 Processing necessary to perform the task

12. (Depends) Assembly Program
Synthesis Phase:
Assembly Statement:
MOVER BREG,ONE
ONE: Address of Memory Word
(Depends) Source Program
MOVER: Machine Operation Code
(Depends) Assembly Program
Considers Two Data Structures:
1) Symbol Table
2) Mnemonics Table

13. Symbol Table:
(Fields)
Name
Address
Mnemonic Table:
(Fields)
mnemonic
opcode

14. Pass Structure of Assemblers:
Two Pass Translation
Single Pass Translation

15. Two Pass Translation
Can handle forward references easily
1st Pass: LC processing, symbols entered in the symbol table
Symbols defined in the program are entered
Performs: Analysis of Source Program
2nd Pass: Synthesizes the target form using the address
information found in the information table
Performs: Synthesis of Target Program
The first pass constructs an intermediate representation of the source
Program for use by the second pass.

16. Design of a Two Pass Assembler
Pass I : 1. Separate the Symbol, mnemonic opcode and
operand fields
2. Build the Symbol table
3. Perform LC Processing
4. Construct Intermediate Representations
Pass 2 : Synthesize the target Program

17. Advanced Assembler Directives:
ORIGIN <address spec>
<symbol> EQU <address spec>
ORIGIN <address spec>
<address spec> or <constants>s
ORIGIN statement is useful when the target program does not consist of consecutive memory words
<address spec> in the ORIGIN statement provides the ability to perform LC processing in a relative rather than absolute manner.

18. EQU
<symbol> EQU <address spec>
where <address spec> is an <operand spec> or <constant>
EQU  Defines the symbol to represent <address spec>
EQU  Associates the name <symbol> with <address spec>