PRB: Some VB 5.0 Programs Do Not Run on Multiprocessor PCs

• Microsoft Visual Basic Professional and Enterprise Editions for Windows, version 5.0

SYMPTOMS

If a Visual Basic 5.0 application attempts to load a 32-bit generic thunking DLL, it might not run properly on a multiprocessor computer.

The term "32-bit generic thunking DLL" describes any 32-bit DLL that links with WOW32.DLL to provide 16-bit code with access to 32-bit code. This type of DLL is loaded by a 16-bit program through the LoadLibraryEx32W() API.

CAUSE

For a 32-bit program to map a 32-bit generic thunking DLL into its address space, you must also map the Windows NT Virtual DOS Machine (NTVDM.EXE) into the address space. The Visual Basic 5.0 Runtime Library, MSVBVM50.DLL, collides with NTVDM.EXE. Both MSVBVM50.DLL and NTVDM.EXE are based at 0x0F000000.

On a uniprocessor computer, the operating system successfully relocates NTVDM.EXE and the program runs correctly. However, on a multiprocessor computer, NTVDM.EXE cannot be relocated. For additional information, please see the following article in the Microsoft Knowledge Base:

Q199671 Relocation of NTVDM.EXE fails on multiprocessor machines

RESOLUTION

A 32-bit generic thunking DLL should be loaded by a 16-bit program only. The loading of a 32-bit thunking DLL into the address space of a 32-bit process is unsupported.

STATUS

This behavior is by design.

MORE INFORMATION

When a Visual Basic 5.0 application makes a call to a function in a 32-bit generic thunking DLL on a multiprocessor computer, the call fails. A query of Err.LastDllError returns the error code 998 "Invalid access to memory location" (ERROR_NOACCESS).

Additional query words:

Keywords : kbDLL kbKernBase kbVBp500
Version : WINDOWS:5.0
Platform : WINDOWS
Issue type : kbprb