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IO.DLL allows seamless port I/O operations for Windows 95/98/NT/2000/XP using the same library.


In the pre-Windows days, it was a relatively simple matter to access I/O ports on a typical PC. Indeed, nearly every language sported a special command for doing so. As Windows emerged and gradually evolved, this flapping in the wind behaviour could no longer be tolerated because of operating system's ability to virtualize hardware.

Virtualizing hardware means that an application (typically a DOS box in Windows) believes it is talking directly to a physical device, but in reality it is talking to a driver that emulates the hardware, passing data back and forth as appropriate. This is how you are able to open dozens of DOS boxes in your Windows session, each one with the strange notion that it has exclusive access to peripherals such as the video adapter, keyboard, sound card and printer.

If one were to rudely bang out data to an I/O port that Windows thought it was in full control of, the "official bad thing" could occur, the severity of which depending upon the exact hardware that was being accessed. Actually, with the virtualization just mentioned, it is quite improbable that Windows would permit anything too nasty from occuring.

Windows 95/98 actually does allow I/O operations be executed at the application level, although you'd be hard pressed to find a language that supports this directly. Typically the programmer will have to resort to assembly language for this kind of low-level control. If you know what you are doing, this can be a quick and easy way to access I/O ports. Of course, not everyone knows, or desires to learn 80x86 assembly programming just because they want to turn on a lamp from their computer. However, the unwillingness to learn assembly language becomes rather trivial when faced with 9x's big brother.

Windows NT/2000/XP, being the secure operating system that it is, does not permit port I/O operations at the application level at all. Period. A program with inline IN and OUT assembly instructions that runs perfectly on Windows 95/98 will fail horribly when it comes to Windows NT/2000/XP.

Windows NT/2000/XP does, however, allow I/O instructions in its kernel mode drivers. A kernel mode driver runs at the most priviledged level of the processor and can do whatever it pleases, including screwing up the system beyond repair, thus writing a kernel mode driver is not for the feint of heart.

If you were to take it upon yourself to wade through the documentation of the Windows NT/2000/XP ddk and piece together a driver that was callable by your application to do the I/O instructions on behalf of your application, you'd probably notice something not too pleasant--this sort of access is painfully slow. The call from application level to system level typically takes about one millisecond. Compare this to the one microsecond that a normal I/O access takes. To further the insult, you are at the whim of the operating system. If it has tasks which it believes are of higher priority than your lowly call to your driver, it will perform them, making precise timing nearly impossible.

Obviously, writing a driver that does acts a proxy for the I/O calls isn't the most ideal solution. There is, however, a solution for NT/2000/XP that allows the same convienience of inline assembly language that 95/98 does.

As mentioned, a kernel mode driver can do whatever it wants. The implication here is that if another kernel mode driver shut off application access to the I/O ports, it should be possible for another kernel mode driver to turn it back on. This is where IO.DLL enters the picture.


IO.DLL is completely free! However, you may not:
  • Charge others for it in any way. For example, you cannot sell it as a stand alone product.
  • Charge for an IO.DLL wrapper, such as an OCX or Delphi control whose purpose is just to put a fancy interface on IO.DLL. I consider these to be "derived works" and they must be provided free of charge.
  • Claim that it is your property.
Also, the author (that's me) cannot be held liable due to io.dll's failure to perform. As with most free stuff, you are on your own.

Source Code and Special Modifications

The source code is available for $1,000 US.

I'm willing to work with people should they require a special modification to IO.DLL. For example, you might have a strict timing requirement of some sort that can only be done in kernel mode. For a fee, I will modify IO.DLL and/or the embedded kernel mode driver for the task at hand.

Description of IO.DLL

IO.DLL provides a useful set of commands for reading and writing to the I/O ports. These commands are consistent between 95/98 and NT/2000/XP. Furthermore, there is no need for the programmer to learn assembly language or muck with kernel mode drivers. Simply link to the DLL and call the functions. It's that easy.

Windows NT/2000/XP is accomodated through the use of a small kernel mode driver that releases the ports as needed to the application. This driver is embedded in the DLL and is installed if Windows NT/2000/XP is determined to be the underlying operating system.

Due to the very minor overhead involved in dynamically linking to IO.DLL, and the optimized functions contained within, access to I/O ports is nearly as fast as if it was written in raw assembler and inlined in your application. This holds true for both Windows 95/98 and Windows NT/2000/XP.

Before moving on, it is probably prudent to mention that the technique employed in IO.DLL for releasing the ports to the application level isn't, strictly speaking, the proper way to do things. The proper way is to have a virtual device driver for Windows 95/98 and a kernel mode driver for Windows NT/2000/XP. This isn't very practical for many people though, nor is it really necessary. There are several successful commercial products on the market that do exactly what IO.DLL does. Let it be noted though that some of them are shady with their explanation of how their product works, meanwhile charging $500 or more for it.

What Ports can IO.DLL Access?

IO.DLL is capable of accessing only those ports that are inherent to the processor itself through the IN and OUT instructions. This means that it cannot access most add-in parallel ports or com ports because they are typically memory-mapped these days rather than IO-mapped. IO.DLL also cannot access virtual devices. Here is a table of what ports can be accessed:

IO AddressDevice
000-01FDMA controller #1
020-03FInterrupt controller
060-06FKeyboard controller
070-07FReal-time clock, CMOS Memory, NMI mask
080 manufacturer's diagnostics checkpoint
080-09FDMA page register
0A0-0BFInterrupt controller #2
0C0-0DFDMA controller #2
0F0-0FFMath Coprocessor
170-177Hard disk (secondary)
1F0-17FHard disk
200-207Game I/O
278-27FLPT 2
2C0-2DFEGA #2
2E8-2EFCOM 4
2F8-2FFCOM 2
300-31FPrototype card
370-377FDC (secondary)
378-37FLPT 1
3A0-3AFbisynchronous port #1
3D0-3DFCGA and EGA
3E8-3EFCOM 3
3F8-3FFCOM 1

Download 46K (Contains all the files)
io.dll 46K

The following two files are for C++ users. There is more info on these in the prototypes section.

io.cpp 1.3K
io.h 1.2K

C/C++ Prototypes

void WINAPI PortOut(short int Port, char Data);
void WINAPI PortWordOut(short int Port, short int Data);
void WINAPI PortDWordOut(short int Port, int Data);
char WINAPI PortIn(short int Port);
short int WINAPI PortWordIn(short int Port);
int WINAPI PortDWordIn(short int Port);
void WINAPI SetPortBit(short int Port, char Bit);
void WINAPI ClrPortBit(short int Port, char Bit);
void WINAPI NotPortBit(short int Port, char Bit);
short int WINAPI GetPortBit(short int Port, char Bit);
short int WINAPI RightPortShift(short int Port, short int Val);
short int WINAPI LeftPortShift(short int Port, short int Val);
short int WINAPI IsDriverInstalled();
To use IO.DLL with Visual C++/ Borland C++, etc, you'll need to use LoadLibrary and GetProcAddress. Yes, it's more of a pain than using a .lib file, but because of name mangling, it's the only reliable way of calling the functions in IO.DLL. I've gone ahead and done the dirty work for you:


Just save these two files and include them in your project. For a Visual C++, you may need to add #include "StdAfx.h" at the top of io.cpp otherwise the compiler will whine at you.

These two files take care of calling LoadLibrary and all the neccessary calls to GetProcAddress, making your life happy once again.

The only step you are required to do is call LoadIODLL somewhere at the beginning of your program. Make sure you do this or you will find yourself faced with all sorts of interesting crashes.

Please let me know if you find any errors in the above two files. They are new and haven't been tested all that much.

Delphi Prototypes

procedure PortOut(Port : Word; Data : Byte);
procedure PortWordOut(Port : Word; Data : Word);
procedure PortDWordOut(Port : Word; Data : DWord);
function PortIn(Port : Word) : Byte;
function PortWordIn(Port : Word) : Word;
function PortDWordIn(Port : Word) : DWord;
procedure SetPortBit(Port : Word; Bit : Byte);
procedure ClrPortBit(Port : Word; Bit : Byte);
procedure NotPortBit(Port : Word; Bit : Byte);
function GetPortBit(Port : Word; Bit : Byte) : WordBool;
function RightPortShift(Port : Word; Val : WordBool) : WordBool;
function LeftPortShift(Port : Word; Val : WordBool) : WordBool; 
function IsDriverInstalled : Boolean;
Important! To use these functions in your Delphi program, the correct calling convention of stdcall is required. For example:

procedure PortOut(Port : Word; Data : Byte); stdcall; external 'io.dll';

Visual Basic Prototypes

Private Declare Sub PortOut Lib "IO.DLL" (ByVal Port As Integer, ByVal Data As Byte)
Private Declare Sub PortWordOut Lib "IO.DLL" (ByVal Port As Integer, ByVal Data As Integer)
Private Declare Sub PortDWordOut Lib "IO.DLL" (ByVal Port As Integer, ByVal Data As Long)
Private Declare Function PortIn Lib "IO.DLL" (ByVal Port As Integer) As Byte
Private Declare Function PortWordIn Lib "IO.DLL" (ByVal Port As Integer) As Integer
Private Declare Function PortDWordIn Lib "IO.DLL" (ByVal Port As Integer) As Long
Private Declare Sub SetPortBit Lib "IO.DLL" (ByVal Port As Integer, ByVal Bit As Byte)
Private Declare Sub ClrPortBit Lib "IO.DLL" (ByVal Port As Integer, ByVal Bit As Byte)
Private Declare Sub NotPortBit Lib "IO.DLL" (ByVal Port As Integer, ByVal Bit As Byte)
Private Declare Function GetPortBit Lib "IO.DLL" (ByVal Port As Integer, ByVal Bit As Byte) As Boolean
Private Declare Function RightPortShift Lib "IO.DLL" (ByVal Port As Integer, ByVal Val As Boolean) As Boolean
Private Declare Function LeftPortShift Lib "IO.DLL" (ByVal Port As Integer, ByVal Val As Boolean) As Boolean
Private Declare Function IsDriverInstalled Lib "IO.DLL" As Boolean

Function Descriptions

Please refer to the prototype for the particular language you are using.

Outputs a byte to the specified port.

Outputs a word (16-bits) to the specified port.

Outputs a double word (32-bits) to the specified port.

Reads a byte from the specified port.

Reads a word (16-bits) from the specified port.

Reads a double word (32-bits) from the specified port.

Sets the bit of the specified port.

Clears the bit of the specified port.

Nots (inverts) the bit of the specified port.

Returns the state of the specified bit.

Shifts the specified port to the right. The LSB is returned, and the value passed becomes the MSB.

Shifts the specified port to the left. The MSB is returned, and the value passed becomes the LSB.

Returns non-zero if io.dll is installed and functioning. The primary purpose of this function is to ensure that the kernel mode driver for NT/2000/XP has been installed and is accessible.

Other Information

An excellent document about the standard parallel port can be found here.