Difference between one pass and two pass assembler: One Pass Assembler Two Pass Assembler Scans entire source file only once Require two passes to scan source file. First pass – responsible for label definition and introduce them in symbol table. Second pass – translates the instructions into assembly language or generates machine code. Generally • Deals with syntax. • Constructs symbol table • Creates label list. • Indentifies the code segment, data segment, stack segment etc… Along with pass1 pass two is also required which • Generates actual Opcode. • Compute actual address of every label. • Assign code address for debugging the information. • Translates operand name to appropriate register or memory code. • Immediate value is translated to binary strings (1’s and 0’s) Cannot resolve forward references of data symbols. Can resolve forward references of data symbols. No object program is written, hence no loader is required Loader is required as object code is generated. Tends to be faster compared to two pass Two pass assembler requires rescanning. Hence slow compared to one pass assembler. Only creates tables with all symbols no address of symbols is calculated. Address of symbols can be calculated
The correct answer and explanation is:
One Pass Assembler vs Two Pass Assembler
A One Pass Assembler and a Two Pass Assembler differ in the number of times they scan the source code to generate machine code. Below is a comparison of the key differences, along with a detailed explanation:
1. Number of Passes:
- One Pass Assembler: Scans the entire source file only once.
- Two Pass Assembler: Requires two passes over the source code.
- First Pass: Responsible for label definition and symbol table creation.
- Second Pass: Translates the instructions into machine code, using the symbol table for resolving labels and addresses.
2. Symbol Table and Label Handling:
- One Pass Assembler:
- Deals with basic syntax checking, constructs the symbol table, and generates a list of labels.
- However, it cannot resolve forward references, i.e., it cannot handle references to labels that are used before they are defined within the code.
- Two Pass Assembler:
- The first pass creates the symbol table and resolves label definitions, allowing the assembler to handle forward references by the time it completes the second pass.
3. Code Generation and Address Calculation:
- One Pass Assembler:
- The assembler constructs the basic structure of the code but does not calculate actual addresses for labels, meaning it cannot generate the final object code in the first pass.
- Two Pass Assembler:
- The second pass generates the machine code, computes the addresses of labels, assigns code addresses for debugging, and resolves the addresses of operands. It translates symbolic addresses into actual memory or register values.
4. Object Code Generation:
- One Pass Assembler:
- Does not generate object code directly, so no loader is needed.
- Two Pass Assembler:
- Generates object code, meaning a loader is required to load the program into memory during execution.
5. Performance:
- One Pass Assembler:
- Tends to be faster because it scans the source code only once and generates the output on the fly.
- Two Pass Assembler:
- Slower because it requires scanning the source code twice and performing additional steps to resolve labels and addresses.
6. Forward Reference Resolution:
- One Pass Assembler:
- Cannot resolve forward references, meaning it cannot handle labels or data symbols used before they are defined in the program.
- Two Pass Assembler:
- Can resolve forward references, as the first pass ensures all labels are defined and available for the second pass to calculate addresses.
Conclusion:
In essence, the One Pass Assembler is simpler and faster, but it has limitations, especially when handling forward references. The Two Pass Assembler, while slower due to its multiple passes over the code, provides a more robust solution for translating assembly code into executable machine code, especially when dealing with complex programs with forward references and label resolution.