Something Can Be Done About It
I just had a Misunderstood word to clear up — what in the heck is the difference between the AMD x64 implementation (AMD64) and the Intel implementation (EM64T). A friend just asked me, I tried to sound important and tell him, but in the end I realized I didn’t know jack either. So there you go. Serves me right for not applying my LRH Study Tech.
Here’s the answer from Answers:
During much of AMD’s history, they have produced processors patterned after Intel’s, but, in an ironic twist of computing history, AMD64 has been adopted (under the name EM64T or IA-32e) by Intel—the original creators of the x86 processor line—in newer versions of its Pentium 4, Pentium D, Pentium Extreme Edition, Celeron D, Xeon, and Core 2 processors.
The EM64T project began with the codename Yamhill, named after the Yamhill River in Oregon‘s Willamette Valley. After several years of denying that this project existed, Intel eventually admitted it existed in early 2004, and gave it the codename CT (Clackamas Technology), also named after an Oregon river, the Clackamas. Then within the space of weeks of the CT announcement, Intel gave it several new names. After the spring 2004 IDF, Intel named it IA-32E (IA-32 Extensions) and a few weeks later devised the name EM64T. Intel’s chairman at the time, Craig Barrett, admitted that this was one of their worst kept secrets. A recent white paper
discussing SSE4 and future extensions refers to the instruction set as “Intel64”.
Intel EM64T improves performance by allowing the system to address more than 4 GiB of both virtual and physical memory. Intel EM64T provides support for: 
EM64T was originally implemented on the E revision (Prescott) of Pentium 4 line of microprocessors, which were supported by i915P (Grantsdale) and i925X (Alderwood) chipsets in June 2004. EM64T’s implementation was largely due to the competitive pressure of AMD‘s AMD64 technology implemented on Opteron and Athlon64 lines of microprocessing units, otherwise known as the K8 core, one year earlier in 2003; and the technology was largely built compatible to AMD64, and the then announced Windows XP Professional x64 Edition supporting AMD64 technology. Intel’s first processor to activate the EM64T technology was the multi-socket processor Xeon
codenamed Nocona. Since the Nocona Xeon itself is directly based on Intel’s desktop processor, the Pentium 4, the Pentium 4 also has EM64T technology built in, although as with Hyper-Threading, this feature was not initially enabled on the then-new Prescott
design, likely because enabling EM64T did not coincide with Intel’s stance on x86-64 extensions at that particular time. Intel has since begun selling EM64T enabled Pentium 4s using the E0 revision of the Prescott core, being sold on the market as the Pentium 4, model F. However, the revision F core was targeted at workstations. Intel’s official launch of EM64T to desktop was the N0 Stepping Prescott-2M. The E0 revision also adds eXecute Disable(XD) support to EM64T, Intel’s name for the NX bit, and has been included in the current Xeon codenamed Irwindale. All 9xx/8xx/6xx/5×6/5×1/3×6/3×1 series CPUs have EM64T enabled, as do the Core 2 CPUs, and as will all future Intel CPUs. EM64T is also present in the last members of the Celeron D line.
The first Intel mobile processor supporting EM64T is the Merom version of the Core 2 processor, which was released on
27 July 2006. None of Intel’s earlier notebook CPUs (Core Duo, Pentium M, Celeron M, Mobile Pentium 4) support EM64T.
There are a small number of differences between each instruction set. Compilers generally produce binaries that target both AMD64 and EM64T, making the differences mainly of interest to compiler developers and operating system developers.
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