Assembler in Computer

An assembler in a computer is a kind of language processor. It translates the assembly language into machine language. However, the program in assembly language may have some pseudo instructions. These instructions cannot be directly translated to the equivalent machine instruction. In this case, we need to scan the assembly code two or multiple times through the assembler.

Although we can classify the assembler into two types, i.e. single-pass assembler and two-pass/multi-pass assembler. In a single-pass assembler, the entire assembly language program is translated to its equivalent machine language program at once.

In a two-pass or multi-pass assembler, the assembly language program has to be scanned twice or multiple times to translate it completely to the equivalent machine language program. Now lets us learn more about the assembler in computer.

Content: Assembler in Computer

1. What is an Assembler?
2. Types of Assembler
3. How Does the Assembler Work?
4. Design of Assembler

What is an Assembler in System Programming?

Assembler is a system program that accepts a program in assembly language and translates it to a relocatable machine code. Along with the machine language program, it also generates some information that is required by the loader. Let us try to understand what is assembly language and machine language.

The assembler analyzes the source program and identifies the symbolic names associated with the instructions or data of the program. It allocates the memory to the program instructions and associated data. So, the assembler knows the memory address of each instruction and data in the assembly code.

Further, the assembler also maintains a data structure that we refer to as the location counter (LC). LC keep track of the addresses that the next instruction or data in the target program would have. As the assembler processes assembly instructions one by one, it updates the location counter by the size of each instruction or data.

The assembler also maintains two tables mnemonic table and a symbol table. The mnemonic table helps in identifying which mnemonic operation code corresponds to which instruction or data. However, the symbol table helps in identifying which symbolic name corresponds to which instruction or data in the program.

Certainly, after analyzing the assembly code, the assembler generates the target code.

Types of Assembler

One-pass Assembler

One pass assembler or single pass assembler scans the program only once to create the equivalent binary program. In this one scan, the assembler substitutes all the symbolic instructions present in the assembly language program with machine code.

Two-pass/Multi-pass Assembler

The two-pass assembler translates an assembly language program into a machine language program in two passes or multiple passes. Moreover, the sequential passes over the assembly language code and resolves the pseudo-op instructions present in the assembly language code.

Pass 1

• Identifies the symbols and opcode and records them in the symbol table.
• Keep a record of the location counter.
• Processes the pseudo instructions.

Pass 2

• Translate the opcode into the corresponding numeric opcode.
• Generate machine code according to the value of literals and symbols.

How Does Assembler Work?

An assembler processes the assembly language code and translates it to the relocatable machine language code. Moreover, how the assembler does this depends on the number of phases used to convert the assembly code to machine code.

If the assembler is one pass assembler, then it would perform the entire conversion of assembly code to machine code in a single pass or single scan.

If the assembler is a two-pass assembler, then in the first pass, the assembler generates a symbol table. Consequently, in the second pass, it generates the machine code. Let us understand the working with the help of an example.

Consider an assembly language code that will be provided to the assembler.

JHON START 0         //identifies the name of the program

     USING *, 15    //register 15 is the base register

         L 1, FIVE  //Load value of FIVE in register 1

         A 1, FOUR  //Add value in FOUR to the value in register 1

        ST 1, TEMP  // Store updated value of register 1 in TEMP

   FOUR DC F ‘4’   //Define constant 4
  
   FIVE DC F ‘5’   //Define constant 5

   TEMP DS 1F     //Define storage of 1 full word

   END

Now in the first pass, when the assembler scans the assembly code above. It defines symbols and literals, traces the location counter and processes pseudo instructions. So, the intermediate code generated after first-pass is:

Additionally, the second pass generates the machine’s instructions and addresses. The figure below particularly illustrates the machine instructions for the intermediate code generated in the first pass.

Design of Assembler

To design an assembler, the programmer must be aware of the problem statement for which it has to design an assembler. The steps to design an assembler.

1. Specify the problem
2. Specify data structure
3. Define the format of the data structure
4. Specify algorithm
5. Look for modularity (capability of one program to be subdivided into independent programming units)
6. Repeat 1 through 5 on modules.

So, this is all about the assembler in computer that translates an assembly language program to a machine language program. Further, we have also learned how the assembler processes the pseudo instruction to generate the machine code.