In our Clear Choice Test of Microsoft's recently released 64-bit edition of Windows Server 2003, we found that when you employ optional, kernel-mode processing features, the operating system flies. When you don't, it runs a bit slower than other 64-bit server operating systems we've tested recently.

These Windows 2003 Server x64 kernel options let certain processes run at the kernel code level - in our test case SSL certificate processing, caching and session handling. When you combine these options with mandated 64-bit hardware drivers and the vast amount of memory that a 64-bit processor can address, you can get some of the best performance we've seen on Intel/AMD architectures.

When we used kernel SSL processing, the number of sustained users climbed by 90% over 32-bit Windows Server 2003 processing. When compared with other 64-bit operating systems (Red Hat Enterprise Linux 4.0 [RHEL 4.0] Advance Server and Solaris 10), Windows Server 2003 x64 has a 15% to 20% performance advantage.

Without the kernel processing options, Windows Server 2003 x64 performed slightly under par with competitive 64-bit operating systems in our testing.

The downside to these performance gains is incompatibility issues in terms of the hardware Windows Server 2003 x64 can run on and some of the applications it can support.

The two generic AMD64 white-box systems we tested were incompatible with Windows Server 2003 x64. One wouldn't start the kernel or boot through a kernel load. The other had constant crashes after installation that seemed to be related to motherboard memory timing and additional SCSI hardware driver issues.

Two systems provided by Microsoft OEM partners - Polywell and HP - had no operational issues. Our primary test server was HP's four-way Opteron DL585 server. HP was the only hardware vendor with a full array of hardware drivers posted at Microsoft's Web site when the 64-bit operating system was released in April. Buyers are captive to OEM hardware providers for now. This obviously limits hardware choice: something we didn't experience with the 64-bit editions of Solaris 10, SuSE SLES 9 or RHEL 4.0.

Old DOS and early 16-bit executables (games, WordPerfect 5.1, and Lotus 123 Version 4) didn't work at all or worked initially but then halted abruptly. Microsoft employs a 32-bit emulator called WOW64 that is automatically invoked to run 32-bit applications. We typically saw equal or slightly better performance of these 32-bit applications on Windows Server 2003 x64 vs. 32-bit Windows Server 2003.