x86-64 (also known as x64) is a 64-bit extension of IA-32, the 32-bit generation of the x86 instruction set. It supports vastly larger amounts of virtual memory and physical memory than is possible on IA-32, allowing programs to store larger amounts of data in memory.
x86-64 also provides 64-bit general purpose registers and numerous other enhancements. The original specification was created by AMD, and has been implemented by AMD, Intel, VIA, and others. It is fully backwards compatible with 16-bit and 32-bit x86 code. Because the full x86 16-bit and 32-bit instruction sets remain implemented in hardware without any intervening emulation, existing x86 executables run with no compatibility or performance penalties, whereas existing applications that are recoded to take advantage of new features of the processor design may achieve performance improvements.
Prior to launch, "x86-64" and "x86_64" were used to refer to the instruction set. Upon release, AMD named it AMD64. Intel initially used the names IA-32e and EM64T before finally settling on Intel 64 for their implementation. Some in the industry, including Apple, use x86-64 and x86_64, while others, notably Sun Microsystems (now Oracle Corporation) and Microsoft, use x64 while the BSD family of OSs and the Debian Linux distribution use AMD64.
The AMD K8 core was the first to implement the architecture; 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 fully software-compatible with AMD's design and specification. VIA Technologies introduced x86-64 in their VIA Isaiah architecture, with the VIA Nano.
The x86-64 specification is distinct from the Intel Itanium (formerly IA-64) architecture, which is not compatible on the native instruction set level with the x86 architecture.
Linux was the first operating system kernel to run the x86-64 architecture in long mode, starting with the 2.4 version in 2001 (prior to the physical hardware's availability). 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 GNU/Linux, 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 (Application Binary Interface), 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.
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.
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