Figuratively speaking, AMD's Opteron processor, introduced in April, is still fighting with one hand tied behind its back: Microsoft hasn't yet shipped its promised native versions of Windows XP and Windows Server 2003 for AMD's 64-bit workstation and server CPU, as it has for Intel's Itanium family. But thanks to its ability to run existing 32-bit code at full speed, the AMD underdog has shown it can compete with -- or even beat -- its rival's Itanium and Xeon in benchmark tests, and caught the eye of a number of workstation vendors.

Architecturally, the Opteron -- AMD's eighth-generation x86 processor -- is closer to the company's seventh-generation Athlon XP desktop chip than to Intel's super-heavyweight, built-from-the-ground-up-for-the-enterprise Itanium; it has 128K of Level 1 cache (64K apiece for instructions and data) and 1MB of Level 2 cache, with no on-chip Level 3 cache (the Itanium 2 offers as much as 6MB). The fastest Opterons currently run at 2.0GHz, compared to 1.5GHz for the Itanium 2 and 3.06GHz for Intel's 32-bit Xeon.

But there's a big advantage to the Opteron's relatively high percentage of desktop-processor DNA: AMD's is the first 64-bit processor capable of running 32-bit operating systems and applications in their native mode. This hybrid architecture, called AMD64, was long known by the codename x86-64, because it's an extension of the venerable x86 instruction set that adds support for 64-bit code and for system memory amounts beyond the 4GB ceiling of all 32-bit processors -- the Opteron has 40-bit (1 terabyte) physical and 48-bit (256 teraybtes) virtual memory-addressing capability.

Have It Both Ways

While 64-bit processing is nothing new, seamless compatibility with 32-bit applications is: AMD's sales pitch is that Opteron adopters can continue to use their existing programs, without any compromise or slowdown in performance, while planning their transition to or awaiting the development of 64-bit versions. By contrast, the Itanium family's EPIC or IA-64 architecture makes a clean break with the x86 past -- and while Intel points out that the library of available IA-64 software is growing all the time, it's still dwarfed by the number of titles (to say nothing of in-house, custom-made applications) that use 32-bit code.

The Itanium 2 can run these applications, but only in an emulation mode whose added translation layers take a big hit out of program performance. So AMD woos companies that have sizable investments in 32-bit applications and want workstations that'll run them at high speed -- even after installing a 64-bit operating system and one or two 64-bit applications.

This best-of-both-worlds sales pitch, of course, wouldn't work if the Opteron wasn't competitive with both Intel's 32-bit Pentium 4 and Xeon and 64-bit Itanium 2 CPUs in real-world technical computing performance, but AMD can cite benchmark results that make a strong case, particularly in the floating-point performance that's critical for the intensive number-crunching of CAD, DCC, and financial modeling applications.

In Standard Performance Evaluation Corp.'s SPECfp_peak2000 test, which measures the peak floating-point performance of 1-way workstation or entry-level server CPUs, AMD's entry in this niche -- the Opteron 100 series -- hangs tough, with the 2.0GHz Opteron 146 scoring 4 percent higher than Intel's 3.2GHz, 800MHz-bus Pentium 4. (On the other hand, the next-fastest 3.0GHz Pentium 4 narrowly outperforms the next-fastest 1.8GHz Opteron 144.)

AMD's 2-way workstation/server chips, the Opteron 200 series, fall between Intel's Xeon and Itanium 2 processors in the SPECfp_rate2000 benchmark, which measures a multiprocessor system's ability to scale while executing floating-point instructions: A pair of 2.0GHz Opteron 246s prove half again as powerful as dual 3.06GHz Xeons with 1MB of L3 cache, but two 1.5GHz Itanium 2s are some 42 percent faster still.