MPL Documentation

Pascal Program Structure

Before you study the building blocks of the Pascal language, let us look at a bare minimum Pascal program structure so you have it as a reference.

A Pascal program basically consists of the following parts:
  • Program name
  • Uses command
  • Type declarations
  • Constant declarations
  • Variables declarations
  • Functions declarations
  • Procedures declarations
  • Main program block
  • Statements and Expressions within each block
  • Comments

Every pascal program generally has a heading statement, a declaration and an execution part strictly in that order. Following format shows the basic syntax for a Pascal program:
Program {name of the program}
uses {comma delimited names of libraries you use}
const {global constant declaration block}
var {global variable declaration block}

function {function declarations, if any}
{ local variables }

procedure { procedure declarations, if any}
{ local variables }

begin { main program block starts}
end. { the end of main program block }

Pascal Hello World Example

The following is a simple pascal program that would print the words: Hello, World!
program HelloWorld;

uses Display;

(* Here the main program block starts *)
   Writeln('Hello, World!');
Looking at various parts of the above program:
  • The first line of the source code is labeled as a program and named HelloWorld.
  • The next line of code uses Display is a preprocessor command, which tells Modern Pascal to include the built-in unit display before going to the actual compilation phase.
  • The next lines enclosed within begin and end statements are the main program block. Every block in Pascal is enclosed within a begin statement and an end statement. However, the end statement indicating the end of the main program is followed by a full stop (.) instead of semicolor (;).
  • The begin statement of the main program block is where the program execution begins.
  • The lines within (*...*) will be ignored by the compiler and it has been put to add a comment in the program.
  • The statement writeln('Hello, World!') uses the writeln function available in Pascal which causes the message to be displayed on the screen.
  • The statement readkey allows the display to pause until the user presses a key. It is part of the display unit. A unit is like a library in Pascal.
  • The last statement end. ends your program.

Executing the Pascal Program

  • Open a text editor and add the above-mentioned source code.
  • Save the file as hello.p
  • Open a command prompt and go to the directory, where you saved the file.
  • type mp2 hello.p at the command prompt and press enter to execute your code.