Meaning of LocalTSRs= in SYSTEM.INI

• Microsoft Windows operating system versions 3.1, 3.11

SUMMARY

In Windows version 3.1, the SYSTEM.INI file [NonWindowsApp] switch, LocalTSRs=, provides for a unique copy of terminate-and- stay-resident (TSR) programs (already loaded before Windows is started) in each virtual machine (VM) under the 386 enhanced mode of Windows.

MORE INFORMATION

Many non-Windows-aware TSR programs that are loaded prior to starting Windows have problems when they are called in different VMs. For instance, many MS-DOS line-editor TSRs (such as DOSEDIT) assume that there is only one keyboard buffer rather than an independent buffer for each VM. When these types of programs are called in different VMs they can cause problems. They may reference a location in memory that does not exist in a VM or they may return data that is valid in one VM but not in the VM for which the call was made. To correct this, 386 enhanced mode will makes a unique copy of the code and data associated with the TSR, assuring that the assumptions made by the TSR of only one environment are valid in others since there is a unique TSR for each environment.

There are some TSRs for which this method of creating uniqueness is not sufficient. Most prominently, network-service provider TSRs are very intimate with MS-DOS (directly modifying MS-DOS internal structures that are common across all VMs) and so must be handled by a more sophisticated virtual device driver. Additionally, it is more memory efficient to use a virtual device driver to regulate access to a TSR if the only problem is that the TSR cannot be reentered in multiple VMs.

Additionally, some TSRs may only need a very small portion of the data of the TSR to be unique in each VM. Using a virtual device driver to make only the required data unique may reduce memory requirements and make switching between VMs faster. Using a virtual device driver, however, requires intimate knowledge of the TSR and how to develop a virtual device driver.