EM64T

Extended Memory 64-bit Technology (EM64T) is Intel's implementation of AMD's AMD64 architecture, an extension to the IA-32 instruction set which adds 64 bit extensions to the x86 architecture. It is almost entirely compatible with AMD64.

The technology is sometimes incorrectly referred to as EMT64. The correct name is EM64T.

Intel's first processor to actively implement the EM64T technology is the processor codenamed Nocona, and is being sold as Intel's latest dual-processor capable Xeon. Since the 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 Intel had not yet perfected it at the time. Intel has since begun selling EM64T enabled Pentium 4's using the E0 revision of the Prescott core, being sold on the market as the Pentium 4, model F. The E0 revision also adds eXecute Disable(XD) support to EM64T, Intel's name for the NX bit, and should be backported in to the Nocona design soon.

EM64T gives Intel processors direct access to memory quantities in excess of 4 Gigabytes and allows larger applications to be run. The AMD64 architecture also adds additional registers and other performance improvements to the instruction set. With 64-bit memory address limits, the theoretical memory size limit would be 16 Billion Gigabytes (16 Exabytes), although the initial implementations do not support the full 64-bit addressability.

The history of the EM64T project is long and convoluted, mainly due to the internal politics of the Intel Corporation. It started out with the codename Yamhill, named after a county in the state of Oregon in the USA. After several years of denying this project existed, Intel eventually admitted it existed in early 2004, and gave it the codename CT (Clackamas Technology); as it turns out Clackamas is the name of a county adjacent to Yamhill county. Then within the space of weeks of the CT announcement, Intel gave it several new names. After the spring 2004 IDF, it gave it the decidedly lacklustre name of IA-32E (IA-32 Extensions), and then a few weeks later it came up with the name EM64T. Intel's chairman, Craig Barrett, had to admit that this was one of their worst kept secrets.

This technology is not compatible with Intel's earlier 64-bit IA-64 technology, upon which the Itanium processor is based. EM64T cannot run the same software written for IA-64. EM64T is an extension of the 32-bit x86 or IA-32 instruction set, while IA-64 is a completely new architecture. Although IA-64 could directly execute IA-32 instructions it was not able to run the existing IA-32 software fast enough for users. Because the AMD64 architecture is an extension of the existing IA-32 architecture, improvements to IA-32 performance also improve 64-bit performance and vice versa.

Intel tried to keep the existence of EM64T secret for a long time for two reasons. First reason was that it did not want to give its customers mixed signals about the future viability of its Itanium IA-64 processors. However, the success of AMD's Opteron and Athlon 64 processors, based on its AMD64 technology, meant that Intel had to respond to the competitive threat. The second reason for Intel's secrecy was that Intel didn't want to admit that it had to copy from its arch-rival AMD. That's why it gave it the brand name EM64T rather than AMD64, even though they are near-identical twins. The EM64T documentation completely neglects to mention the origin of the instruction set. For this reason it is also known as iAMD64.

See also

• AMD64
• PowerPC 970