Section 12: Subroutines192
Keystrokes Display    
C (  
  300 _    
  The distance.
  2.25 _    
  The rate.
You can continue to execute the local label programs any number of times using the local label keys without executing the main program each time. All you do is press A ( Σ+ ), B ( 1/X ), or C (   ) in USER mode. But when the calculator is positioned outside of the SPEED program, pressing the local keys search only the current program. If they are not found, the function printed on or above the key is executed.
Application Module Function Execution
When you execute programs that are contained in an application module, the prompt in the program is changed from XEQ name to XROM name . This lets you know that the program is in the application module, not in program memory.
  1. Look closely at the program for finding roots r1 and r2 of a quadratic equation (page 182). Can you see other instructions that could be replaced by a subroutine? (Look at lines 3 through 17 and lines 20 through 24.) Modify the program by using another subroutine and run it to find the roots of x2 + x – 6 = 0; of 3x2 + 2x –1 = 0.
    (Answers: –3.0000, 2.0000; –1.0000, 0.3333.)
    Did you save any more lines of program memory?
  2. The surface area of a sphere can be calculated according to the equation A = 4 π r2, where r is the radius. The formula for finding the volume of a sphere is V = (4 π r3) ÷ 3. This may also be expressed as V = (r × A) ÷ 3.
    Create and load a program to calculate the area A of a sphere given its radius r. Name the program SAREA and include an initialization routine to prompt for the value of the radius. Then create and load a second program to calculate the volume V of a sphere, using the equation V = (r × A) ÷ 3. Name this second program VOLUME and include an XEQ SAREA to use SAREA as a subroutine to calculate area.
    Run the two programs to find the area and volume of the planet Earth, a sphere with a radius of about 3963 miles and of the Earth’s moon, a sphere with a radius of about 1080 miles.
Earth area = 197,359,487.5 square miles,
Earth volume = 2.6071188 × 1011 cubic miles;
Moon area = 14,657,414.69 square miles,
Moons volume = 5,276,699,290 cubic miles.)