x86-64 is the 64-bit version of the x86 instruction set. It introduces two new modes of operation, 64-bit mode and compatibility mode, along with a new 4-level paging mode. With 64-bit mode and the new paging mode, it supports vastly larger amounts of virtual memory and physical memory than is possible on its 32-bit predecessors, allowing programs to store larger amounts of data in memory. x86-64 also expands general-purpose registers to 64-bit, as well extends the number of them from 8 to 16, and provides numerous other enhancements. Floating point operations are supported via mandatory SSE2-like instructions, and x87/MMX style registers are generally not used ; instead, a set of 32 vector registers, 128 bits each, is used. In 64-bit mode, instructions are modified to support 64-bit operands and 64-bit addressing mode. The compatibility mode allows 16- and 32-bit user applications to run unmodified coexisting with 64-bit applications if the 64-bit operating system supports them. As the full x86 16-bit and 32-bit instruction sets remain implemented in hardware without any intervening emulation, these older executables can run with little or no performance penalty,
while newer or modified applications can take advantage of new features of the processor design to achieve performance improvements. Also, a processor supporting x86-64 still powers on in real mode for full backward compatibility, as x86 processors have done since 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 VIA Isaiah architecture, with the VIA Nano.
The x86-64 architecture is distinct from the Intel Itanium architecture, which is not compatible on the native instruction set level with the x86 architecture. Operating systems and applications compiled for one cannot be run on the other.



AMD64 was created as an alternative to the radically different IA-64 architecture, which was designed by Intel and Hewlett Packard. Originally announced in 1999 while a full specification became available in August 2000, the AMD64 architecture was positioned by AMD from the beginning as an evolutionary way to add 64-bit computing capabilities to the existing x86 architecture, as opposed to Intel's approach of creating an entirely new 64-bit architecture with IA-64.
The first AMD64-based processor, the Opteron, was released in April 2003.


AMD's processors implementing the AMD64 architecture include Opteron, Athlon 64, Athlon 64 X2, Athlon 64 FX, Athlon II, Turion 64, Turion 64 X2, Sempron, Phenom, Phenom II, FX, Fusion/APU and Ryzen/Epyc.

Architectural features

The primary defining characteristic of AMD64 is the availability of 64-bit general-purpose processor registers, 64-bit integer arithmetic and logical operations, and 64-bit virtual addresses. The designers took the opportunity to make other improvements as well. Some of the most significant changes are described below.
; 64-bit integer capability
; Additional registers
; Additional XMM registers
; Larger virtual address space
; Larger physical address space
; Larger physical address space in legacy mode
; Instruction pointer relative data access
; SSE instructions
; No-Execute bit
; Removal of older features

Virtual address space details

Canonical form addresses

Although virtual addresses are 64 bits wide in 64-bit mode, current implementations do not allow the entire virtual address space of 264 bytes to be used.
This would be approximately four billion times the size of the virtual address space on 32-bit machines. Most operating systems and applications will not need such a large address space for the foreseeable future, so implementing such wide virtual addresses would simply increase the complexity and cost of address translation with no real benefit. AMD, therefore, decided that, in the first implementations of the architecture, only the least significant 48 bits of a virtual address would actually be used in address translation.
In addition, the AMD specification requires that the most significant 16 bits of any virtual address, bits 48 through 63, must be copies of bit 47. If this requirement is not met, the processor will raise an exception. Addresses complying with this rule are referred to as "canonical form." Canonical form addresses run from 0 through 00007FFF'FFFFFFFF, and from FFFF8000'00000000 through FFFFFFFF'FFFFFFFF, for a total of 256 TiB of usable virtual address space. This is still 65,536 times larger than the virtual 4 GiB address space of 32-bit machines.
This feature eases later scalability to true 64-bit addressing. Many operating systems take the higher-addressed half of the address space for themselves and leave the lower-addressed half for application code, user mode stacks, heaps, and other data regions. The "canonical address" design ensures that every AMD64 compliant implementation has, in effect, two memory halves: the lower half starts at 00000000'00000000 and "grows upwards" as more virtual address bits become available, while the higher half is "docked" to the top of the address space and grows downwards. Also, enforcing the "canonical form" of addresses by checking the unused address bits prevents their use by the operating system in tagged pointers as flags, privilege markers, etc., as such use could become problematic when the architecture is extended to implement more virtual address bits.
The first versions of Windows for x64 did not even use the full 256 TiB; they were restricted to just 8 TiB of user space and 8 TiB of kernel space. Windows did not support the entire 48-bit address space until Windows 8.1, which was released in October 2013.

Page table structure

The 64-bit addressing mode is a superset of Physical Address Extensions ; because of this, page sizes may be 4 KiB or 2 MiB. Long mode also supports page sizes of 1 GiB. Rather than the three-level page table system used by systems in PAE mode, systems running in long mode use four levels of page table: PAE's Page-Directory Pointer Table is extended from four entries to 512, and an additional Page-Map Level 4 Table is added, containing 512 entries in 48-bit implementations. A full mapping hierarchy of 4 KiB pages for the whole 48-bit space would take a bit more than 512 GiB of memory.
Intel has implemented a scheme with a 5-level page table, which allows Intel 64 processors to support a 57-bit virtual address space. Further extensions may allow full 64-bit virtual address space and physical memory by expanding the page table entry size to 128-bit, and reduce page walks in the 5-level hierarchy by using a larger 64 KiB page allocation size that still supports 4 KiB page operations for backward compatibility.

Operating system limits

The operating system can also limit the virtual address space. Details, where applicable, are given in the "Operating system compatibility and characteristics" section.

Physical address space details

Current AMD64 processors support a physical address space of up to 248 bytes of RAM, or 256 TiB. However,, there were no known x86-64 motherboards that support 256 TiB of RAM.
The operating system may place additional limits on the amount of RAM that is usable or supported. Details on this point are given in the "Operating system compatibility and characteristics" section of this article.

Operating modes

The architecture has three primary modes of operation, long mode, legacy mode, and real mode.

Long mode

Long mode is the architecture's intended primary mode of operation; it is a combination of the processor's native 64-bit mode and a combined 32-bit and 16-bit compatibility mode. It is used by 64-bit operating systems. Under a 64-bit operating system, 64-bit programs run under 64-bit mode, and 32-bit and 16-bit protected mode applications run under compatibility mode. Real-mode programs and programs that use virtual 8086 mode at any time cannot be run in long mode unless those modes are emulated in software. However, such programs may be started from an operating system running in long mode on processors supporting VT-x or AMD-V by creating a virtual processor running in the desired mode.
Since the basic instruction set is the same, there is almost no performance penalty for executing protected mode x86 code. This is unlike Intel's IA-64, where differences in the underlying instruction set mean that running 32-bit code must be done either in emulation of x86 or with a dedicated x86 coprocessor. However, on the x86-64 platform, many x86 applications could benefit from a 64-bit recompile, due to the additional registers in 64-bit code and guaranteed SSE2-based FPU support, which a compiler can use for optimization. However, applications that regularly handle integers wider than 32 bits, such as cryptographic algorithms, will need a rewrite of the code handling the huge integers in order to take advantage of the 64-bit registers.

Legacy mode

Legacy mode is the mode used by 32-bit or 16-bit "protected mode" operating systems. In this mode, the processor acts like an older x86 processor, and only 16-bit and 32-bit code can be executed. Legacy mode allows for a maximum of 32 bit virtual addressing which limits the virtual address space to 4 GiB. 64-bit programs cannot be run from legacy mode.

Real mode

is the initial mode of operation when the processor is initialized. It is backwards compatible with the original 8086 and 8088 processors. Real mode is primarily used today by operating system bootloaders, which are required by the architecture to configure [|virtual memory details] before transitioning to higher modes.

Intel 64

Intel 64 is Intel's implementation of x86-64, used and implemented in various processors made by Intel.


Historically, AMD has developed and produced processors with instruction sets patterned after Intel's original designs, but with x86-64, roles were reversed: Intel found itself in the position of adopting the ISA which AMD had created as an extension to Intel's own x86 processor line.
Intel's project was originally codenamed Yamhill. After several years of denying its existence, Intel announced at the February 2004 IDF that the project was indeed underway. Intel's chairman at the time, Craig Barrett, admitted that this was one of their worst-kept secrets.
Intel's name for this instruction set has changed several times. The name used at the IDF was CT ; within weeks they began referring to it as IA-32e and in March 2004 unveiled the "official" name EM64T. In late 2006 Intel began instead using the name Intel 64 for its implementation, paralleling AMD's use of the name AMD64.
The first processor to implement Intel 64 was the multi-socket processor Xeon code-named Nocona in June 2004. In contrast, the initial Prescott chips did not enable this feature. Intel subsequently began selling Intel 64-enabled Pentium 4s using the E0 revision of the Prescott core, being sold on the OEM market as the Pentium 4, model F. The E0 revision also adds eXecute Disable to Intel 64, and has been included in then current Xeon code-named Irwindale. Intel's official launch of Intel 64 in mainstream desktop processors was the N0 stepping Prescott-2M.
The first Intel mobile processor implementing Intel 64 is the Merom version of the Core 2 processor, which was released on July 27, 2006. None of Intel's earlier notebook CPUs implement Intel 64.


Intel's processors implementing the Intel64 architecture include the Pentium 4 F-series/5x1 series, 506, and 516, Celeron D models 3x1, 3x6, 355, 347, 352, 360, and 365 and all later Celerons, all models of Xeon since "Nocona", all models of Pentium Dual-Core processors since "Merom-2M", the Atom 230, 330, D410, D425, D510, D525, N450, N455, N470, N475, N550, N570, N2600 and N2800, all versions of the Pentium D, Pentium Extreme Edition, Core 2, Core i9, Core i7, Core i5, and Core i3 processors, and the Xeon Phi 7200 series processors.

VIA's x86-64 implementation

introduced their first implementation of the x86-64 architecture in 2008 after five years of development by its CPU division, Centaur Technology.
Codenamed "Isaiah", the 64-bit architecture was unveiled on January 24, 2008, and launched on May 29 under the VIA Nano brand name.
The processor supports a number of VIA-specific x86 extensions designed to boost efficiency in low-power appliances.
It is expected that the Isaiah architecture will be twice as fast in integer performance and four times as fast in floating-point performance as the previous-generation VIA Esther at an equivalent clock speed. Power consumption is also expected to be on par with the previous-generation VIA CPUs, with thermal design power ranging from 5 W to 25 W.
Being a completely new design, the Isaiah architecture was built with support for features like the x86-64 instruction set and x86 virtualization which were unavailable on its predecessors, the VIA C7 line, while retaining their encryption extensions.

Differences between AMD64 and Intel 64

Although nearly identical, there are some differences between the two instruction sets in the semantics of a few seldom used machine instructions, which are mainly used for system programming. Compilers generally produce executables that avoid any differences, at least for ordinary application programs. This is therefore of interest mainly to developers of compilers, operating systems and similar, which must deal with individual and special system instructions.

Recent implementations

In supercomputers tracked by TOP500, the appearance of 64-bit extensions for the x86 architecture enabled 64-bit x86 processors by AMD and Intel to replace most RISC processor architectures previously used in such systems, as well as 32-bit x86, even though Intel itself initially tried unsuccessfully to replace x86 with a new incompatible 64-bit architecture in the Itanium processor.
, the main non-x86 CPU architecture which is still used in supercomputing is the Power ISA used by IBM POWER microprocessors, with SPARC being far behind in numbers on TOP500, while in 2011 a Fujitsu SPARC64 VIIIfx based supercomputer without co-processors reached number one. The first ARM-based supercomputer appeared on the list in 2018 and, in recent years, non-CPU architecture co-processors have also played a big role in performance. Intel's Xeon Phi "Knights Corner" coprocessors, which implement a subset of x86-64 with some vector extensions, are also used, along with x86-64 processors, in the Tianhe-2 supercomputer.

Operating system compatibility and characteristics

The following operating systems and releases support the x86-64 architecture in long mode.


DragonFly BSD

Preliminary infrastructure work was started in February 2004 for a x86-64 port. This development later stalled. Development started again during July 2007
and continued during Google Summer of Code 2008 and SoC 2009. The first official release to contain x86-64 support was version 2.4.


first added x86-64 support under the name "amd64" as an experimental architecture in 5.1-RELEASE in June 2003. It was included as a standard distribution architecture as of 5.2-RELEASE in January 2004. Since then, FreeBSD has designated it as a Tier 1 platform. The 6.0-RELEASE version cleaned up some quirks with running x86 executables under amd64, and most drivers work just as they do on the x86 architecture. Work is currently being done to integrate more fully the x86 application binary interface, in the same manner as the Linux 32-bit ABI compatibility currently works.


x86-64 architecture support was first committed to the NetBSD source tree on June 19, 2001. As of NetBSD 2.0, released on December 9, 2004, NetBSD/amd64 is a fully integrated and supported port.
32-bit code is still supported in 64-bit mode, with a netbsd-32 kernel compatibility layer for 32-bit syscalls. The NX bit is used to provide non-executable stack and heap with per-page granularity.


has supported AMD64 since OpenBSD 3.5, released on May 1, 2004. Complete in-tree implementation of AMD64 support was achieved prior to the hardware's initial release because AMD had loaned several machines for the project's hackathon that year. OpenBSD developers have taken to the platform because of its support for the NX bit, which allowed for an easy implementation of the W^X feature.
The code for the AMD64 port of OpenBSD also runs on Intel 64 processors which contains cloned use of the AMD64 extensions, but since Intel left out the page table NX bit in early Intel 64 processors, there is no W^X capability on those Intel CPUs; later Intel 64 processors added the NX bit under the name "XD bit". Symmetric multiprocessing works on OpenBSD's AMD64 port, starting with release 3.6 on November 1, 2004.


It is possible to enter long mode under DOS without a DOS extender, but the user must return to real mode in order to call BIOS or DOS interrupts.
It may also be possible to enter long mode with a DOS extender similar to DOS/4GW, but more complex since x86-64 lacks virtual 8086 mode. DOS itself is not aware of that, and no benefits should be expected unless running DOS in an emulation with an adequate virtualization driver backend, for example: the mass storage interface.


was the first operating system kernel to run the x86-64 architecture in long mode, starting with the 2.4 version in 2001. Linux also provides backward compatibility for running 32-bit executables. This permits programs to be recompiled into long mode while retaining the use of 32-bit programs. Several Linux distributions currently ship with x86-64-native kernels and userlands. Some, such as Arch Linux, SUSE, Mandriva, and Debian allow users to install a set of 32-bit components and libraries when installing off a 64-bit DVD, thus allowing most existing 32-bit applications to run alongside the 64-bit OS. Other distributions, such as Fedora, Slackware and Ubuntu, are available in one version compiled for a 32-bit architecture and another compiled for a 64-bit architecture. Fedora and Red Hat Enterprise Linux allow concurrent installation of all userland components in both 32 and 64-bit versions on a 64-bit system.
x32 ABI, introduced in Linux 3.4, allows programs compiled for the x32 ABI to run in the 64-bit mode of x86-64 while only using 32-bit pointers and data fields.
Though this limits the program to a virtual address space of 4 GB it also decreases the memory footprint of the program and in some cases can allow it to run faster.
64-bit Linux allows up to 128 TB of virtual address space for individual processes, and can address approximately 64 TB of physical memory, subject to processor and system limitations.


Mac OS X 10.4.7 and higher versions of Mac OS X 10.4 run 64-bit command-line tools using the POSIX and math libraries on 64-bit Intel-based machines, just as all versions of Mac OS X 10.4 and 10.5 run them on 64-bit PowerPC machines. No other libraries or frameworks work with 64-bit applications in Mac OS X 10.4.
The kernel, and all kernel extensions, are 32-bit only.
Mac OS X 10.5 supports 64-bit GUI applications using Cocoa, Quartz, OpenGL, and X11 on 64-bit Intel-based machines, as well as on 64-bit PowerPC machines.
All non-GUI libraries and frameworks also support 64-bit applications on those platforms. The kernel, and all kernel extensions, are 32-bit only.
Mac OS X 10.6 is the first version of macOS that supports a 64-bit kernel. However, not all 64-bit computers can run the 64-bit kernel, and not all 64-bit computers that can run the 64-bit kernel will do so by default.
The 64-bit kernel, like the 32-bit kernel, supports 32-bit applications; both kernels also support 64-bit applications. 32-bit applications have a virtual address space limit of 4 GB under either kernel.
OS X 10.8 includes only the 64-bit kernel, but continues to support 32-bit applications.
macOS 10.15 includes only the 64-bit kernel and no longer supports 32-bit applications.
The 64-bit kernel does not support 32-bit kernel extensions, and the 32-bit kernel does not support 64-bit kernel extensions.
macOS uses the universal binary format to package 32- and 64-bit versions of application and library code into a single file; the most appropriate version is automatically selected at load time. In Mac OS X 10.6, the universal binary format is also used for the kernel and for those kernel extensions that support both 32-bit and 64-bit kernels.


10 and later releases support the x86-64 architecture.
For Solaris 10, just as with the SPARC architecture, there is only one operating system image, which contains a 32-bit kernel and a 64-bit kernel; this is labeled as the "x64/x86" DVD-ROM image. The default behavior is to boot a 64-bit kernel, allowing both 64-bit and existing or new 32-bit executables to be run. A 32-bit kernel can also be manually selected, in which case only 32-bit executables will run. The isainfo command can be used to determine if a system is running a 64-bit kernel.
For Solaris 11, only the 64-bit kernel is provided. However, the 64-bit kernel supports both 32- and 64-bit executables, libraries, and system calls.


x64 editions of Microsoft Windows client and server—Windows XP Professional x64 Edition and Windows Server 2003 x64 Edition—were released in March 2005. Internally they are actually the same build, as they share the same source base and operating system binaries, so even system updates are released in unified packages, much in the manner as Windows 2000 Professional and Server editions for x86. Windows Vista, which also has many different editions, was released in January 2007. Windows 7 was released in July 2009. Windows Server 2008 R2 was sold in only x64 and Itanium editions; later versions of Windows Server only offer an x64 edition.
Versions of Windows for x64 prior to Windows 8.1 and Windows Server 2012 R2 offer the following:
Under Windows 8.1 and Windows Server 2012 R2, both user mode and kernel mode virtual address spaces have been extended to 128 TiB. These versions of Windows will not install on processors that lack the CMPXCHG16B instruction.
The following additional characteristics apply to all x64 versions of Windows:
Both PlayStation 4 and Xbox One and their successors incorporate AMD x86-64 processors, based on the Jaguar microarchitecture. Firmware and games are written in x86-64 code; no legacy x86 code is involved.

Industry naming conventions

Since AMD64 and Intel 64 are substantially similar, many software and hardware products use one vendor-neutral term to indicate their compatibility with both implementations. AMD's original designation for this processor architecture, "x86-64", is still sometimes used for this purpose, as is the variant "x86_64". Other companies, such as Microsoft and Sun Microsystems/Oracle Corporation, use the contraction "x64" in marketing material.
The term IA-64 refers to the Itanium processor, and should not be confused with x86-64, as it is a completely different instruction set.
Many operating systems and products, especially those that introduced x86-64 support prior to Intel's entry into the market, use the term "AMD64" or "amd64" to refer to both AMD64 and Intel 64.
x86-64/AMD64 was solely developed by AMD. AMD holds patents on techniques used in AMD64; those patents must be licensed from AMD in order to implement AMD64. Intel entered into a cross-licensing agreement with AMD, licensing to AMD their patents on existing x86 techniques, and licensing from AMD their patents on techniques used in x86-64. In 2009, AMD and Intel settled several lawsuits and cross-licensing disagreements, extending their cross-licensing agreements.