Platform Restrictions for MS-DOS-Based Graphics Programs

• Microsoft Windows NT operating system, version 3.1
• Microsoft Windows NT Advanced Server, version 3.1
• Microsoft Windows NT Workstation version 3.5, 3.51, and 4.0
• Microsoft Windows NT Server version 3.5, 3.51, and 4.0

The text below discusses the hardware platform restrictions Windows NT imposes on MS-DOS-based graphics applications.

Intel x86-Based Computers

An MS-DOS-based graphical application cannot run in a window on a computer with an Intel microprocessor. This restriction is caused by the overhead involved in trapping register and memory accesses from the VGA areas to the VDM (virtual MS-DOS machine) to emulate the graphics hardware in software; the graphics performance would be too low for the application to be useful. Therefore, when an application switches into graphics mode, the VDM switches to full-screen mode.

In full-screen mode, most applications can run with native performance directly on the installed video adapter. Windows maps VGA memory to the appropriate place in the VDM and maps the relevant registers from the application to the video adapter. Note that not all miniports map every video card extended register, only enough registers to provide VGA compatibility. This behavior may prevent some applications (such as IBM PC3270 and Aldus Persuasion Player) from working with all SVGA cards. In general, such programs experience the same problems that they do running with 16-bit Microsoft Windows version 3.1.

In some cases, an application may work correctly only if you start it from a full screen Command Prompt (which you can choose in an appropriate program information file [PIF]). This usually occurs because the application checks for the installed video card. The check fails when the application is in a window because only the standard VGA emulation is available to the BIOS; the extensions on the installed card are not available.

If you press ALT+ENTER, Windows NT displays a window that contains a frozen image of the full-screen window. Windows NT halts the application and indicates its halted state by adding "-FROZEN" to the window title). If you press ALT+ENTER again, Windows NT resumes the program full screen.

An application changes to full screen mode only after it has system focus. If an application runs in text mode without focus and reaches a point where it switches to graphics mode, it does not switch to full screen until it receives the focus. Until this occurs, Windows NT adds the "-FROZEN" suffix to the window title.

RISC-Based Computers

A graphical MS-DOS-based application can run in a window on a machine with a RISC microprocessor. When the application changes to graphics mode, the size of the window changes to maintain the correct aspect ratio. The MS-DOS window on a RISC-basic computer emulates a Video Seven VGA card. If you see two mouse images while the window is open (one the application produces and the Windows NT mouse pointer), choose Hide Mouse Pointer from the window system menu and use the application mouse pointer. To restore the system pointer image, press any system hot key.