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                                                       Chapter 13
                                            UNITS IN TURBO PASCAL


When Nicklaus Wirth originally defined Pascal, it was intended to
be a very small language to be used primarily for teaching
programming concepts to computer neophytes.  A program would be
contained in a single file and compiled in its entirety each time
it was compiled.  There was no provision for splitting a program
up into smaller parts, compiling each part separately, and linking
all of the parts together into a final completed package.

Since human beings make mistakes, and because the entire program
must be recompiled each time any mistake is discovered, pure Pascal
is unsuitable for very large programs.  Seeing this problem, many
compiler writers have defined some method by which a large program
could be broken down into smaller parts and separately compiled.


This chapter will define and illustrate the way Borland
International has chosen to allow TURBO Pascal to be broken up into
smaller pieces to permit compilation of smaller portions of a
program.  This allows you to write a much larger program since it
does not have to be compiled all at once.


PART OF A PROGRAM
_________________________________________________________________

Load the program named AREAS.PAS and display it   ===============
on your monitor.  This is the first example of       AREAS.PAS
a TURBO Pascal unit and although it is similar    ===============
to a program in many ways, it has a few
differences which must be pointed out.  We will
start by pointing out the major sections, then get into the details
of each section.

You will first notice that this program begins with the reserved
word unit instead of our usual program, followed by the unit name,
Areas.  In line 10, the reserved word interface is used and all of
the statements following it down to the next reserved word
implementation, are part of the interface with any program outside
of this unit.  The reserved word, implementation, defines the
beginning of the definitions and executable parts of the private
portion of the unit.

Finally, in lines 48 through 50, we find what appears to be a
program block just like we have been using all through this
tutorial, but actually is not.  We will see in a few paragraphs
that this is the initialization section and does a very specific
job for us even though somewhat different than what we have become
used to.


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                               Chapter 13 - Units in Turbo Pascal

THE INTERFACE PART
_________________________________________________________________

Following the unit name we have a section of code in lines 10
through 15 that define the interface of this module to the outside
world.  Anything defined here is available to the outside world and
can be used by any other program provided it has a "uses Areas;"
statement in it.  Constants, types, and variables could also be
defined here, and if they were, they too would be available to any
user program, but in this case, only the four functions are made
available. It should be fairly obvious that the functions calculate
the areas of four different geometric shapes.

These four functions are available for use in any program in much
the same way that any of the standard Pascal functions are
available for use.  The only difference is that a uses clause must
be included in order to use these functions.


THE IMPLEMENTATION PART
_________________________________________________________________

From line 16 through line 47 we have the implementation part as
delineated by the reserved word implementation and the beginning
of the initialization block.  The implementation part is the actual
workhorse of the unit since it contains all of the executable code
for the four functions defined above.

Lines 26 through 31 contain the code needed to generate the area
of a circle, and this code is no different than the code that would
be used if this function were placed in the declaration part of any
Pascal program.  There is a difference in the function header since
the formal parameters are not repeated here, but are defined only
in the interface part in this example.  TURBO Pascal allows you to
either drop the formal parameters here or include them if you think
the code would be more readable.  If you include them, they must
be exactly as shown in the interface part or you will get a compile
error.



A LOCAL PROCEDURE
_________________________________________________________________

In lines 20 through 24, we have a procedure that is used within one
of the four functions, namely the first.  It is really a stupid
procedure since it really wastes time setting up linkage for the
procedure call and does nothing that couldn't be done just as easy
with a simple multiply, but it does illustrate that you can use
another procedure within the unit body.  The procedure
Mult_Two_Numbers cannot be used outside of this unit because it is
not included in the interface part of the unit.  It is, in effect,
invisible to the outside world.


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                               Chapter 13 - Units in Turbo Pascal

The variable My_Pi would be more correctly represented as a
constant but it is defined as a variable to illustrate the use of
the body of the unit later.  Since My_Pi is not defined in the
interface part of the unit, it also is invisible to the outside
world and in fact protected from accidental corruption by a
misplaced statement in another program.  The procedure
Mult_Two_Numbers and the variable My_Pi for all practical purposes
have an impenetrable barrier around them protecting them from
unauthorized use or modification by the outside world, but the
functions internal to this unit have free access to them just as
in any other program.


WHAT IS THE BODY USED FOR?
_________________________________________________________________

Lines 48 through 50 constitute the body of the unit and although
they appear to consist of another executable program that can be
called and used, they actually perform another very specific and
useful purpose.  This is actually an initialization section and all
of the statements in this part of the unit are executed once and
only once, and they are executed when the main program is loaded.
This is done automatically for you by the system.  There is no way
provided for you to call the statements in the body after the
program has begun execution.  This is why the variable My_Pi was
defined as a variable, so we could use this section to initialize
it to a useful value as an illustration.

The body can actually have function and procedure calls that are
executed when the program is loaded, as well as loops or
conditional statements.

If you would like to execute some statements during initialization
and again during the execution of the program one or more times,
you can write a procedure or function to accomplish your desires
and call it at the appropriate times in the main program.


SELECTIVE NAMING OF FUNCTIONS AND PROCEDURES
_________________________________________________________________

If you will study the interface part of this unit you will find
that everything you need to use this unit is contained within it,
provided that you know enough about plane geometry to understand
the functions.  You should strive for this understanding in all of
your interfaces so that the implementation doesn't even require
consultation.  Keep in mind, that if you need to, you can include
comments to further define the functions in the interface part of
the unit.

At this time, you should compile this unit.  You will have to
compile it to disk rather than only to memory so it will be
available for use later in this chapter.  You do this by using the
menus to change the Compile/Destination to the Disk option.  Note

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                               Chapter 13 - Units in Turbo Pascal

that it will not generate an .EXE file but instead a .TPU file.
This is Borland's filename extension for a unit.


ANOTHER UNIT
_________________________________________________________________

Load the file named PERIMS.PAS for another       ================
example of a unit.  This is similar to the last     PERIMS.PAS
except that it does not contain an internal      ================
procedure, and it is composed of three
procedures that calculate the perimeters of
geometric shapes, all of which are visible to the outside world
because they are included in the interface part of the unit.  Once
again, we have a private variable named My_Pi and a block of code
(actually a single statement) to initialize the value of My_Pi when
the unit is loaded.

Be sure you compile this unit to disk in the same manner as the
last and they will be ready for use.  Note that it is not a
requirement that a unit be composed of only functions or only
procedures.  They can be freely mixed in a unit.  It was only done
this way in this example because it was convenient.

Now that we have several functions and procedures that can be used
to calculate the areas or perimeters of several different shapes,
we need a program to illustrate their use, so if you load and
display the program named GARDEN.PAS you will have an example of
their use.


HOW DO WE USE OUR DEFINED UNITS?
_________________________________________________________________

GARDEN.PAS is a very simple program that uses    ================
one of the functions and one of the procedures.     GARDEN.PAS
The only thing you must do is add the names of   ================
the units prior to using the external functions
or procedures.  Lines 16 and 17 each use one of
our newly defined routines.  As you can see, there is nothing magic
about the new routines, and once you include the unit names in a
uses statement, the new routines are in a sense, an extension to
the Pascal language.  Compile and run this program and see that it
really does what you expect it to do.


ONE MORE EXAMPLE OF UNIT USE
_________________________________________________________________

Load and display the program named SHAPES.PAS    ================
for another example of using a predefined unit.     SHAPES.PAS
In line 3, this program includes our new unit    ================
named Areas so all four of the area functions
are available, and in fact, all four are used

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within the body of the program.  This program should not be
difficult for you to understand and you will be left to study it
on your own.  The supplied unit named Crt is also included in the
uses clause to allow the use of the keyboard subprograms in lines
15 and 16.


MULTIPLE USES OF AN IDENTIFIER
_________________________________________________________________

Suppose we wanted to move the variable named My_Pi to the interface
section in both of the units we defined earlier.  Then in the
program named GARDEN.PAS when we included both of the units in the
uses statement, both variables named My_Pi would be available for
use so we would have a bit of a problem defining which one we
really meant to use.  TURBO Pascal has a way to tell the system
which one you wish to use by using a qualifier in much the same way
that you use a field of a record.  The variable name Areas.My_Pi
would refer to that variable from the unit named Areas, and the
name Perims.My_Pi would refer to the variable from the unit named
Perims.

You could even define a new variable of the same name in your main
program and refer to it by the qualified name Garden.My_Pi if you
chose to.  This is not recommended as it would get very confusing
to you.  The compiler would be very happy to compile and run such
a program, because it would not get confused.

It is not illustrated in the example program, but this technique
applies to procedure and function names as well.  If you used the
same procedure name in two different units, you could specify which
procedure you intend to use by using the dot notation with the unit
name and the procedure name.  Unit_Name.Procedure_Name would
therefore refer to the procedure named Procedure_Name that is a
part of the unit named Unit_Name.


WHY USE UNITS?
_________________________________________________________________

There are basically three reasons to use units in your programming.
First, some programs are so large that they should be broken up
into smaller chunks for ease of handling and reasonable compilation
size.  In fact some are so large that they cannot be compiled all
at one time since TURBO Pascal has an upper limit of 64K of code
which can be compiled at once.  Most other Pascal compilers have
a similar limit also.

Secondly, once you complete the code to perform a certain job, you
may wish to use the same code in another program to do the same
job.  If you put the code in a unit, it is ready to simply call and
use again in the same manner that we reused Areas in SHAPES.PAS.
This is becoming a rather important topic in software engineering
usually referred to as "Reusable Software".

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                               Chapter 13 - Units in Turbo Pascal

THIS IS INFORMATION HIDING
_________________________________________________________________

Finally, it is sometimes important to hide a portion of code from
the rest of the program to assure that it cannot be unduly modified
by an error somewhere else in the program.  This too is becoming
an important area of software engineering and is usually referred
to as information hiding.



PROGRAMMING EXERCISE
_________________________________________________________________

1.   Move My_Pi to the interface in both units and change one of
     the values slightly to see if you can read in the right one
     at the right time.  Define another variable of the same name
     in your main program and see if you can differentiate between
     all three values.  Note that, due to the nature of this
     exercise, no answer is given for it on the distribution disk.



































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