Editing X86 (section)

===Long mode===
{{Main|Long mode}}

In the mid 1990s, it was obvious that the 32-bit address space of the x86 architecture was limiting its performance in applications requiring large data sets. A 32-bit address space would allow the processor to directly address only 4&nbsp;GB of data, a size surpassed by applications such as [[Video editing software|video processing]] and [[database engine]]s. Using 64-bit addresses, it is possible to directly address 16&nbsp;[[Exbibyte|EiB]] of data, although most 64-bit architectures do not support access to the full 64-bit address space; for example, AMD64 supports only 48&nbsp;bits from a 64-bit address, split into four paging levels.

In 1999, [[AMD]] published a (nearly) complete specification for a [[64-bit computing|64-bit]] extension of the x86 architecture which they called ''x86-64'' with claimed intentions to produce. That design is currently used in almost all x86 processors, with some exceptions intended for [[embedded system]]s.

Mass-produced ''x86-64'' chips for the general market were available four years later, in 2003, after the time was spent for working prototypes to be tested and refined; about the same time, the initial name ''x86-64'' was changed to ''AMD64''. The success of the AMD64 line of processors coupled with lukewarm reception of the IA-64 architecture forced Intel to release its own implementation of the AMD64 instruction set. Intel had previously implemented support for AMD64<ref>{{Cite web |url=http://www.geek.com/intels-yamhill-technology-x86-64-compatible/ |title=Intel's Yamhill Technology: x86-64 compatible {{!}}Geek.com<!-- Bot generated title --> |access-date=July 18, 2008 |archive-date=September 5, 2012 |archive-url=https://archive.today/20120905073732/http://www.geek.com/intels-yamhill-technology-x86-64-compatible/ |url-status=dead }}</ref> but opted not to enable it in hopes that AMD would not bring AMD64 to market before Itanium's new IA-64 instruction set was widely adopted. It branded its implementation of AMD64 as ''EM64T'', and later rebranded it ''Intel 64''.

In its literature and product version names, Microsoft and Sun refer to AMD64/Intel 64 collectively as ''x64'' in the Windows and [[Solaris (operating system)|Solaris]] operating systems. [[Linux distribution]]s refer to it either as "x86-64", its variant "x86_64", or "amd64". [[Berkeley Software Distribution|BSD]] systems use "amd64" while [[macOS]] uses "x86_64".

Long mode is mostly an extension of the 32-bit instruction set, but unlike the 16–to–32-bit transition, many instructions were dropped in the 64-bit mode. This does not affect actual binary backward compatibility (which would execute legacy code in other modes that retain support for those instructions), but it changes the way assembler and compilers for new code have to work.

This was the first time that a major extension of the x86 architecture was initiated and originated by a manufacturer other than Intel. It was also the first time that Intel accepted technology of this nature from an outside source.