x64

x64, often written x86-64, is the 64-bit extension of the x86 architecture. It defines a 64-bit instruction set and 64-bit virtual addresses while preserving backward compatibility with 32-bit x86 software. The architecture originated with AMD as AMD64, released in 2003 to extend IA-32, and was later adopted by Intel as Intel 64 (initially EM64T). Today the terms AMD64 and x86-64 are commonly used to refer to the same family.

In operation, x64 provides 64-bit general-purpose registers (such as RAX, RBX, RCX, RDX, RSI, RDI, RBP, RSP, and R8 through R15), a 64-bit program counter, and expanded addressing and data paths. It introduces long mode, enabling 64-bit addressing and registers, while maintaining a compatibility mode that allows 64-bit processors to run existing 32-bit x86 code. Most operating systems offer both 64-bit native editions and 32-bit compatibility support for older software.

Addressing and memory management in x64 systems typically feature a large virtual address space of 2^64 possible addresses, although practical limits depend on the processor, operating system, and hardware. Memory is managed with paging structures that have evolved from four levels to five levels in recent generations, increasing the maximum addressable space.

x64 is widely used in desktop and server computing and is supported by major operating systems including Windows, Linux, and macOS on x86-64 hardware. The architecture supports extensive software ecosystems, performance features such as larger registers and SIMD extensions, and serves as the baseline for most modern 64-bit software.