Neither Microsoft nor Intel planned to leave its 32-bit users in the dust. Both 64-bit Windows and the Itanium architecture provide ample backward-compatibility with the 32-bit world, making it easier to implement these new products in your environment. The next two sections cover the software compatibility provided by Windows and the hardware compatibility provided by Intel's architecture.
Ever since the first version of Windows NT, administrators have become familiar with WOW, the Windows On Windows subsystem. Essentially, WOW emulates 16-bit Windows, allowing Windows NT to run 16-bit Windows applications for backward-compatibility. Although WOW was more commonly used on clients than on servers, where native 32-bit applications reign supreme, WOW is making a fresh appearance in 64-bit Windows. All editions of 64-bit Windows include WOW64, a 32-bit Windows subsystem that hosts 32-bit Windows applications. These applications run in what is basically an emulated 32-bit Windows environment, so they tend to be a bit slower than if they were running on 32-bit hardware in a 32-bit operating environment. Also, 64-bit Windows doesn't include the original WOW, making it tougher to run any 16-bit applications you still have laying around. Certain 16-bit applications will still run, perhaps most notably certain 16-bit Setup utilities used to distribute some older 32-bit applications.
So, by and large, any application written for 32-bit Windows will work fine (albeit a bit more slowly) with 64-bit Windows. You won't be running the 32-bit edition of SQL Server on 64-bit Windows, but you can certainly run 32-bit utilities and administrative tools. And, in case you're wondering, Microsoft will probably release a 64-bit edition of SQL Server around the same time as Windows Server 2003, and Microsoft is hard at work on 64-bit editions of its other popular .NET Enterprise Servers (such as SQL Server), Visual Studio, and more.
Speaking of Visual Studio, software developers don't need to wait for a 64-bit edition to start creating 64-bit applications. They can develop 64-bit applications on 32-bit Windows computers and compile them as native 64-bit applications; 64-bit Windows itself was, after all, written under 32-bit Windows. A Visual Studio 6 service pack has introduced a special 64-bit thunking layer, which enables developers to write 64-bit code on a 32-bit operating system.
64-bit Windows must boot from a GPT partition, but it allows you to use the older MBR scheme to partition other disks. This feature enables you to easily add existing MBR-based disks to your Itanium servers, primarily for migration and coexistence purposes. Because 32-bit Windows can't read GPT-based disks, MBR-based disks provide a convenient common ground for the two architectures. For example, removable drives such as Zip and Jaz drives can't be partitioned with GPT; the 64-bit architecture considers these to be superfloppies and supports only MBR-based partitioning. Other removable drives, including USB and IEEE-1394 (FireWire), must also be partitioned with MBR and can be easily transported between 64-bit and 32-bit machines.
Be very careful when using 32-bit disk management utilities in 64-bit Windows. These utilities won't be aware of the GPT partitioning scheme and might try to use MBR techniques instead, thus corrupting your GPT-based disks. You should use only 64-bit disk management utilities that are compatible with both MBR- and GPT-based disks.
The GPT scheme includes a "dummy" MBR, which gives GPT-unaware applications something to work with that can't be hurt. However, you should still avoid using older disk management utilities to eliminate any possible risk of disk corruption.
As we've implied, most of your 32-bit peripheral equipment will work fine on 64-bit servers, especially external USB and IEEE-1394-based devices. Early Itanium servers still use PCI devices, as the new Infiniband hardware is just coming to market. SCSI and IDE disks work fine in Itanium machines, as do other IDE and SCSI peripherals such as CD-ROM and DVD-ROM drives.