Windows 3.1 Standard Mode and the VCPI

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Windows 3.1 standard mode is compatible with expanded memory managers such as EMM386, 386MAX, and QEMM386. However, there are several problems with the Virtual Control Program Interface (VCPI) used by these products to provide access to extended memory in standard mode. Under most circumstances, users of machines with 386 or higher processors and with more than 2 megabytes (MB) of memory installed should use enhanced mode rather than standard mode.

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Windows 3.1 can run in standard mode when an expanded memory manager (EMM) is active. An EMM uses the paging mechanism of the 386 or higher processor to map extended memory blocks, as EMS pages, into the first megabyte of address space where no real memory is present. To do so, the EMM puts the processor into protected mode, and keeps the emulated upper memory block (UMB) and EMS page frames always available for processes designed for real mode (such as MS-DOS), installed device drivers, and terminate-and-stay-resident (TSR) programs. When one of these processes is running, the processor is in virtual-8086 (v86) mode, with paging on and the address mapping specified by the EMM.

In this configuration, the EMM services request to enter and leave protected mode using the VCPI. The EMM may also allocate extended memory. The standard mode MS-DOS Extender, DOSX, uses VCPI to switch to protected mode and to v86 mode. Therefore, in standard mode, the EMM remains active and continues to run "underneath" Windows. In enhanced mode, the EMM is deactivated for the duration of the Windows session.

Several questions have arisen regarding "VCPI support" under standard-mode Windows. If no EMM is installed, more extended memory and more total memory is available to Windows. An EMM uses a considerable amount of extended memory to store itself and its tables. The system incurs additional overhead because paging must be active at all times. Unless an EMM is absolutely necessary, possibly because an MS-DOS-based application will not run without EMS, Windows will probably run better without an EMM. Windows enhanced mode runs on a machine with 2 MB of RAM memory installed if no EMM is present.

Windows standard mode does not support an MS-DOS-based application that is a DPMI (DOS Protected Mode Interface) client. However, if the EMM also provides DPMI services, DOSX will not interfere with these services.

The standard-mode task switcher attempts to arbitrate extended memory use between Windows-based and MS-DOS-based applications. It performs this arbitration by hooking the XMS function dispatcher. This does not work properly if an EMM is installed, primarily because the extended memory portion of the address space in standard mode is not accessible to the MS-DOS portion of the switcher. For this reason, when an EMM is present in standard mode, users will experience the following types of problems:

  • Performance degradation when the system switches between tasks because the task switcher cannot use XMS memory for swap space.
  • Applications, such as AutoCad and Lotus 1-2-3, that include an MS-DOS extender will not run.
Because the standard-mode task switcher can't access Windows's extended memory, it can't use extended memory for swap space when an EMM is present. Users may experience some performance degradation switching between MS-DOS-based applications in this configuration. Enhanced mode is recommended to run many MS-DOS-based applications under Windows.

Windows standard mode does not specifically prevent VCPI applications from running. However, because the task switcher cannot effectively use extended memory provided by an EMM, an MS-DOS-based application that uses extended memory probably will fail to run in the standard-mode MS-DOS box. This applies equally to applications that use XMS, VCPI, and DPMI. While it may be possible to run an MS-DOS-based application that uses extended memory in an MS-DOS box under standard-mode Windows, there is no general solution to the problems involved.

The VCPI specification is maintained by Phar Lap Software, Inc., and Quarterdeck Office Systems. Windows 3.1 standard mode complies with version 1.10 of the VCPI specification. Because many commercial EMM products can be configured to not provide EMS, DOSX will attempt to use VCPI if the following are true:

  • The INT 67h vector is not null
  • A device named "EMMXXXX0" is present
  • The VCPI detection call (INT 67h, AX=DE00h) succeeds
In particular, DOSX does not require the presence of a LIM 4.0 (Lotus/Intel/Microsoft expanded memory specification) driver or a LIM 3.2 page frame.


Article ID: 82298 - Last Review: October 26, 2013 - Revision: 2.0
Applies to
  • Microsoft Windows Software Development Kit 3.1
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