x86-64
AMD's backwards-compatible 64-bit extension of the x86 instruction set, announced 1999, first shipped in Opteron (2003), later adopted by Intel as EM64T / Intel 64. The dominant 64-bit ISA for PCs and a leading one for servers.
x86-64 (also called AMD64, Intel 64, EM64T, or simply x64) is a 64-bit extension of the x86 instruction set architecture, designed by AMD and first publicly announced in 1999 with the full specification released in August 2000. The first silicon shipped in the AMD Opteron server processor on April 22, 2003, followed by the Athlon 64 on the desktop later that year. The architecture's defining feature is backwards compatibility. Existing 16-bit and 32-bit x86 code runs unmodified on x86-64 processors at full native speed, while new 64-bit code gets sixteen 64-bit general-purpose registers (up from eight 32-bit registers), a much larger virtual address space (currently up to 256 TiB), mandatory SSE2 for floating-point work, and a four-level paging structure. This evolutionary path stood in deliberate contrast to Intel's clean-sheet IA-64 approach. x86-64 won the 64-bit transition decisively. Intel, having bet on Itanium, was forced to license and implement AMD's extension as EM64T (later Intel 64), shipping it first in the Nocona Xeon in June 2004. Within a few years x86-64 had displaced both IA-32 and Itanium in servers and was sweeping into desktops; today it is the dominant ISA for personal computers and one of two dominant ISAs in servers alongside ARM64. The success of x86-64 is the canonical case study in the power of incremental, compatibility-preserving architecture changes over revolutionary clean-sheet designs.