4.2 Assembly Language: The Two-Pass Assembler Process

This section details the stages involved in the assembly process for a two-pass assembler. A two-pass assembler is a common type of assembler that performs the translation of assembly language instructions into machine code in two distinct passes. Each pass has a specific role in the overall conversion.

Overview of the Two-Pass Process

The two-pass assembler typically involves the following two main stages:

1. First Pass (Pass 1): This pass focuses on analyzing the source code and building the symbol table. The symbol table stores information about identifiers (labels, variables, etc.) used in the assembly program.
2. Second Pass (Pass 2): This pass uses the information gathered in the first pass (primarily the symbol table) to perform the actual translation of assembly instructions into machine code. It also handles addressing modes and generates the final object code.

Detailed Stages

First Pass (Pass 1) - Symbol Table Generation

The first pass is crucial for understanding the structure of the assembly program. It performs the following steps:

1. Scanning and Label Recognition: The assembler reads the source code line by line. It identifies and records all the labels (names given to memory locations) encountered in the code.
2. Symbol Table Creation: A symbol table is created. This table is a data structure that stores information about each identifier. For each label, the symbol table typically records:
   • Name: The identifier (label name).
   • Address: The memory address assigned to the label (this might be tentative in the first pass).
   • Type: The type of the identifier (e.g., label, variable, constant).
3. Address Calculation (Tentative): The assembler calculates a tentative memory address for each label. This initial address might be based on the order in which the labels appear in the source code. This address is often adjusted in the second pass.

The symbol table is a vital output of the first pass and is used by the second pass to resolve labels and determine the correct memory addresses.

Second Pass (Pass 2) - Code Generation

The second pass utilizes the information from the symbol table to generate the machine code. It involves the following steps:

1. Code Generation: The assembler reads the source code again. Using the information in the symbol table, it translates each assembly instruction into its equivalent machine code representation. This involves:
   • Identifying the opcode (operation code) of the instruction.
   • Determining the operands (data or memory addresses) required by the instruction.
   • Generating the corresponding machine code bytes for the opcode and operands.
2. Address Resolution: The assembler uses the symbol table to resolve the memory addresses associated with labels. It replaces the tentative addresses calculated in the first pass with the final, correct addresses. This is particularly important for labels that are not sequential in the source code.
3. Relocation: If the program is assembled into a non-contiguous memory area, the assembler performs relocation. This involves adjusting the addresses in the generated machine code to reflect the actual memory layout.
4. Output: The assembler writes the generated machine code to an output file (the object code). This object code can then be linked with other object files to create an executable program.

Summary Table of Stages

The two-pass assembly process is a fundamental technique in computer science for converting human-readable assembly language into machine-executable code. The symbol table plays a critical role in the process, enabling the assembler to correctly translate instructions and generate the final object code.