X86 Architecture

X86 Architecture

x86 is a series of computer microprocessor instruction set architectures based on the Intel 8086 CPU. The 8086 was introduced during 1978 as a fully 16-bit extension of Intel's 8-bit based 8080 microprocessor and also introduced memory segmentation to overcome the 16-bit addressing barrier of such designs. The term x86 derived from the fact that early successors to the 8086 also had names ending with "86". Many additions and extensions have been added to the x86 instruction set over the years, almost consistently with full backward compatibility. The architecture has been implemented in processors from Intel, Cyrix, Advanced Micro Devices, VIA, and many other companies.

The term is not synonymous with IBM PC compatibility as this implies a multitude of other computer hardware; embedded systems as well as general-purpose computers used x86 chips before the PC-compatible market started, some of them before the IBM PC itself.

As the term became common after the introduction of the 80386, it usually implies binary compatibility with the 32-bit instruction set of the 80386. This may sometimes be emphasized as x86-32 or x32 to distinguish it either from the original 16-bit "x86-16" or from the 64-bit x86-64. Although most x86 processors used in new personal computers and servers have 64-bit capabilities, to avoid compatibility problems with older computers or systems, the term x86-64 (or x64) is often used to denote 64-bit software, with the term x86 implying only 32-bit.

Although the 8086 was primarily developed for embedded systems and small 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 has replaced midrange computers and Reduced instruction set computer (RISC) based processors in a majority of servers and workstations as well. A large amount of software, including operating systems (OSs) such as DOS, Windows, Linux, BSD, Solaris, and Mac OS X functions with x86-based hardware.

Modern x86 is relatively uncommon in embedded systems, however, and small low power applications (using tiny batteries) as well as low-cost microprocessor markets, such as home appliances and toys, lack any significant x86 presence. Simple 8-bit and 16-bit based architectures are common here, 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 itself, 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 (alias Intel 8800), 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) and the scalability of x86 chips such as the eight-core Intel Xeon and 12-core AMD Opteron is underlining x86 as an example of how continuous refinement of established industry standards can resist the competition from completely new architectures.

Note: In the following text, all instances of the prefixes kilo/mega/giga/tera are to be understood in the binary (powers of 2) sense. See the article on the IEC prefixes (kibi/mebi/gibi/tebi) for details.