Working of Assembler

Assembler divides tasks into two passes:

Pass-1

• Define symbols and literals and remember them in the symbol table and literal table respectively.
• Keep track of the location counter.
• Process pseudo-operations.
• Defines a program that assigns the memory addresses to the variables and translates the source code into machine code.

Pass-2

• Generate object code by converting symbolic op-code into respective numeric op-code.
• Generate data for literals and look for values of symbols.
• Defines a program that reads the source code two times.
• It reads the source code and translates the code into object code.

Firstly, We will take a small assembly language program to understand the working in their respective passes. Assembly language statement format:

[Label] [Opcode] [operand]

Example:  M  ADD  R1, ='3'
where, M - Label; ADD - symbolic opcode; 
R1 - symbolic register operand; (='3') - Literal

Assembly Program:
Label  Op-code   operand   LC value(Location counter)
JOHN   START     200
       MOVER     R1, ='3'    200
       MOVEM     R1, X       201
L1     MOVER     R2, ='2'    202
       LTORG                 203
X      DS        1           204
       END                   205

Let’s take a look at how this program is working:

1. START: This instruction starts the execution of the program from location 200 and the label with START provides a name for the program. (JOHN is the name of the program).
2. MOVER: It moves the content of literal(=’3′) into register operand R1.
3. MOVEM: It moves the content of the register into memory operand(X).
4. MOVER: It again moves the content of literal(=’2′) into register operand R2 and its label is specified as L1.
5. LTORG: It assigns an address to literals(current LC value).
6. DS(Data Space): It assigns a data space of 1 to Symbol X.
7. END: It finishes the program execution.

Assembler is a program for converting instructions written in low-level assembly code into relocatable machine code and generating along information for the loader. It is necessary to convert user-written programs into machinery code. This is called a translation of a high-level language to a low-level that is machinery language. This type of translation is performed with the help of system software. An Assembler can be defined as a program that translates an assembly language program into a machine language program. Self-assembler is a program that runs on a computer and produces the machine codes for the same computer or same machine. It is also known as a resident assembler. A cross-assembler is an assembler that runs on a computer and produces machine codes for other computers.