INFO: Running a Windows-Based Application in its Own VDM

• Microsoft Win32 Software Development Kit (SDK) for Windows NT, versions 3.5, 3.51, 4.0

SUMMARY

Under Windows NT, 16-bit Windows-based applications run as separate threads under a special environment called WOW (Windows on Win32). Under Windows NT version 3.1 and earlier, WOW supports running all 16-bit Windows-based applications in one virtual machine (VM). These applications share an address space, just as they do on Windows. But under Windows NT version 3.5 through 4.0, WOW supports running a Windows-based application in its own VM, which gives the application its own address space.

To programmatically start a Windows-based application in its own VM, start the application with CreateProcess() and then specify the flag CREATE_SEPARATE_WOW_VDM.

To specify a Win16 application started from the command prompt to run in its own address space, use the following syntax:

   start /SEPARATE <filename>

NOTE: This option is not the default, nor is there any way to make it the default.

MORE INFORMATION

Allowing a 16-bit Windows-based application to run in a separate address space provides for more robust operation, because the application is isolated from other Windows-based applications. However, the downside is twofold:

• WOW VMs require approximately 2500K of private memory on x86 machines.
• There is no shared memory between WOW VMs. Therefore, 16-bit Windows- based applications that rely on shared memory cannot be run in separate VMs. As an alternative, use DDE or OLE, because they can be used by an application in one VM to communicate with an application in another VM.