1.6 Processing a High-Level Language Program

Before it can be processed, a high-level language program must be entered at the terminal, then translated, and finally loaded into memory for execution. A number of software tools--programs that help us develop other programs--assist in this task. These are described next, and the process is summarized in Figure 1.6.

Figure 1.6
Preparing a Program for Execution

Figure 1.6

Steps in Preparing a Program for Execution

Use an editor program to enter the source program into memory and save it on disk as a source file.
Use a compiler program to translate the source program into machine language. Often, one or more statements in the source file will contain a syntax error or semantic error. This means that these statements do not correspond exactly to the rules of the high-level language. In this case, the compiler will cause some error messages to be displayed. Use the editor program to make changes to your source file, and compile it again.
When the source program is error-free, the compiler saves its machine-language translation as an object file.
Call the binder program (sometimes called linker) to combine your object program with additional object files needed for your program to execute. These may be system files such as input/output modules, or other application modules (packages, in Ada terms) that you or others have written. Generally, the binder needs only to be told the name of your main program; it then proceeds to save the final result as an executable program on disk.

Executing a Program

To execute a program, the CPU examines each program instruction in memory and sends out the command signals required to carry out the instruction. Although the instructions are normally executed in sequence, as we will see later, it is possible to have the control unit skip over some instructions or execute some instructions more than once.

During execution, data can be entered into memory and manipulated in some specified way. Special program instructions are used for requesting the user to enter input data, then reading the data into memory. After some manipulation of the input data, instructions are used for displaying or printing result values--program output--from memory.

Figure 1.7 shows the effect of executing a payroll program stored in memory. The first step of the program requires entering data into memory that describe the employee. In the second step, the employee data are manipulated by the central processor and the results of computations are stored in memory. In the final step, the computational results may be displayed as payroll reports or employee payroll checks.

Figure 1.7
Flow of Information During Program Execution

Figure 1.7

Exercises for Section 1.6

Self-Check

What is the role of a compiler? What is a syntax error? In which file would a syntax error be found?
What is the difference between the source file and the object file? Which do you create and which does the compiler create? Which one is processed by the linker? What does the loader do?