Index Addressing Mode

Summary

This video tutorial explains the concept of index addressing mode in computer organization and architecture. It covers how operands are addressed in memory by adding the content of an index register to an offset, forming the effective address. The video provides a step-by-step breakdown of memory references during instruction execution, including how many are needed for different operations. Real-life examples, such as processing student data stored in memory, demonstrate the practical use of index addressing mode for handling arrays and structured data. The tutorial also highlights the efficiency of this addressing mode for accessing consecutive memory locations.

Takeaways

• 😀 **Index Addressing Mode** involves calculating the effective address of an operand by adding an offset to the contents of an index register.
• 😀 The **effective address** is the address of the operand, and it is computed by adding the value in the index register to the offset specified in the instruction.
• 😀 **Index Register**: In general, a CPU uses a general-purpose register (e.g., `R1`) as the index register, but some processors use dedicated registers for indexing (e.g., base registers).
• 😀 **Displacement (Offset)** is the signed value representing the distance from the base address. It can be positive or negative, indicating relative memory positions.
• 😀 The **ALU** (Arithmetic Logic Unit) performs the addition operation between the index register and the offset to compute the effective address inside the CPU.
• 😀 **Memory References**: In index addressing mode, a minimum of two memory references are required: one to fetch the instruction and one to retrieve the operand from memory using the effective address.
• 😀 **Instruction Fetch**: The instruction is first fetched from memory, and then the operands are retrieved based on the calculated effective address.
• 😀 A **third memory reference** is required if the instruction also involves writing the result back to memory (e.g., `ADD R2, 20(R1)` requires three memory accesses).
• 😀 Index addressing mode is particularly useful when accessing data structures like arrays, where data elements are stored sequentially in memory.
• 😀 An example involving **student data** illustrates how index addressing mode can be used to calculate addresses for student test marks stored in consecutive memory locations.

Q & A

What is index addressing mode in computer architecture?

In index addressing mode, the effective address of an operand is calculated by adding an offset (displacement) to the value stored in an index register. This mode allows the operand to be located at a memory address relative to a base address stored in the register.

What is the difference between index addressing mode and register indirect mode?

In register indirect mode, the effective address is determined by the content of a register that directly holds the address of the operand. In index addressing mode, the effective address is obtained by adding an offset to the value in the index register, providing more flexibility in locating operands in memory.

What role does the ALU (Arithmetic Logic Unit) play in index addressing mode?

The ALU performs the addition operation to calculate the effective address by adding the content of the index register and the offset. The result is the memory address of the operand, which is then used for data retrieval.

What is an effective address in the context of index addressing mode?

The effective address is the final memory location where the operand is stored. It is the sum of the base address (from the index register) and the offset (displacement).

What happens to the value in the index register after the effective address is calculated?

The value in the index register remains unchanged after the calculation of the effective address. The index register is only used to calculate the address, not to store the result.

How many memory references are required for an index addressing mode instruction?

At least two memory references are required: one to fetch the instruction itself and another to access the operand from memory once the effective address is calculated.

What is the significance of the offset (displacement) in index addressing mode?

The offset (displacement) represents the distance between the base address and the operand’s memory location. It can be both positive or negative, depending on whether the operand is located before or after the base address.

Can index addressing mode be used with any general-purpose register?

Yes, in index addressing mode, you can use any general-purpose register as the index register. However, some processors may have dedicated index registers or base registers for this purpose.

Why is index addressing mode useful when dealing with arrays or structures in memory?

Index addressing mode is particularly useful when accessing data stored sequentially, like elements in an array or fields in a structure. The base address of the array or structure can be stored in the index register, and the offset can be used to access each individual element.

How does index addressing mode help in accessing data like student test scores?

In an example where student data is stored sequentially in memory, index addressing mode allows easy access to each student's test scores by calculating the memory location relative to the start of the student's data (base address) and adding the appropriate offset for each test score.