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The "Microsoft Windows Resource Kit" (WRK) guide for Windows 3.1 incorrectly states on page 248:
Windows 3.1 supports VCPI in both standard mode and 386 enhanced mode.Windows 3.1 is a Virtual Control Program Interface (VCPI) client in standard mode (using DOSX.EXE). Windows 3.1 in 386 enhanced mode is a DOS Protected Mode Interface (DPMI) server (using WIN386.EXE); it does not support VCPI.
It is possible to run some VCPI applications in standard mode. It is not possible to run VCPI applications in 386 enhanced mode.
INT15 and Extended Memory Specification (XMS) are two methods for allocating extended memory. INT15 is rarely used anymore. It can be used for data storage or VCPI (early specification). MS-DOS 5.0's HIMEM.SYS version 2.77 or later has the /INT15=xxxx switch for running these older applications. Windows in standard and 386 enhanced modes does not use INT15 for itself or MS-DOS applications run from Windows. Some MS-DOS applications that can switch from INT15 VCPI to XMS VCPI because INT15 is not available, can run from Windows' standard mode.
XMS can be used for data storage, DPMI calls, and VCPI calls. Windows standard mode uses XMS as data storage and VCPI (DOSX.EXE is a VCPI client). Windows in 386 enhanced mode can use XMS for data storage and DPMI (WIN386.EXE is a DPMI server).
Standard mode can run some VCPI applications because when the MS-DOS- based application is run, Windows, except for DOSX.EXE, is swapped to disk or XMS data. This leaves the MS-DOS application to use any XMS calls or usage it needs. Windows in standard mode understands how to allocate memory from a VCPI provider (server), or a DPMI provider.
Extended MS-DOS Standard Mode 386 Enhanced Mode Memory Use Windows Windows ---------- ------ ------------- ----------------- INT15 data Yes, with No No /INT15= INT15 VCPI Yes, with No No /INT15= XMS data Yes Yes Yes XMS VCPI Yes Yes No XMS DPMI Yes N/A Yes
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