AMD64

AMD64 is a 64-bit extension to the x86 instruction set designed by AMD and introduced in 2003 with the Opteron server processors and the Athlon 64 client line. It provides 64-bit virtual addressing, a 64-bit general-purpose register set, and a largely backward-compatible execution environment that allows running 32-bit x86 code alongside 64-bit applications.

Architecturally, AMD64 introduces a mode called long mode that enables 64-bit operation. In 64-bit mode, registers RAX through R15 are 64 bits wide. The memory management system uses a four-level paging scheme (PML4, PDPT, PD, PT) with 4 KiB pages and optional large pages of 2 MiB or 1 GiB. The architecture defines 2^48 virtual addresses (256 TiB per process) and up to 2^52 physical addresses (up to 4 PiB of RAM). The instruction set includes 64-bit extensions and modern SIMD capabilities, such as SSE2 and beyond.

Compatibility and modes allow the processor to run 32-bit x86 code in a compatibility mode while executing 64-bit code in long mode, facilitating a smooth transition to 64-bit software. AMD64 is commonly referred to as x86-64, while Intel markets a compatible implementation under the name Intel 64 (formerly EM64T).

Adoption and impact: AMD64 has become the de facto standard for 64-bit x86 computing, supported by major operating systems such as Windows, Linux, and macOS on desktop and server platforms. It remains the foundation for contemporary AMD and Intel 64-bit processors and has shaped the development of subsequent generations of x86-64 technology.