2.9 Programs in Memory

In this section we will look at a new sample program and see what happens to memory when this program is loaded and then executed.

Example 2.6

A payroll program is shown in Program 2.7.

Program 2.7
A Simple Payroll Calculation

WITH Ada.Text_IO; 
WITH Ada.Float_Text_IO;
PROCEDURE Weekly_Pay IS
------------------------------------------------------------------------
--| Simple payroll calculation
--| Author: Michael B. Feldman, The George Washington University 
--| Last Modified: July 1995                                     
------------------------------------------------------------------------

  SUBTYPE NonNegFloat IS Float RANGE 0.0 .. Float'Last;
 
  Tax : CONSTANT NonNegFloat := 25.0;
  Hours : NonNegFloat;
  Rate  : NonNegFloat;
  Gross : NonNegFloat;
  Net   : NonNegFloat;
 
BEGIN -- Weekly_Pay

  Ada.Text_IO.Put (Item => "Enter hours worked > ");
  Ada.Float_Text_IO.Get (Item => Hours);
  Ada.Text_IO.New_Line;
  Ada.Text_IO.Put (Item => "Enter hourly rate > ");
  Ada.Float_Text_IO.Get (Item => Rate);   
  Ada.Text_IO.New_Line;
 
  Gross := Hours * Rate;
  Net := Gross - Tax;
   
  Ada.Text_IO.Put (Item => "Gross pay is $ ");
  Ada.Float_Text_IO.Put (Item => Gross, Fore => 6, Aft => 2, Exp => 0);
  Ada.Text_IO.New_Line;
   
  Ada.Text_IO.Put (Item => "Net pay is   $ ");
  Ada.Float_Text_IO.Put (Item => Net, Fore => 6, Aft => 2, Exp => 0);
  Ada.Text_IO.New_Line;

END Weekly_Pay;

Sample Run

Enter hours worked > 40

Enter hourly rate > 6.50

Gross pay is $    260.00

Net pay is   $    235.00

This program computes an employee's gross pay and net pay using the algebraic formulas

gross pay = hours worked x hourly rate

net pay = gross pay - tax amount

These formulas are written as the Ada assignment statements

    Gross := Hours * Rate; 
    Net := Gross - Tax;

in the program. New values of Hours and Rate are read each time the program is executed; a constant Tax of $25.00 is always deducted.

Near the top of the program, there is a line reading

    SUBTYPE NonNegFloat IS Float RANGE 0.0 .. Float'Last;

The purpose of this line, called a subtype declaration, is to indicate to the compiler that certain variables and constants will have floating-point values that must not be negative. The variables of the program are declared as being of type NonNegFloat because it does not make sense for an employee's hours worked or hourly wage to be negative. We will return to this question in Section 2.10.

Program 2.7 first reads the data representing hours worked and hourly rate and then computes gross pay as their product. Next, it computes net pay by deducting a constant tax amount of 25.00. Finally, it displays the computed values of gross pay and net pay. Note in the program how the results are formatted.

In the sample run, note that we have entered 40 as an integer token. Recall that this is allowed if the value is a whole number and that the value is still stored internally as a float value.

Memory Area for the Payroll Program

Figure 2.6(a) shows the payroll program loaded into memory and the program memory area before execution of the program body.

Figure 2.6
Memory for Payroll Program

Figure 2.6

The question mark in memory cells Hours, Rate, Gross, and Net indicates that these variables are undefined (value unknown) before program execution begins. During program execution, the data values 40.0 and 4.50 are read into the variables Hours and Rate, respectively. After the assignment statements shown earlier are used to compute values for Gross and Net, all variables are defined as shown in Figure 2.6(b).