Introduction of Assembler

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.

Types of Assembler

The assembler generates instructions by evaluating the mnemonics (symbols) in the operation field and finding the value of symbols and literals to produce machine code. On the basis of this functionality, assembler has two types:

• Single-Pass Assembler: If an assembler does all this work in one scan then it is called a single-pass assembler.
• Multiple-Pass Assembler: If it does it in multiple scans then called a multiple-pass assembler.

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.

Working of Pass-1

Define Symbols and literal tables with their addresses. Note: Literal address is specified by LTORG or END.

Step-1: START 200

(here no symbol or literal is found so both table would be empty)

Step-2: MOVER R1, ='3' 200

( ='3' is a literal so a literal table is made)

Step-3: MOVEM R1, X 201

X is a symbol referred before its declaration so it is stored in the symbol table with a blank address field.

Step-4: L1 MOVER R2, ='2' 202

L1 is a label and ='2' is a literal so store them in respective tables

Step-5: LTORG 203

Assign an address to the first literal specified by LC value, i.e., 203

Step-6: X DS 1 204

It is a data declaration statement i.e. X is assigned a data space of 1. But X is a symbol that was referred to earlier in step 3 and defined in step 6. This condition is called a Forward Reference Problem where the variable is referred prior to its declaration and can be solved by back-patching. So now the assembler will assign X the address specified by the LC value of the current step.

Step-7: END 205

The program finishes execution and the remaining literal will get the address specified by the LC value of the END instruction. Here is the complete symbol and literal table made by pass-1 of the assembler.

Now tables generated by pass 1 along with their LC value will go to pass 2 of the assembler for further processing of pseudo-opcodes and machine op-codes.

Working of Pass-2

Pass-2 of the assembler generates machine code by converting symbolic machine-opcodes into their respective bit configuration(machine understandable form). It stores all machine-opcodes in the MOT table (op-code table) with symbolic code, their length, and their bit configuration. It will also process pseudo-ops and will store them in the POT table(pseudo-op table). Various Databases required by pass-2:

1. MOT table(machine opcode table)
2. POT table(pseudo opcode table)
3. Base table(storing value of base register)
4. LC ( location counter)

Take a look at the flowchart to understand:

As a whole assembler works as:

Conclusion

Assembler is a program that converts Assembly Language into machine language. There are two types of assemblers on the basis of a number of phases used to convert to machine code.

• One-Pass Assembler.
• Multi-Pass or Two-Pass Assembler.

One-Pass Assembler converts the whole conversion of assembly code into machine code in one pass or one go. On the other hand, Multi-Pass-Assembler takes multiple passes to convert assembly code into machine language.