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x86-64 ○˒｜Definition｜1st｜20261025155446-00-⌔
x86-64
x86-64 (also known as AMD64, x86_64, x64 and, rarely, Intel 64)1 is a 64-bit extension of the x86 instruction set. It was announced in 1999 and first available in the AMD Opteron family in 2003. It introduces two new operating modes: 64-bit mode and compatibility mode, along with a new four-level paging mechanism.
In 64-bit mode, x86-64 supports significantly larger amounts of virtual and physical memory compared to its 32-bit predecessors, allowing computer programs to utilize more memory. The architecture increases the number of general-purpose registers from 8 to 16, and extends each of the register’s width (capacity) to 64 bits.
Floating-point arithmetic is supported through mandatory SSE2 instructions in 64-bit mode. While registers for older floating-point implementations—the x87 and MMX —are still available, they are generally superseded by 128-bit vector registers for the SSE instruction set. These vector registers are capable of storing two double-precision, or up to four single-precision floating-point numbers, along with various integer formats.
In 64-bit mode, instructions are modified to support 64-bit operands and 64-bit addressing mode.
The x86-64 architecture defines a compatibility mode that allows 16-bit and 32-bit user applications to run unmodified alongside 64-bit applications, provided the 64-bit operating system supports them.23 Since the full x86-32 instruction sets remain implemented in hardware without the need for emulation, these older executables can run with little or no performance penalty,4 while newer or modified applications can take advantage of new features of the processor design to achieve performance improvements. Also, processors supporting x86-64 still power on in real mode to maintain backward compatibility with the original 8086 processor, as has been the case with x86 processors since the introduction of protected mode with the 80286.
The original specification, created by AMD and released in 2000, has been implemented by AMD, Intel, and VIA. The AMD K8 microarchitecture, in the Opteron and Athlon 64 processors, was the first to implement it. This was the first significant addition to the x86 architecture designed by a company other than Intel. Intel was forced to follow suit and introduced a modified NetBurst family which was software-compatible with AMD’s specification. VIA Technologies introduced x86-64 in their Isaiah microarchitecture, with the VIA Nano.
The x86-64 architecture was quickly adopted for desktop and laptop personal computers and servers which were commonly configured for 16 GiB (gibibytes) of memory or more. It has effectively replaced the discontinued Intel Itanium architecture (formerly IA-64), which was originally intended to replace the x86 architecture. x86-64 and Itanium are not compatible on the native instruction set level, and operating systems and applications compiled for one architecture cannot be run on the other natively.
In October 2024, the x86 Ecosystem Advisory Group was formed jointly by Intel and AMD. Goals included the enhancement of software consistency and standardizing x86 interfaces and features.5
Printed 2026-10-25.
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Link to original Footnotes
Various names are used for the instruction set. Prior to the launch, x86-64 and x86_64 were used, while upon the release AMD named it AMD64. Intel initially used the name EM64T before finally settling on “Intel 64” for its implementation. Some in the industry, including Apple,^{[2]}$$^{[3]}$$^{[4]} use x86-64 (or x86_64), while others, notably Sun Microsystems (now Oracle Corporation) and Microsoft, use x64. The BSD family of OSs and several Linux distributions (like Debian, Gentoo) use AMD64, as does Microsoft Windows internally.^{[9]}$$^{[10]} ↩
AMD Corporation (December 2016). “Volume 2: System Programming” (PDF). AMD64 Architecture Programmer’s Manual. AMD Corporation. Archived (PDF) from the original on July 13, 2018. Retrieved March 25, 2017. ↩
In practice, 64-bit operating systems generally do not support 16-bit applications, although modern versions of Microsoft Windows contain a limited workaround that effectively supports 16-bit InstallShield and Microsoft ACME installers by silently substituting them with 32-bit code. ↩
“IBM WebSphere Application Server 64-bit Performance Demystified” (PDF). IBM Corporation. September 6, 2007. p. 14. Archived (PDF) from the original on January 25, 2022. Retrieved April 9, 2010. Figures 5, 6 and 7 also show the 32-bit version of WAS runs applications at full native hardware performance on the POWER and x86-64 platforms. Unlike some 64-bit processor architectures, the POWER and x86-64 hardware does not emulate 32-bit mode. Therefore applications that do not benefit from 64-bit features can run with full performance on the 32-bit version of WebSphere running on the above mentioned 64-bit platforms. ↩
“Intel and AMD Form x86 Ecosystem Advisory Group to Accelerate Innovation for Developers and Customers”. Intel Corporation. October 15, 2024. Retrieved June 11, 2026. ↩
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