Here, this tutorial gives you step-by-step instructions along with GIF image screen shots to show you how to easily make your own DLL. This tutorial is also in response to the hundreds of email Boondog gets each month about writing VB apps for the 8255 PC Interface Card. This simple tutorial will show you how to get started.

Dynamically Linked Libraries (DLL)

Why do I need DLLs?

If you have migrated from DOS' QuickBasic to VB, you soon realize that QuickBasic's INPUT and OUTPUT (or Turbo C's inportb and outportb) functions were not implemented in VB. These functions are crucial for PC hardware developers and programmers because they allowed you to read and write to ports. Thus without INPUT or OUPUT you can't read from or write to your device.

There is a way around this, using a DLL. As the name implies, DLLs allows VB to link (a step before compiling) code (libraries you coded up in another language like Delphi, Borland C++ or Microsoft's Visual C++) during run-time (dynamically). VC++ has port I/O (input and output) read/write functions. Also VC++'s compiler allows you to create DLLs (in addition to executable EXE files). Thus you:

      1. Write VC++ code that uses these read/write functions
      2. Compile it into a DLL (instead of an executable EXE file) file
      3. Call your functions from VB

Writing your own DLL or just using one?

Download 8255.ZIP which contains 8255.def, 8255.cpp and the 8255.dll files.

You just copy the 8255.DLL file to your C:/windows directory. You can then have your VB program use them. But if you are curious then writing a DLL is very easy. The steps in this tutorial specifically use Visual C++ 5.0, but easy enough to mimic in Delphi, Visual C++ 4.0 and Borland C++.

Writing the DLL

8255.def listing:

-----------------------------------------------------------------------
LIBRARY 8255
DESCRIPTION DLL FOR 8255 CARD

EXPORTS
    	Out8255        @1
        In8255         @2
-----------------------------------------------------------------------

The name of your DLL library is given on the first line. It is 8255. The second line is just a comment. Exports list the names of the functions you will eventually define in your VC++. These functions are: Out8255 and In8255. If you eventually wish to add more functions, give the name of your function and the next number, like MyFunction @3.

8255.cpp listing:

-----------------------------------------------------------------------
//   FILE: 8255.cpp
//   AUTH: P.OH/Boondog Automation
//   DATE: 07/01/98
//   DESC: CPP source file for 8255 DLL - compiled with Microsoft Visual C++ 5.0

#include 
#include   // contains Visual C++'s inp and out functions

// ------------------------------------------------------
//   FUNC: Out8255
//   DESC: uses Microsoft's Visual C++ _outp() function
//         to output a PortData to PortAddress
// ------------------------------------------------------

short _stdcall Out8255( int PortAddress, int PortData )
{
  short Dummy; 
  // Need Dummy since _outp officially returns int 
  // short is a 16-bit integer in Win32 C++
  // whereas int is 32-bit integer Win32 C++
  // use (short) to force returning 16-bit integer
  // back to VB
  Dummy = (short)(_outp( PortAddress, PortData ));

  return(Dummy);

}; // end of Out8255


// ----------------------------------------------------
//   FUNC: In8255
//   DESC: uses Microsoft's Visual C++ _inp() function
//         to read PortAddress
// ----------------------------------------------------

short _stdcall In8255( int PortAddress ) 
{
  short PortData; 
  // short is a 16-bit integer in Win32 C++
  // whereas int is 32-bit integer in Win32 C++
  // use (short) to force returning 16-bit integer 
  // back to VB
  
  PortData = (short)(_inp( PortAddress ));
  return( PortData );

}; /* end of In8255 */

8255.cpp defines In8255 and Out8255. It uses VC++'s _inp() and _out() functions. The single underscore before inp and outp are needed. This underscore notation refers to downloadly compatiable functions that were defined in older versions of VC++.

Step 1: Write your .def and .cpp files
Create a directory called c:/port. Type and save the DEF file as 8255.def and the CPP file as 8255.cpp - if you want, just cut, paste and save these files, or just download and save them.

Step 2: Visual C++ 5.0
Bring up Visual C++. Select FILE - NEW as seen in Figure 1:

Figure 1

Step 3: Create Your Project
This brings up the NEW window. Make sure the Projects tab is selected and choose Win32 Dynamic-Link Library (Figure 2). Make sure that Location is c:/port (you can click the ... button to explore your directories). This is why you created a directory c:/port in Step 1. Type 8255 in the Project Name field. Click on OK. You should now see the result (Figure 3). If not, click on the classes tab. You have just created a project called 8255.

Figure 2

Figure 3

Step 4: Add your .cpp file
Left click on 8255 classes once. This selects it and turns it blue. Next, right click your mouse and choose Add Files to Project (Figure 4).

Figure 4

An Insert Files to Project (Figure 5) will pop up. Make sure the Files of Type is set to C++ files. Next, choose the 8255.cpp file and hit OK

Figure 5

Step 5: Add your .def file
Similar to Step 4, click on 8255 classes once. Right click and select Add Files to Project. Again the Insert File into Project window pops up (Figure 6). This time make sure the Files of Type is set for Definition (.def) files. Click on 8255.def and hit OK. Your 8255 Project now has the 8255.def and 8255.cpp files added (Figure 7). Save everything by clicking FILE - SAVE ALL.

Figure 6

Figure 7

Step 6: Build your .dll file
On the top bar click on Build and select Build 8255.dll (Figure 8). This will start compiling and create th2 8255.dll file. If you didn't mistype and lines in 8255.def or 8255.cpp (or just cut/paste/save or download/save) VC++ will respond with 0 errors. Your new 8255.dll file is now saved in c:/port/8255/debug (Figure 9).

Figure 8

Figure 9

Step 7: Copy your .dll file to c:/windows
Copy your new DLL file to your c:/windows directory. You can do this from DOS (my habit), by bringing up MS-DOS prompt and cd`ing to c:/port/8255/Debug (Figure 10). Of course, you can drag-and-drop from Explorer. Congradulations! Your .dll is ready to use in your Visual Basic programs! Easy right?

Figure 10

Using Your DLL in Your VB Programs

Private Declare Function Out8255 Lib "8255.dll" _
     (ByVal PortAddress As Integer, _
      ByVal PortData As Integer) As Integer

Private Declare Function In8255 Lib "8255.dll" _
     (ByVal PortAddress As Integer) As Integer

where Dummy, Cntrl are integers. Here, the decimal value 128 is written to the port address assigned to Cntrl. It is necessary to use associate an integer variable (I called it "Dummy" here) with the Out8255 function. This is because the DLL was built using VC++'s outp(), which returns an integer (1 if successfully accomplished and and 0 if failed).

To read from a port, use In8255:

where PortValue and PortSelected are integers. The 8-bit number at port PortSelected will be assigned (in decimal form) to PortValue.

The Count Program

The above figure is a screen shot of the COUNT program. The user types in the 8255 Card's base address (608 in this case), selects a port and hits OK. The program starts counting.

Designing Count's Form

Count's VB code

------------------------------------------------------------------------

Option Explicit
'Declare use of the DLL
Private Declare Function Out8255 Lib "8255.dll" (ByVal PortAddress As Integer, ByVal PortData As Integer) As Integer
Private Declare Function In8255 Lib "8255.dll" (ByVal PortAddress As Integer) As Integer
'Declare variables
Dim BaseAddress As Integer: ' 8255 Base Address
Dim Dummy As Integer: ' Dummy variable used with DLL
Dim PortA As Integer: ' 8255 Port A address
Dim PortB As Integer: ' 8255 Port B address
Dim PortC As Integer: ' 8255 Port C address
Dim Cntrl As Integer: ' 8255 Control Address
Dim Number As Integer: ' decimal number to count from 1 to 255
Dim Start As Integer: ' Start flag
Dim Msg As String
Dim Style As Integer
Dim Response As Integer
Dim PortSelected As Integer


Private Sub cmdGo_Click()

If Start = 0 Then
    ' user clicked GO button first time
    If txt8255Address.Text = "" Then
        ' Base address was not defined
        Msg = "Enter a Base Address! e.g. 608"   ' Define message.
        Style = vbOK + vbExclamation ' Define buttons.
        Response = MsgBox(Msg, Style)
        Exit Sub
    End If
    
    Start = 1: ' Go button enabled; start counting
        
    cmdGo.Caption = "Pause"
    ' Assign values for all addresses
    BaseAddress = Val(txt8255Address.Text)
    PortA = BaseAddress
    PortB = BaseAddress + 1
    PortC = BaseAddress + 2
    Cntrl = BaseAddress + 3

    ' determine which port to output to
    ' default is Port A
    If optPortA.Value = True Then
        PortSelected = PortA
        Print PortSelected
        End If
    If optPortB.Value = True Then
        PortSelected = PortB
        Print PortSelected
        End If
    If optPortC.Value = True Then
        PortSelected = PortC
        Print PortSelected
        End If
    
    ' configure all ports for output
    Dummy = Out8255(Cntrl, 128)
    ' initialize all Ports to 0
    Dummy = Out8255(PortA, 0)
    Dummy = Out8255(PortB, 0)
    Dummy = Out8255(PortC, 0)
Else
    Start = 0: ' user clicked GO button again
    cmdGo.Caption = "Go!"
End If
End Sub

Private Sub cmdEnd_Click()

Beep
'txtOutputWindow.Text = "Stopped"
Dummy = Out8255(PortA, 0)
Dummy = Out8255(PortB, 0)
Dummy = Out8255(PortC, 0)
' quit program
End
End Sub

Private Sub Form_Load()
' Program is loaded with these values
    txtOutputWindow.Text = "Enter Base Address"
    Start = 0: 'Counting action not started
    Number = 0: ' Number to start with
    optPortA.Value = True ' Default port is A
End Sub

Private Sub tmrTimer_Timer()

If Start = 1 Then
    Number = Number + 1
    Dummy = Out8255(PortSelected, Number)
    txtOutputWindow.Text = "Number = " + Str(Number)
    If Number = 255 Then
        Beep
        txtOutputWindow.Text = "Finished"
        Dummy = Out8255(PortSelected, 0)
        Start = 0
        Number = 0
        cmdGo.Caption = "Go!"
    Exit Sub
    End If
Else
    Exit Sub
End If

End Sub

Count's Program Description

The DIP Program

The look and functionality of DIP is very similar to the Count program you earlier saw. You can re-use many of Count's objects.

DIP's VB code

Option Explicit
'Declare use of the DLL
Private Declare Function Out8255 Lib "8255.dll" (ByVal PortAddress As Integer, ByVal PortData As Integer) As Integer
Private Declare Function In8255 Lib "8255.dll" (ByVal PortAddress As Integer) As Integer
'Declare variables
Dim BaseAddress As Integer: ' 8255 Base Address
Dim Dummy As Integer: ' Dummy variable used with DLL
Dim PortA As Integer: ' 8255 Port A address
Dim PortB As Integer: ' 8255 Port B address
Dim PortC As Integer: ' 8255 Port C address
Dim Cntrl As Integer: ' 8255 Control Address
Dim PortValue As Integer: ' decimal value read at port
Dim Start As Integer: ' Start flag
Dim Msg As String
Dim Style As Integer
Dim Response As Integer
Dim PortSelected As Integer

Private Sub cmdEnd_Click()
Beep
'txtOutputWindow.Text = "Stopped"
' quit program
End
End Sub

Private Sub cmdGo_Click()
If Start = 0 Then
    ' user clicked GO button first time
    If txt8255Address.Text = "" Then
        ' Base address was not defined
        Msg = "Enter a Base Address! e.g. 608"   ' Define message.
        Style = vbOK + vbExclamation ' Define buttons.
        Response = MsgBox(Msg, Style)
        Exit Sub
    End If
    
    Start = 1: ' Go button enabled; start counting
        
    cmdGo.Caption = "Pause"
    ' Assign values for all addresses
    BaseAddress = Val(txt8255Address.Text)
    PortA = BaseAddress
    PortB = BaseAddress + 1
    PortC = BaseAddress + 2
    Cntrl = BaseAddress + 3

    ' determine which port to output to
    ' default is Port A
    If optPortA.Value = True Then
        PortSelected = PortA
    End If
    If optPortB.Value = True Then
        PortSelected = PortB
    End If
    If optPortC.Value = True Then
        PortSelected = PortC
    End If
    
    ' configure all ports for input
    Dummy = Out8255(Cntrl, 155)
    ' initialize all Ports to 0
Else
    Start = 0: ' user clicked GO button again
    cmdGo.Caption = "Go!"
End If
End Sub


Private Sub Form_Load()
' Program is loaded with these values
    txtOutputWindow.Text = "Enter Base Address"
    Start = 0: 'Counting action not started
    optPortA.Value = True ' Default port is A
End Sub

Private Sub tmrTimer_Timer()
If Start = 1 Then
    PortValue = In8255(PortSelected)
    txtOutputWindow.Text = "Value = " + Str(PortValue)
End If

End Sub

DIP's Program Description

Conclusion

Hopefully the step-by-step tutorial on DLL creation sheds light on how they are made. This is in response to the flood of questions on DLL programming on the VB usenet groups.

One caveat is that if you use the wrong memory address (e.g you mistype the address) you can cause Windows 95 to freeze. As a developer/programmer, you might wish to use some error checking in your VB program to make sure that you always write/read to your custom-build card's address.

Boondog hopes that you have found this tutorial useful. Hopefully a new world of VB/Win95 programming/PC interfacing brings you a lot of excitement and fun.

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Click here to email me: paul@cs.columbia.edu