x8664Amd64

x86-64, also known as AMD64, is a 64-bit extension to the x86 instruction set developed by AMD. It was introduced in 2003 with the Opteron and Athlon 64 processors and quickly became the industry standard for 64-bit x86 computing. The design preserves backward compatibility with 16- and 32-bit x86 software, providing a long mode that enables native 64-bit execution while offering a compatibility mode to run 32-bit code.

A key feature of x86-64 is its expanded general-purpose registers, now including rax, rbx, rcx, rdx, rsi, rdi, rbp, rsp and the additional r8 through r15, all accessible in 64-bit form. The architecture supports a 64-bit virtual address space and large physical address space through multilevel paging and other memory-management enhancements. While the architectural limit is 2^64 bytes, practical implementations commonly use 48-bit virtual addressing in current CPUs, with extensions under active development.

The instruction set has been extended with numerous enhancements, including advanced vector extensions (SSE, SSE2, and later AVX), as well as new instructions and prefixes to support 64-bit operation and improved performance. Long mode introduces a separate 64-bit operating state, while a compatibility mode allows 32-bit operating systems and applications to run without modification.

Adoption and naming: AMD64 is widely used in desktops, laptops, and servers. Intel adopted the same extension, branding it Intel 64 (formerly EM64T). The term x86-64 is now the common descriptor for this 64-bit extension of the x86 architecture, reflecting its broad industry acceptance and compatibility with existing software ecosystems.