Two Pass Assembler | LDV/LOAD Relative Instruction | Symbol Table | A/AS Level | By ZAK

Two Pass Assembler:
A Two Pass Assembler is a type of assembler that processes the source code of a program in two distinct passes:
1. First Pass:
- Analysis: The first pass involves reading the source code and analyzing it. This pass does not produce any machine code.
- Symbol Table Creation: It creates a symbol table, which is a data structure that stores the addresses of various symbols (like labels and variables) used in the source code.
- Identification of Labels: The assembler identifies all labels and calculates their addresses. These addresses are stored in the symbol table.
2. Second Pass:
- Machine Code Generation: During the second pass, the assembler converts the source code into machine code or object code.
- Symbol Resolution: It uses the symbol table created in the first pass to resolve all symbol references and addresses.
- Assembling Instructions: It assembles instructions and replaces symbols and labels with their corresponding addresses.
LDV/LOAD Relative Instruction:
LDV or LOAD is a type of instruction commonly used in assembly language programming. While the specifics can vary depending on the assembly language, generally:
- LOAD Relative Instruction: This instruction is used to load data into a register from a memory location. The 'relative' aspect typically means that the address of the data is given relative to some point, like the program counter (PC) or another register.
Symbol Table:
- Symbol Table in Assemblers: In the context of assemblers, a symbol table is a crucial data structure. It maps symbolic names (like variable names, constants, routine names) to their actual addresses or values in memory.
- Functionality:
- During the first pass of a two-pass assembler, the symbol table is created and filled with addresses and labels.
- In the second pass, it's used to resolve the addresses of these symbols, allowing the assembler to generate the correct machine code.
- Importance: The symbol table is essential for linking and loading in later stages of the compilation process, as it provides a clear reference of where each symbol is located in the compiled code.