9. Pass - 2 Assembler of Two-pass assembler in System Software by Dr. Mahesh Huddar

Summary

This video explains the two-pass assembler process used in converting assembly language into object programs. In Pass 1, the assembler reads the source code, creates a header record with program details, initializes text records, and processes each instruction while checking opcodes against the opcode table and handling operands. Pass 2 focuses on finalizing the object code by writing the last text record and adding an end record. The tutorial emphasizes understanding the mechanics of assembly language processing and provides practical insights for programmers.

Takeaways

• 😀 A two-pass assembler converts assembly language into an object program through two distinct phases.
• 📜 In Pass 1, the assembler generates an intermediate file that captures the structure of the source assembly program.
• 🔍 In Pass 2, the assembler reads the intermediate file line by line to produce the final object code.
• 🚦 The process starts by checking if the first opcode is 'START', which indicates the beginning of the program.
• 🗂️ The header record is created, including the program name, starting address, and the length calculated from Pass 1.
• 📝 Each opcode is checked against the operation table (OPTAB) to determine if it corresponds to a valid instruction.
• 🔗 If an operand contains a symbol, the assembler looks it up in the symbol table (SYMTAB) to retrieve its address.
• ⚠️ If no symbol is found in the operand field or the opcode is not in OPTAB, the assembler assigns a default value (0) and may flag an error.
• 📏 The assembler ensures that each text record does not exceed 69 columns, finalizing records as necessary.
• 📅 Upon encountering the 'END' opcode, the final text record is written, along with an 'END' record, completing the assembly process.

Q & A

What is the purpose of a two-pass assembler?

A two-pass assembler converts assembly language programs into object code, facilitating the execution of the programs on a machine.

What does the first pass of the assembler do?

The first pass reads the source assembly program and generates an intermediate file containing necessary information for the second pass.

In pass two, what is the first action taken by the assembler?

The assembler reads the first line of the intermediate file and checks if the opcode is 'start'. If so, it initiates the process of writing the header record.

What information is included in the header record?

The header record includes the program name, the starting address (e.g., 4000), and the length of the program.

How does the assembler handle opcodes during pass two?

The assembler checks if each opcode is present in the opcode table. If it exists, it processes the associated operand and writes the corresponding object code.

What is the significance of checking the operand field?

The operand field indicates whether there is a symbol. If a symbol is present, the assembler looks it up in the symbol table to determine its address. If not, a default address of 0 is used.

What happens if an opcode is not found in the opcode table?

If an opcode is not found, it is treated as an assembly directive. The assembler then checks if it is a 'byte' or 'word' directive and converts it to object code accordingly.

What limitation is placed on the text records during the object code writing process?

Text records cannot exceed 69 columns. If adding new object code exceeds this limit, the current text record is completed, and a new one is initialized.

What does the assembler do at the end of the input?

Upon reaching the end of the input, the assembler writes the final text record and then adds the end record, which includes the starting address of the program.

What is the final message conveyed at the end of the video?

The video concludes by encouraging viewers to like, share, and subscribe for more educational content.