Let's begin with a simple calculation. You have to type the "1" key, then the "+" key and then again the "1" key. The result is 2.00, with two digits behind the dot, because the spinbutton "precision" is set to 2. The white field "1+1" is the input field and the gray field with "2.00" the output field. There's no need for a "=" button. You can also type "1+1" into the input field using your keyboard.
Now the smaller 4allCalc dialog window is activated and the width of the window was enlarged by the user. The "V", "W", "X", "Y" edit fields contain variable values or other formulas. 4allCalc replaces the X and Y characters in the main entry field with the occurences of the X and Y field. The image shows 4allCalc with the financial annuity formula. Text between [brackets] is not interpretated and is an explanation for the formula. Formulas can be saved permanently.
Another window offers you scientific functions. "Pi" and "Eul" are constants, the other buttons refer to functions like "sin(45)".
The Regression functionality enables you calculating a variable value if the result ("300") is known. Because of rounding effects, it's possible that the regression doesn't hit the result exactly.
Now you see the OS/2 version of 4allCalc with activated programmers functionality. Because "unsigned char" is selelected, an overflow is indicated. In the input field, two binary values were added.