Struggling with Assembly Programming – Seeking Assistance

Title: Fixing a Misbehaving Assembly Code for Multiplication

Hello! Today, I’m excited to share an interesting challenge I encountered while working with an assembly language project. This task got me working on debugging and understanding some low-level operations—specifically, multiplication using bitwise operations in assembly. The journey was a thorough learning curve, and I’d like to guide you through what went wrong and how I resolved it. Let’s dive into the world of register manipulation and bitwise arithmetic!

The Problem

I was working on a piece of assembly code intended to perform multiplication without using a direct multiply instruction. This approach is quite common in low-level optimizations or systems where multiplication needs to be controlled at a very granular level, e.g., in microcontrollers without a dedicated multiply instruction.

The goal was simple: multiply two numbers (0xAABB0011 by 0xBBAA00CC) using the addition and bit-shifting technique. However, the code wasn’t functioning as expected. The result in registers was incorrect, and it seemed like an eternal loop was happening.

Analyzing the Code

First, here’s a breakdown of the code’s intended operation:

• Registers Setup: The code initializes registers r0 and r1 for the multiplicand, r2 for the multiplier, and r3 and r4 for storing the product. r5 is used as a counter.

• Loop for Multiplication: The loop uses the least significant bit (LSB) of r2 to decide whether to add the multiplicand to the product. It then shifts r2 and the multiplicand (spread across r0 and r1) accordingly.

The Original Solution’s Issues

Upon examination, I noted a few issues with the setup:

1. Infinite Loop: There’s no termination condition for the loop based on r5. Since we’re multiplying 32-bit numbers, this loop should ideally run 32 times, which wasn’t explicitly checked.

1. Shift and Add Logic: There was incorrect handling of the ADDEQ and ADCS instructions that might have messed up the intended calculations at various points.

Correcting the Approach

Here is how I approached fixing it:

• Loop Control: I implemented a clear exit strategy for the loop. The loop must only execute 32 times (the length of our words being multiplied).

CMP r5, #32
BGE end_loop

• Logic Correction: I meticulously revised the shift-and-add logic to correctly handle both halves of the product and ensure no overflow goes unnoticed.

loop:
    MOV r6, r2
    AND r6, #1
    CMP r6, #1
    ADDEQ r3, r3, r0
    ADCS r4, r4, r1

    LSL r0, r0, #1
    ORR r0, r0, r1, LSR #31
    LSL r1, r1, #1

    LSR r2, r2, #1
    ADD r5, r5, #1
    CMP r5, #32
    BLT loop
end_loop:

Conclusion

After incorporating these changes, the code not only exited properly but also calculated the correct product for the input values. This exercise was a fantastic refresher on the intricacies of bitwise operations and low-level programming. It underscores the fact that assumptions in assembly programming can lead to subtle bugs, which require a return to basic principles and careful step-by-step analysis.

This resolution not only fixed the immediate issue but also deepened my appreciation for the robustness needed in systems programming. Sometimes, going back to the basics can illuminate the most enlightening solutions!