MSP430FR59941: Calculation error

Welcome to floating point. floats and doubles are only approximations, i.e., they are not infinite precision, and you can expect round off and conversion errors. As a Kernigham and Plauger say: "Working with floating point is like moving sand piles. Every time you move one you lose a little sand and add a little dirt."

You need to work with the significant figures you have.

If you really want better precision with numbers like this, you might need to switch to fixed point, which is faster anyway.

Pleasee read the C programming faq about floating point:

https://c-faq.com/fp/index.html

Just for the record I tried the same thing in C# with MSVS 2022.

Here is the relevant code:

int Status_ALU = 0;
            double alu_calculation = 0.000;

            alu_calculation = 7.7 + 2.4;
            richTextBox1.AppendText("=10.1 :: " + alu_calculation.ToString() + '\n');

            if (alu_calculation == 10.100000)
            {
                
                alu_calculation = 0.000;
                alu_calculation = 9.8 - 5.6;
                richTextBox1.AppendText("=4.2 :: " + alu_calculation.ToString() + '\n');

                if (alu_calculation == 4.2)
                {
                    Status_ALU = 1;
                }
            }
            else
            {
                Status_ALU = 0;
            }

            if (Status_ALU == 1)
            {
                alu_calculation = 5.9 * 2.7;
                richTextBox1.AppendText("=15.93 :: " + alu_calculation.ToString() + '\n');

                if (alu_calculation == 15.93)
                {
                    Status_ALU = 1;
                }
            }
            else
            {
                Status_ALU = 0;
            }

            alu_calculation = 5.9 * 2.7;
            richTextBox1.AppendText("=15.93 :: " + alu_calculation.ToString() + '\n');

And here are the results: