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Legacy apps at home with XP

Faithe Wempen Tech Republic

Published: 16 Jul 2002 09:09 BST

For more than a decade, the programmers behind Microsoft Windows have been caught in a double bind. On one hand, Windows needs to be the latest and greatest OS; on the other hand, it needs to maintain a certain amount of backward compatibility. The problem has been handled in different ways over the years.

While it's true that there's no DOS kernel behind Windows XP, and therefore no guarantee of compatibility with 16-bit programs, XP's support of legacy applications through a new Compatibility Mode is more effective than any of the attempts in previous Windows versions. I'll explain how this feature can make nearly any legacy application work with Windows XP.

First of two parts This is the first section of a two-part article on the use of legacy applications in Windows XP. Next week's installment will discuss setting up a Compatibility Mode and running MS-DOS in XP.

What constitutes a legacy application?
A legacy application is one that's designed to run under an older version of the OS. Legacy applications can include 32-bit programs designed for Windows 9x, Windows NT, or Windows 2000; they can also include 16-bit Windows applications designed for Windows 3.x and MS-DOS applications. Basically, legacy is a big basket into which everything is thrown that wasn't specifically designed for Windows XP.

Most legacy applications work fine in Windows XP with the default settings, and you don't have to do anything special to make them work. However, you may run into the occasional maverick program that breaks the rules, usually because of some programming quirk created by a shortsighted programmer who never dreamed that his program would live longer than the version of Windows for which he wrote it. Some applications check the OS version at startup, for example, and won't run unless they find a version they recognise. Others rely on certain system files included with a particular OS or a particular method of accessing the video subsystem or some other hardware.

To be fair, the refusal of a legacy application to run under Windows XP isn't always due to shortsighted programming. There might have been a very good reason at the time to limit a certain application to a fixed set of OSs. Programmers couldn't have guessed in the mid-1990s that Windows XP was on its way and that it would make irrelevant what was then a compelling application. There are also programs of a certain type that have legitimate reasons for not running under Windows XP, such as older disk utilities. See the advice below.

Don't even try it... Before we get into making legacy applications work under Windows XP, let's reflect for a moment upon the old adage that just because you can do something doesn't mean you should. Any older applications that modify the disk structure, such as old versions of Norton Utilities, old versions of Partition Magic, antivirus utilities, and so on, should never be run on Windows XP. Make sure you get a version that specifically supports Windows XP-when you're working with programs that alter the disk structure or content, you don't want to fool around. Even if such a program seems to run okay on Windows XP, I wouldn't trust it with my data. Ditto for command-line utilities from MS-DOS, such as DriveSpace (or its earlier incarnation, DoubleSpace), and third-party compression utilities, such as Stacker. (Anybody remember Stacker? Come on, it's not that old.)If you've decided to ignore the warning in the preceding paragraph, at least heed this one: Do not try to coax such an older disk utility into running under Windows XP by using Compatibility Mode or any special settings. If the program doesn't want to run under XP, it's probably for a very good reason

How Compatibility Mode works Compatibility issues are nothing new. Many of you probably remember the days of MS-DOS's SETVER utility, which enabled MS-DOS to trick older applications into believing that an earlier version of MS-DOS was running. And you may recall in Windows 9x a setting for MS-DOS programs that enabled Windows to convince an application that Windows wasn't running when, in fact, it was.

It's in the same vein that the new Compatibility Mode feature in Windows XP operates but much more smoothly and with better options. Compatibility Mode in Windows XP doesn't actually run the older OS that you specify; it just tricks the program into believing that the specified OS is running by changing certain cues that the program receives. Compatibility Mode can emulate Windows 95, Windows 98, Windows NT 4, and Windows 2000. It doesn't emulate MS-DOS. (I'll talk about MS-DOS programs later.)

In addition to the behind-the-scenes compatibility it sets up, Compatibility Mode can enable a program to use a 256-color depth and a 640 x 480 screen resolution. These video modes have been retired from regular Windows XP operation; you won't find them available in Display Properties anymore. So if you need to run a program that requires that particular color depth or resolution, Compatibility Mode is the way to get access to those.

You can set up Compatibility Mode in Windows XP in two ways. The wizard method takes a rather long time, but it provides some help for figuring out the right settings to use. The other method is quick and dirty, but you run the risk of getting the settings wrong.

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