A 64-bit upgrade to Intel's 32-bit x86 (IA-32) architecture. Originally designated as EM64T (Extended Memory 64 Technology), Intel 64 added a set of 64-bit instructions that were compatible with AMD's 64-bit instructions (see AMD64). Introduced in 2004, Xeons were the first CPUs to incorporate Intel 64 along with other enhancements such as the PCI Express bus and DDR2 memory. With the introduction of Intel 64, Intel offers 64 bits in two CPU families: the 32/64-bit x86 (IA-32) line and the 64-bit Itanium (IA-64) line. See IA-64, x64, x86 and 64-bit computing. Legacy Mode PCs with Intel 64 CPUs that run only 32-bit software (OS, drivers and applications) will not experience any performance increase. Compatibility Mode When only the operating system and drivers have been recompiled for 64 bits, and the applications have not, the applications may experience only a slight performance increase. 64-Bit Mode PCs take full advantage of the Intel 64 architecture when the operating system, all drivers and the applications have been recompiled for 64 bits.