x86 (also known as 80x86^[2] or the 8086 family^[3]) is a family of complex instruction set computer (CISC) instruction set architectures^[a] initially developed by Intel based on the Intel 8086 microprocessor and its 8088 variant. 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 processors. 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,^[b] some of them before the IBM PC (1981) debut.

As of June 2022^[update], most desktop and laptop computers sold are based on the x86 architecture family,^[4] 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.^[5] The fastest supercomputer in the TOP500 list for June 2022 was the first exascale system, Frontier,^[6] built using AMD Epyc CPUs based on the x86 ISA; it broke the 1 exaFLOPS barrier in May 2022.^[7]

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 a binary compatibility also 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 probably also 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,^[c] applied as a kind of system-level prefix. An 8086 system, including coprocessors such as 8087 and 8089, and simpler Intel-specific system chips,^[d] was thereby described as an iAPX 86 system.^[8][e] There were also terms iRMX (for operating systems), iSBC (for single-board computers), and iSBX (for multimodule boards based on the 8086-architecture), all together under the heading Microsystem 80.^[9][10] However, this naming scheme was quite temporary, lasting for a few years during the early 1980s.^[f]

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,^[11] 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 x86 operating systems are using x86-based hardware.

Modern x86 is relatively uncommon in embedded systems, however, and small low power applications (using tiny batteries), and low-cost microprocessor markets, such as home appliances and toys, lack significant x86 presence.^[g] Simple 8- and 16-bit based architectures are common here, as well as simpler RISC architectures like 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 (a project originally named the Intel 8800^[12]), 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 (which Intel eventually responded to with a compatible design)^[13] and the scalability of x86 chips in the form of modern multi-core CPUs, is underlining x86 as an example of how continuous refinement of established industry standards can resist the competition from completely new architectures.^[14]

Chronology

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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,^[h] AMD, TI, STM, Fujitsu, OKI, Siemens, Cyrix, Intersil, C&T, NexGen, UMC, and DM&P started to design or manufacture^[i] x86 processors (CPUs) 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 (which, unlike numbers, could be trademarked) 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 (M1) and 6x86MX (MII) 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 (K5), 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 (FPU) and (the then crucial) pin-compatibility, while the K5 had somewhat disappointing performance when it was (eventually) 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.^[j] 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 (formerly IDT), 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 (in 2008) 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.^[k] 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 (currently inactive).^[17] 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^[18] of x86 processors (e.g. KX-7000), which are not quite as fast as AMD or Intel chips but are still state of the art,^[19] had been planned for 2021; as of March 2022 the release had not taken place, however.^[20]

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 (later known as i386) which gradually replaced the earlier 16-bit chips in computers (although typically not in embedded systems) during the following years; this extended programming model was originally referred to as the i386 architecture (like its first implementation) but Intel later dubbed it IA-32 when introducing its (unrelated) 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".^[21]

In 2023, Intel proposed a major change to the architecture referred to as x86-S (with S standing for "simplification"), which aims to remove support for legacy execution modes and instructions. A processor implementing this proposal would start execution directly in long mode and would only support 64-bit operating systems. 32-bit code would only be supported for user applications running in ring 3, and would use the same simplified segmentation as long mode.^[22][23]

Basic properties of the architecture

The x86 architecture is a variable instruction length, primarily "CISC" design with emphasis on backward compatibility. The instruction set is not typical CISC, however, but basically an extended version of the simple eight-bit 8008 and 8080 architectures. Byte-addressing is enabled and words are stored in memory with little-endian byte order. Memory access to unaligned addresses is allowed for almost all instructions. The largest native size for integer arithmetic and memory addresses (or offsets) is 16, 32 or 64 bits depending on architecture generation (newer processors include direct support for smaller integers as well). Multiple scalar values can be handled simultaneously via the SIMD unit present in later generations, as described below.^[l] Immediate addressing offsets and immediate data may be expressed as 8-bit quantities for the frequently occurring cases or contexts where a −128..127 range is enough. Typical instructions are therefore 2 or 3 bytes in length (although some are much longer, and some are single-byte).

To further conserve encoding space, most registers are expressed in opcodes using three or four bits, the latter via an opcode prefix in 64-bit mode, while at most one operand to an instruction can be a memory location.^[m] However, this memory operand may also be the destination (or a combined source and destination), while the other operand, the source, can be either register or immediate. Among other factors, this contributes to a code size that rivals eight-bit machines and enables efficient use of instruction cache memory. The relatively small number of general registers (also inherited from its 8-bit ancestors) has made register-relative addressing (using small immediate offsets) an important method of accessing operands, especially on the stack. Much work has therefore been invested in making such accesses as fast as register accesses—i.e., a one cycle instruction throughput, in most circumstances where the accessed data is available in the top-level cache.

Floating point and SIMD

A dedicated floating-point processor with 80-bit internal registers, the 8087, was developed for the original 8086. This microprocessor subsequently developed into the extended 80387, and later processors incorporated a backward compatible version of this functionality on the same microprocessor as the main processor. In addition to this, modern x86 designs also contain a SIMD-unit (see SSE below) where instructions can work in parallel on (one or two) 128-bit words, each containing two or four floating-point numbers (each 64 or 32 bits wide respectively), or alternatively, 2, 4, 8 or 16 integers (each 64, 32, 16 or 8 bits wide respectively).

The presence of wide SIMD registers means that existing x86 processors can load or store up to 128 bits of memory data in a single instruction and also perform bitwise operations (although not integer arithmetic^[n]) on full 128-bits quantities in parallel. Intel's Sandy Bridge processors added the Advanced Vector Extensions (AVX) instructions, widening the SIMD registers to 256 bits. The Intel Initial Many Core Instructions implemented by the Knights Corner Xeon Phi processors, and the AVX-512 instructions implemented by the Knights Landing Xeon Phi processors and by Skylake-X processors, use 512-bit wide SIMD registers.

Current implementations

During execution, current x86 processors employ a few extra decoding steps to split most instructions into smaller pieces called micro-operations. These are then handed to a control unit that buffers and schedules them in compliance with x86-semantics so that they can be executed, partly in parallel, by one of several (more or less specialized) execution units. These modern x86 designs are thus pipelined, superscalar, and also capable of out of order and speculative execution (via branch prediction, register renaming, and memory dependence prediction), which means they may execute multiple (partial or complete) x86 instructions simultaneously, and not necessarily in the same order as given in the instruction stream.^[24] Some Intel CPUs (Xeon Foster MP, some Pentium 4, and some Nehalem and later Intel Core processors) and AMD CPUs (starting from Zen) are also capable of simultaneous multithreading with two threads per core (Xeon Phi has four threads per core). Some Intel CPUs support transactional memory (TSX).

When introduced, in the mid-1990s, this method was sometimes referred to as a "RISC core" or as "RISC translation", partly for marketing reasons, but also because these micro-operations share some properties with certain types of RISC instructions. However, traditional microcode (used since the 1950s) also inherently shares many of the same properties; the new method differs mainly in that the translation to micro-operations now occurs asynchronously. Not having to synchronize the execution units with the decode steps opens up possibilities for more analysis of the (buffered) code stream, and therefore permits detection of operations that can be performed in parallel, simultaneously feeding more than one execution unit.

The latest processors also do the opposite when appropriate; they combine certain x86 sequences (such as a compare followed by a conditional jump) into a more complex micro-op which fits the execution model better and thus can be executed faster or with fewer machine resources involved.

Another way to try to improve performance is to cache the decoded micro-operations, so the processor can directly access the decoded micro-operations from a special cache, instead of decoding them again. Intel followed this approach with the Execution Trace Cache feature in their NetBurst microarchitecture (for Pentium 4 processors) and later in the Decoded Stream Buffer (for Core-branded processors since Sandy Bridge).^[25]

Transmeta used a completely different method in their Crusoe x86 compatible CPUs. They used just-in-time translation to convert x86 instructions to the CPU's native VLIW instruction set. Transmeta argued that their approach allows for more power efficient designs since the CPU can forgo the complicated decode step of more traditional x86 implementations.

Addressing modes

Addressing modes for 16-bit processor modes can be summarized by the formula:^[26][27]

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