Following Task of An Assembler
An Assembler Must do the Following Task :-
1.
Generate Instruction :-
a) Evaluate the mnemonic in the
operation field to produce the machine code.
b) Evaluate the sub – fields find the
value of each symbols, process literals and assign address.
c) Process pseudo operation.
2.
Pseudo Operation/Assembler Directives :- It is an assembly language instructions that specifies an
operation of the assembler. It direct the assembler during the assembly process
for which no machine code is generate. The directive is a statement which gives
direction to the assembler.
Example :-
Using :- It indicates to the assembler which general register to use as a base and
what its contains will be. This is necessary because no special register are set
aside for addressing.
3.
Equation (EQU) :- This pseudo operation allows the
programmers to define variables.
Example :- BASE EQU 15
END
Here if any one wish to use
a different base register, He would need only to change the EQU
statement.
4.
START :-
It tells the
assembler where the beginning of the program is and allows the user to give a
name to the program.
Example :- PROGRAMMING START --> identifies name of a program
(PROGRAMMING)
5.
END :- It tells the assembler where the last cards of
programs has been reached.
Example :-
i.
LTORG :- It tells the assembler to place the
encountered literals at an earlier location. This pseudo operation is used in
the case of a very long program.
ii.
ORIGIN :- It
helps in placing the machine code in the specified location while translating
the instruction into machine codes by the assembler.
6.
Forward Reference :- A forward reference of a program entity is a reference to the
entity which precedes its defecation in the program.
While processing a statement containing a forward reference a language
processor does not passes all relevant information concerning the referenced
entity. This creates difficulties in synthesizing the equivalent largest
statement. This problem can be solve by postponing the generation of target
code. Until more information becomes available postponing the generation of
target code reduce memory requirement of the language processor and simplify
its organization.
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