Intel's 64-bit CPUs. Also known as "x64," "AMD64" and "Intel 64," x86-64 is the 64-bit version of Intel's x86 architecture. Introduced in 1999 and originally designated as EM64T (Extended Memory 64 Technology), x86-64 is a set of 64-bit instructions compatible with the ones AMD had previously announced (see
AMD64). In 2004, Xeons were the first Intel CPUs to incorporate x86-64 along with enhancements such as the PCI Express bus and DDR2 memory.
IA-32 and IA-64
The x86-64 line uses the IA-32 designation because it is backward compatible with Intel's 32-bit CPUs. The Itanium line used the IA-64 designation because it was designed as a 64-bit CPU. See
IA-32,
IA-64,
Itanium,
x86 and
64-bit computing.
Legacy Mode
PCs with x86-64 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 x86-64 architecture when the operating system, all drivers and the applications have been recompiled for 64 bits.