Addressing Modes Lecture Notes

Introduction

• Topic: Addressing Modes in Computer Organization and Architecture
• Importance: Key topic for competitive and academic exams

What is Addressing Mode?

• Definition: Addressing mode defines how to interpret the operand bits in an instruction.
• Components of an Instruction:
  • Opcode: Represents the operation to be performed (e.g., addition, subtraction).
  • Operand: Represents the data to be operated on.

Benefits of Addressing Modes

• Efficiency: Allows for a reduction in the number of bits required for instructions.
  • Example: If memory has 1M words, addressing requires 20 bits. Using registers reduces this to 4 bits (for 16 registers).
• Variable Handling: Facilitates the use of variables and pointers in programming.
• Dynamic Data Retrieval: Helps in accessing data at runtime, especially when using loops or user input.

Types of Addressing Modes

1. Implied Addressing Mode
2. Immediate Addressing Mode
3. Register Addressing Mode
4. Indirect Addressing Mode
5. Direct Addressing Mode
6. Auto Increment/Decrement Addressing Mode
7. Relative Addressing Mode
8. Indexed Addressing Mode
9. Base Register Addressing Mode

Differences Between RISC and CISC Systems

• RISC (Reduced Instruction Set Computers): Typically have 4 to 6 addressing modes.
• CISC (Complex Instruction Set Computers): May have many addressing modes, varying by computer organization.

Additional Benefits of Addressing Modes

• Pointers: Allow for indirect addressing, where the operand can point to a memory location.
• Looping: Supports automatic increment/decrement for control variables in loops.
• Relocation: Ensures that addresses can be adjusted when a program is executed in different memory locations, enhancing flexibility.

Conclusion

• Addressing modes are essential for effective data management and instruction execution in programming.
• Next steps: Detailed exploration of each addressing mode in future sessions.