This document discusses elements of assembly language programming including mnemonic operation codes, symbolic operands, data declaration, instruction format, assembly language statements like imperative, declarative, and assembler directives. It also summarizes the basic steps to develop an assembler including identifying necessary information, designing data structures, and determining necessary processing. Finally, it outlines the pass structure of an assembler including initializing data structures, reading statements, searching tables, evaluating operands, and generating target code.

1. ASSEMBLER
ASSEMBLER
Chapter 4

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Elements of Assembly
Elements of Assembly
Language Programming
Language Programming
Assembly Language: Assembly Language is a machine dependent, low level
programming language which is specific to certain computer system.
Features that simplify programming:
qMnemonic Operation Codes(Opcodes)
Ex: MOVER or MOVEM
qSymbolic Operand:
Ex: DS – Declare as storage
DC – Declare as Constant
qData Declaration:
Ex: X DC ‘-10.5’
Statement Format:
[label] <opcode> <operand spec> [< operand spec >,…..]

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A Simple Assembly
A Simple Assembly
Language
Language
 Possible Operand Forms:
 AREA – Refers to memory word with which the name is
associated.
 AREA + 5 – Refers to memory word 5 words away from
AREA.
 AREA(4) – Indexing with index register 4.
 AREA + 5(4) – By combining above two.

 Each statement has 2 operands, the first operand is always a register
(AREG, BREG, CREG & DREG). The second operand refers to a
memory word using a sysmbolicname & an optional
displacement.



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Instruction Format:
Sign Opcode Reg Memory Operand
Ex: Page 88, Fig 4.3
Assembly Language Statements:
1.Imperative statements
Ex: ADD BREG, ONE
1.Declarative statements
Ex: [Label] DS <constant> [Label] DC ‘<value>’
DS (Declare as Storage): Allots memory word of block value same as tha
of the constant mentioned in the statement.
DC (Declare as constant): Constructs memory word containing the value.
Use of constants: Immediate operands and Literals
1.Assembler directives
Ex: Start <constant>

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A Simple Assembly
scheme
Steps to develop a design specification for an assembler
1. Identify the information necessary to perform a task.
2. Design 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.
Analysis & Synthesis Phase:



Analysis Phase Synthesis Phase
Mnemonic Opcode Length
ADD 01 1
SUB 02 1
Symbol Address
N 113
Source
Program
Target
Program
Data Access
Control Transfer
Mnemonic Table
Symbol Table

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Mnemonics Table is a fixed table which is merely accessed by the analysis and
synthesis phases, while the Symbol table is constructed during analysis and
used during synthesis
Analysis Phase Synthesis Phase

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Pass Structure of
Assembler
Start
Initialize Data Structures
Read Next Statement
Label Present ?
Yes
No
Enter (Lable, LC) in SYMTAB
Search in OPTAB
E

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M/C Instruction ?
LC = LC + Length
DC/DS ?
No
Yes
Type of Directive
Allocate literals update LITTAB POOLTAB
Evaluate operand ADD
Correct SYMTAB E
LTORG ORIGIN END
Calculate Storage Size
No
Yes
Literal Used ?
Enter into LITTAB
No
Yes
A
B C D

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A B C D
END ?
Go to Pass II
Update LC
Generate Intermediate Code
E
Yes
No

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Pass II Structure of
Assembler
Start
Initialize target code area/file set END FLAG=OFF
Read Next Statement
Search in OPTAB
M/C Instruction ?
A
B
Yes
No