How Does ADDI Work in MIPS Assembly Language

MIPS assembly language, a simple and powerful low-level language, includes the addi instruction to perform arithmetic operations, specifically adding an immediate value (a constant) to a register value and storing the result in a destination register. Understanding the addi instruction is crucial for anyone delving into MIPS programming. Let's explore the intricacies of how this instruction operates.

Understanding the ADDI Instruction

At its core, the addi instruction follows a specific format:

addi destination source immediate

Breaking down the components of the addi instruction:

Destination: The register where the result will be stored. Source: The register that contains the value to which the immediate value will be added. Immediate: A 16-bit signed integer that will be added to the value in the source register.

How the ADDI Instruction Works

Fetch the Value from the Source Register: The first step in executing the addi instruction is to fetch the value from the source register. Sign-Extend the Immediate Value: The immediate value (a 16-bit integer) is sign-extended to 32 bits. If the immediate value is negative, it is extended with 1s. If it is positive, it is extended with 0s. Add the Extended Immediate to the Source Value: The sign-extended immediate value is added to the value in the source register. Store the Result in the Destination Register: The sum is then stored in the destination register.

Example

addi t0 t1 10

In this example:

The value in register t1 is taken. The immediate value 10 is added to it. The result is stored in register t0.

Important Notes

It's worth noting that the immediate value is limited to 16 bits, meaning it can range from -32768 to 32767. Additionally, if the result of the addition exceeds the range of a 32-bit signed integer, it can lead to overflow, but this is not detected by the addi instruction.

The Significance of ADDI in MIPS Assembly Programming

The addi instruction is a fundamental building block of MIPS assembly programming. It is particularly useful for:

Incrementing counters Adjusting addresses Performing simple calculations

Real-World Application

Consider a scenario where a loop counter needs to be incremented. Using the addi instruction, you can easily increment the counter without writing complex code. Here's an example:

addi $t0 $t0 1  # Increment the counter in register $t0

This instruction will effectively increase the value in the $t0 register by 1.

Conclusion

Understanding the addi instruction is not just useful for theoretical purposes; it is essential for practical programming tasks. Whether you are working on a microcontroller or a complex system, the addi instruction forms the foundation of many arithmetic operations in MIPS assembly language.