X86


x86 is a family of complex instruction set computer instruction set architectures initially developed by Intel, based on the 8086 microprocessor and its 8-bit-external-bus variant, the 8088. The 8086 was introduced in 1978 as a fully 16-bit extension of Intel's 8-bit 8080 microprocessor, with memory segmentation as a solution for addressing more memory than can be covered by a plain 16-bit address. The term "x86" came into being because the names of several successors to Intel's 8086 processor end in "86", including the 80186, 80286, 80386 and 80486. Colloquially, their names were "186", "286", "386" and "486".
The term is not synonymous with IBM PC compatibility, as this implies a multitude of other computer hardware. Embedded systems and general-purpose computers used x86 chips before the PC-compatible market started, some of them before the IBM PC debut.
, most desktop and laptop computers sold are based on the x86 architecture family, while mobile categories such as smartphones or tablets are dominated by ARM. At the high end, x86 continues to dominate computation-intensive workstation and cloud computing segments.

Overview

In the 1980s and early 1990s, when the 8088 and 80286 were still in common use, the term x86 usually represented any 8086-compatible CPU. Today, however, x86 usually implies binary compatibility with the 32-bit instruction set of the 80386. This is due to the fact that this instruction set has become something of a lowest common denominator for many modern operating systems and also probably because the term became common after the introduction of the 80386 in 1985.
A few years after the introduction of the 8086 and 8088, Intel added some complexity to its naming scheme and terminology as the "iAPX" of the ambitious but ill-fated Intel iAPX 432 processor was tried on the more successful 8086 family of chips, applied as a kind of system-level prefix. An 8086 system, including coprocessors such as 8087 and 8089, and simpler Intel-specific system chips, was thereby described as an iAPX 86 system. There were also terms iRMX, iSBC, and iSBX, all together under the heading Microsystem 80. However, this naming scheme was quite temporary, lasting for a few years during the early 1980s.
Although the 8086 was primarily developed for embedded systems and small multi-user or single-user computers, largely as a response to the successful 8080-compatible Zilog Z80, the x86 line soon grew in features and processing power. Today, x86 is ubiquitous in both stationary and portable personal computers, and is also used in midrange computers, workstations, servers, and most new supercomputer clusters of the TOP500 list. A large amount of software, including a large list of are using x86-based hardware.
Modern x86 is relatively uncommon in embedded systems, however; small low power applications, and low-cost microprocessor markets, such as home appliances and toys, lack significant x86 presence. Simple 8- and 16-bit based architectures are common here, as well as simpler RISC architectures like ARM and RISC-V, although the x86-compatible VIA C7, VIA Nano, AMD's Geode, Athlon Neo and Intel Atom are examples of 32- and 64-bit designs used in some relatively low-power and low-cost segments.
There have been several attempts, including by Intel, to end the market dominance of the "inelegant" x86 architecture designed directly from the first simple 8-bit microprocessors. Examples of this are the iAPX 432, the Intel 960, Intel 860 and the Intel/Hewlett-Packard Itanium architecture. However, the continuous refinement of x86 microarchitectures, circuitry and semiconductor manufacturing would make it hard to replace x86 in many segments. AMD's 64-bit extension of x86 and the scalability of x86 chips in the form of modern multi-core CPUs, make x86 an example of how continuous refinement of established industry standards can resist the competition from completely new architectures.
For some advanced features, x86 may require a license from Intel, though some do not need it; x86-64 may require an additional license from AMD. The Pentium Pro processor has been on the market for about years as of and so cannot be subject to patent claims. The i686 subset of the x86 architecture is therefore fully open. The Opteron 1000-series processors have equally been on the market for about years as of and so also cannot be subject to patent claims. The AMD K8 subset of the x86 architecture is therefore fully open.

Chronology

The table below lists processor models and model series implementing various architectures in the x86 family, in chronological order. Each line item is characterized by significantly improved or commercially successful processor microarchitecture designs.

History

Designers and manufacturers

At various times, companies such as IBM, VIA, NEC, AMD, TI, STM, Fujitsu, OKI, Siemens, Cyrix, Intersil, C&T, NexGen, UMC, and DM&P started to design or manufacture x86 processors intended for personal computers and embedded systems. Other companies that designed or manufactured x86 or x87 processors include ITT Corporation, National Semiconductor, ULSI System Technology, and Weitek.
Such x86 implementations were seldom simple copies but often employed different internal microarchitectures and different solutions at the electronic and physical levels. Quite naturally, early compatible microprocessors were 16-bit, while 32-bit designs were developed much later. For the personal computer market, real quantities started to appear around 1990 with i386 and i486 compatible processors, often named similarly to Intel's original chips.
After the fully pipelined i486, in 1993 Intel introduced the Pentium brand name for their new set of superscalar x86 designs. With the x86 naming scheme now legally cleared, other x86 vendors had to choose different names for their x86-compatible products, and initially some chose to continue with variations of the numbering scheme: IBM partnered with Cyrix to produce the 5x86 and then the very efficient 6x86 and 6x86MX lines of Cyrix designs, which were the first x86 microprocessors implementing register renaming to enable speculative execution.
AMD meanwhile designed and manufactured the advanced but delayed 5k86, which, internally, was closely based on AMD's earlier 29K RISC design; similar to NexGen's Nx586, it used a strategy such that dedicated pipeline stages decode x86 instructions into uniform and easily handled micro-operations, a method that has remained the basis for most x86 designs to this day.
Some early versions of these microprocessors had heat dissipation problems. The 6x86 was also affected by a few minor compatibility problems, the Nx586 lacked a floating-point unit and pin-compatibility, while the K5 had somewhat disappointing performance when it was introduced.
Customer ignorance of alternatives to the Pentium series further contributed to these designs being comparatively unsuccessful, despite the fact that the K5 had very good Pentium compatibility and the 6x86 was significantly faster than the Pentium on integer code. AMD later managed to grow into a serious contender with the K6 set of processors, which gave way to the very successful Athlon and Opteron.
There were also other contenders, such as Centaur Technology, Rise Technology, and Transmeta. VIA Technologies' energy efficient C3 and C7 processors, which were designed by the Centaur company, were sold for many years following their release in 2005. Centaur's 2008 design, the VIA Nano, was their first processor with superscalar and speculative execution. It was introduced at about the same time as Intel introduced the Intel Atom, its first "in-order" processor after the P5 Pentium.
Many additions and extensions have been added to the original x86 instruction set over the years, almost consistently with full backward compatibility. The architecture family has been implemented in processors from Intel, Cyrix, AMD, VIA Technologies and many other companies; there are also open implementations, such as the Zet SoC platform. Nevertheless, of those, only Intel, AMD, VIA Technologies, and DM&P Electronics hold x86 architectural licenses, and from these, only the first two actively produce modern 64-bit designs, leading to what has been called a "duopoly" of Intel and AMD in x86 processors.
However, in 2014 the Shanghai-based Chinese company Zhaoxin, a joint venture between a Chinese company and VIA Technologies, began designing VIA based x86 processors for desktops and laptops. The release of its newest "7" family of x86 processors, which are not quite as fast as AMD or Intel chips but are still state of the art, had been planned for 2021; as of March 2022 the release had not taken place, however.

From 16-bit and 32-bit to 64-bit architecture

The instruction set architecture has twice been extended to a larger word size. In 1985, Intel released the 32-bit 80386 which gradually replaced the earlier 16-bit chips in computers during the following years; this extended programming model was originally referred to as the i386 architecture but Intel later dubbed it IA-32 when introducing its IA-64 architecture.
In 1999–2003, AMD extended this 32-bit architecture to 64 bits and referred to it as x86-64 in early documents and later as AMD64. Intel soon adopted AMD's architectural extensions under the name IA-32e, later using the name EM64T and finally using Intel 64. Microsoft and Sun Microsystems/Oracle also use term "x64", while many Linux distributions, and the BSDs also use the "amd64" term. Microsoft Windows, for example, designates its 32-bit versions as "x86" and 64-bit versions as "x64", while installation files of 64-bit Windows versions are required to be placed into a directory called "AMD64".

Continued support for 16-bit and 32-bit execution modes

In 2023, Intel proposed a major change to the architecture referred to as X86S. The S in X86S stood for "simplification", which aimed to remove support for legacy execution modes and instructions.
A processor implementing this proposal would have lacked support for legacy mode, started execution directly in long mode and provided a way to switch to 5-level paging without going through the unpaged mode.
The new architecture would have removed support for 16-bit and 32-bit operating systems. 32-bit code would have only been supported for user applications running in ring 3, and would have used the same simplified segmentation as long mode.
Specific removed features would have included:
The draft specification received multiple updates, reaching version 1.2 by June 2024. It was eventually abandoned as of December 2024, following the formation of the x86 Ecosystem Advisory Group by Intel and AMD.