How to Multiply Two 8-bit Numbers in Assembly Using Only Add Instructions

Learn how to perform multiplication of two 8-bit numbers with the ATmega328's assembly instruction set using only addition operations.
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Understanding Multiplication in Assembly

When working with assembly language, especially on platforms like the ATmega328, one might encounter tasks that require implementing mathematical operations using limited resources. A common exercise is to multiply two 8-bit numbers while only using add instructions. This can be particularly challenging for beginners, but with the right approach, it is definitely manageable!

The Challenge

You are tasked with multiplying two numbers, specifically 0b00111010 (58 in decimal) and 0b01010101 (85 in decimal). The resulting product is a 16-bit number and must be computed using only addition commands. Many learners find themselves struggling with this problem, particularly when it comes to handling the carry bits and ensuring accurate results across multiple iterations.

Step-by-Step Solution

To solve this problem, we will break it down into a series of steps, utilizing assembly instructions effectively. Here is how you can achieve multiplication through bit-shifting and addition.

Preparing the Registers

First, we need to prepare our registers to ensure we have sufficient space for the result and can manipulate bits correctly. We'll use the following registers:

r16 for the first number (the multiplicand)

r18 for the second number (the multiplier)

r0 and r1 for storing the result (lower byte and upper byte respectively)

r17 to track the shifting value of r16

Code Breakdown

Here’s a step-by-step breakdown of the corrected assembly code to perform the multiplication:

[[See Video to Reveal this Text or Code Snippet]]

Key Concepts Explained

Bit Shifting: Each iteration processes one bit of the multiplier. The least significant bit (LSB) is checked to determine whether to add the multiplicand (r16) to the result.

Adding with Carry: The adc (Add with Carry) instruction allows us to consider carry bits produced in previous additions, which is crucial for accurate multiplication.

Using Clear Registers: We start with cleared registers to avoid unintended carry from previous computations. This ensures a clean slate for our arithmetic operations.

Conclusion

Multiplying two 8-bit numbers in assembly can seem daunting at first, especially when limited to addition instructions. However, by understanding how bit manipulation and addition can work together, you can build a functional multiplication routine. This exercise not only enhances your understanding of assembly programming but also gives you insight into low-level data operations—a valuable skill for any programmer or engineer. Happy coding!