How To Access PC Bus Devices in Visual Basic for Windows

MORE INFORMATION

MS-DOS did not implement any memory protection; this meant that BASIC applications were able to write to any memory location directly, giving fast access to the hardware of the PC, including custom hardware that plugged into the expansion ports. In a multitasking environment it was clearly unacceptable for an application to write to memory owned by another application. The hardware port addresses are deemed to belong to the system. Accordingly, applications are unable to write directly to the PC bus addresses.

It is often desirable to port MS-DOS BASIC code to a Windows environment. However, programs that attempt to control custom devices will fail to port directly.

Effectively, there are two potential solutions:

1. Use a custom Virtual Device Driver (VxD). It is likely that the manufacturers of more popular custom devices will have a VxD. However, there may be no VxD, or the VxD interface may be structured differently.
   
   Versions of Visual Basic prior to 5.0 may require extra steps to use a VxD. Versions prior to 5.0 did not have the AddressOf operator needed by many VxDs to use Callback procedures. The workaround is have the Visual Basic program call a C DLL which would in turn call the VxD.
2. Use a third-party DLL or OCX that emulates the functionality. There are a number available commercially or as shareware.