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x86-64 Architecture

x86-64 is the 64-bit extension of the x86 instruction-set architecture used by most desktop and server PCs. It defines the instructions, registers, operating modes, memory rules, and protection mechanisms that software and operating systems rely on when they run on an x86-64 processor.

itComputer architecture and hardware

x86-64 Architecture

x86-64 is a 64-bit instruction-set architecture, or ISA, built as an extension of 32-bit x86. AMD introduced the architecture as AMD64. Intel implements a closely compatible form called Intel 64. Microsoft commonly calls the shared platform x64.

An ISA is the contract visible to software. It defines instructions, registers, data types, addressing, execution modes, exceptions, and system-control facilities. A processor's microarchitecture is the internal design that carries out that contract. Two processors can run the same x86-64 binary while using different pipelines, caches, execution units, and prediction strategies.

The software-visible machine

In 64-bit mode, x86-64 provides sixteen general-purpose registers. The inherited registers become 64 bits wide, such as RAX and RSP, and R8 through R15 add eight more. RIP identifies the next instruction, while RFLAGS records status and control bits. XMM and wider vector registers support floating-point and vector work when the relevant instruction-set extensions are available.

Registers have overlapping names. RAX names all 64 bits, EAX names the low 32 bits, AX names the low 16 bits, and AL names the low 8 bits. Writing a 32-bit general-purpose subregister clears the upper 32 bits of its 64-bit parent. An 8-bit or 16-bit write does not perform that clearing.

Instructions use variable-length encodings. Prefixes, an opcode, addressing fields, displacements, and immediate values can contribute to one instruction. The REX prefix selects 64-bit operand size and exposes the added registers for many legacy encodings. This density preserves compatibility, but it makes instruction boundaries harder to find without decoding from a known boundary.

Long mode and compatibility

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