Configuring the Program Environments

Chapter 23, "Tweaking the GUI," and Chapter 24, "Configuration via Control Panel Applets," covered quite a few of the adjustments that you can make to the Windows XP user environment. Those chapters also addressed a variety of settings that affect the operation of Windows on a more rudimentary level, such as Properties sheets for printers and other devices you might have installed on a typical system or network. In addition to all these settings and properties, Windows XP allows for fine-tuning of program and system handling under the various operating system environments that Windows XP can control.

As discussed in Chapter 1, "Introducing Windows XP Professional," and Chapter 2, "Getting Your Hardware and Software Ready for Windows XP," in addition to standard 32-bit Windows applications, Windows XP can also run programs designed for MS-DOS and Windows 3.x. If you download and install the "Services for UNIX" (Interix) package, it can additionally run POSIX-compliant character-based applications.

How does Windows XP handle programs written for different operating systems? Windows XP has a feature called environment subsystems, which enables Windows XP to emulate a particular operating system's operation and translate a program's request for services into Windows XP commands. Each subsystem basically is an emulator responsible for providing the API for each operating system and then translating the API calls to the 32-bit Windows subsystem, which passes the commands onto Windows XP for processing. For MS-DOS applications, this includes monitoring the program's attempts to access hardware registers and memory locations that had special meaning to MS-DOS, and taking the appropriate action.

Some optimization and configuration of the various environments are possible, typically to allow for higher compatibility with nonWindows XP programs. Or, in some cases, the adjustments possible are purely aesthetic or convenience factors. The following sections describe options for configuring applications and subsystems. They also describe some specific programs or classes of programs (such as DOS-based TSRs) and how best to run them under Windows XP.

In the following sections, you'll learn about settings for the following subsystems:

• Windows 16-bit

• DOS

32-bit Windows tuning was covered earlier in this chapter in the section, "Tuning Windows Performance with the System Applet."

VDMThe Virtual DOS Machine

Windows XP provides support for old MS-DOS and Windows 3.x applications through a program subsystem called the Virtual DOS Machine, or VDM. VDM is a program that mimics the hardware and software of a computer running MS-DOS. This program then loads and executes MS-DOS and Windows 3.x programs in a controlled environment. The "virtual" part signifies that an old program running in the VDM thinks it can directly control hardware like the video card and keyboard, when in fact, the VDM intercepts all hardware control attempts and uses Windows XP to carry out the desired operations safely. An illegal or dangerous hardware operation can simply be rejected. No user application program can directly manipulate hardware on Windows XPthat's why it's so sturdy. When you run a Windows 3.x application or an MS-DOS program, Windows XP runs it through the VDM where it can do whatever it wants without the risk of actually crashing the computer.

The Virtual DOS Machine can be configured by several settings and setup files, which I'll discuss in the following sections.