X8664

X86-64, sometimes written as x8664, is a 64-bit extension of the x86 instruction set that preserves backwards compatibility with 32-bit x86 code. It introduces a 64-bit operating mode and expands the width of the general-purpose registers, enabling larger address spaces and improved performance for 64-bit software.

Origin and naming: The extension was developed by AMD and marketed as AMD64 when introduced in 2003 with the Opteron and Athlon 64 processors. Intel later adopted the technology, branding it Intel 64 (formerly EM64T). The architecture is commonly referred to as x86-64 or x64.

Key features: Long mode provides 64-bit mode and a large virtual address space, together with a compatibility mode for running 32-bit code. It adds 64-bit general-purpose registers (RAX, RBX, RCX, etc.), expands instruction decoders, and supports modern extension sets such as SSE, AVX, and beyond. The page-table structure uses four levels, enabling large memory configurations; most current implementations expose a 48-bit virtual address space (with up to 52-bit physical addresses on some CPUs).

Adoption and impact: x86-64 has become the standard for desktop and server computing, with major operating systems and toolchains supporting it. It enables large memory footprints, improved performance for 64-bit applications, and enhanced security features. The architecture remains backward compatible with existing x86 software through compatibility mode.