An independent analysis by In-Stat/MDR's Microprocessor Report indicates that the 64-bit x86 architectures from AMD and Intel are almost, but not quite, identical. Near-100% software compatibility is possible, but in some cases, programs written for one 64-bit architecture may not run properly on the other 64-bit architecture, according to the report.

Microprocessor Report says it compared all the new instructions, modified instructions, deleted instructions, and modifications to the register files-including control registers, system registers, and registers visible to application programs. Memory-addressing schemes and many other architectural features, such as data-addressing modes, context-switching behavior, interrupt handling, and support for existing 16- and 32-bit x86 execution modes were also compared. In every case, Intel had patterned its 64-bit x86 architecture after AMD64 in almost every detail.

However, Microprocessor Report says it discovered a few differences that even AMD and Intel were unaware of. Some of these differences may be resolved in future 64-bit x86 processors, or even in future steppings of x86 processors already announced or on the market. In other cases, software can easily adapt to the differences by executing slightly different code, after first probing the CPU to learn which 64-bit extensions it supports.

“We found nothing to contradict Intel's promise that its 64-bit x86 processors will run the 64-bit operating systems developed for AMD64,” noted Tom R. Halfhill, a Senior Editor with In-Stat/MDR's Microprocessor Report. “At the same time, Intel's reluctance to make a blanket guarantee about mutual 64-bit software compatibility is justified by the minor differences we discovered. Of course, that's always the case when Intel or AMD introduces new x86 extensions-such as Intel's SSE3 media extensions, which aren't yet supported by AMD.”

Despite the differences, Intel clearly derived its 64-bit architecture by reading AMD's prerelease documentation for AMD64 and by testing AMD64 processors, according to Halfhill. “Intel's reverse-engineering of AMD64 marks a major turning point in the historical relationship between the companies,” Halfhill says. “Although AMD has in the past introduced some innovations to the x86 architecture-the 3DNow multimedia extensions being a prime example-this is the first time AMD has truly steered the direction of the world's most important microprocessor architecture, which Intel invented in 1978 and has closely guarded for 26 years.”

The article, “AMD and Intel Harmonize on 64” can be found in the March 29th edition of In-Stat/MDR's Microprocessor Report, a subscription newsletter for makers and users of high-performance microprocessors.

 
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