x86

x86
Name	x86
Designer	Intel
Bits	16-bit, 32-bit, 64-bit
Introduced	1978
Version	x86-64 (2003)
Encoding	Variable (1 to 15 bytes)
Endianness	Little-endian
Page size	4 KB, 2 MB, 1 GB
Extensions	MMX, SSE, AVX, AES-NI
Predecessor	Intel 8080
Successor	Itanium (non-x86)

x86. It is a family of complex instruction set computer (CISC) instruction set architectures initially developed by Intel and now produced by multiple manufacturers. The architecture originated with the 16-bit Intel 8086 microprocessor in 1978 and has evolved through 32-bit and 64-bit generations, becoming the dominant architecture for personal computers, workstations, and servers. Its longevity is largely due to its extensive backward compatibility, ensuring software written for earlier processors continues to run on modern CPUs.

History

The lineage of the architecture began with the Intel 8086 and its cost-reduced sibling, the Intel 8088, which was chosen by IBM for its seminal IBM Personal Computer. This decision cemented the architecture's place in the PC compatible market. Successive generations, including the Intel 80286, Intel 80386, and Intel 80486, introduced protected mode, 32-bit computing, and integrated floating-point unit capabilities. The introduction of the Pentium brand in 1993 marked a shift to superscalar and pipelining designs. Competition from Advanced Micro Devices (AMD) intensified with processors like the AMD K5 and culminated in AMD's development of the 64-bit x86-64 extension, which was later adopted by Intel as Intel 64. Other significant competitors have included VIA Technologies and, historically, Cyrix.

Architecture

The architecture is based on a CISC design philosophy, featuring a relatively small set of processor registers and a memory segmentation model in its earlier forms. The Intel 80386 introduced a flat protected mode memory model and paging for virtual memory. Modern implementations use extensive internal microcode to translate complex instructions into simpler micro-operations (μops) that are executed on highly pipelined, out-of-order execution, superscalar cores. Key architectural components include the floating-point unit (FPU), integrated initially in the Intel 80486DX, and sophisticated branch prediction and speculative execution units to enhance performance.

Instruction set

The instruction set is notable for its variable-length encoding and support for a wide array of addressing modes. Base instructions handle integer arithmetic, logic, control flow, and string operations. Over decades, numerous extensions have been added, starting with the MMX instructions for SIMD on integers. This was followed by Streaming SIMD Extensions (SSE), SSE2, SSE3, and SSE4 for floating-point and packed data processing. Later extensions include Advanced Vector Extensions (AVX), AVX-512 for wider vectors, and specialized instructions like AES-NI for encryption and Intel VT-x for virtualization.

Operating modes

Processors support multiple operating modes to maintain compatibility. Real mode emulates the environment of the original Intel 8086, used during system startup. Protected mode, introduced with the Intel 80286 and enhanced in the Intel 80386, enables multitasking, memory protection, and the flat memory model used by modern operating systems like Microsoft Windows and Linux. Virtual 8086 mode allows the running of real-mode applications within a protected mode environment. With the advent of x86-64, a new long mode was introduced, which supports 64-bit operating systems and applications while retaining a legacy mode for running 32-bit and 16-bit software.

Implementations

While Intel remains a primary designer, the architecture is implemented by several major semiconductor companies. Advanced Micro Devices produces the Ryzen and EPYC lines, and historically created the Athlon and Opteron processors. Intel's notable lines include the Core, Xeon, and legacy Pentium brands. Other implementations have come from VIA Technologies with its VIA C7 processors, and historically from Cyrix and Transmeta. Modern implementations focus on multi-core designs, simultaneous multithreading (marketed as Hyper-Threading by Intel), and integrating graphics processing unit (GPU) functionality onto the same die.

Software compatibility

The defining feature of the platform is its exceptional backward compatibility, often described as the "x86 ecosystem." This allows contemporary CPUs to run operating systems and applications designed for processors decades older. Major operating systems built for the architecture include Microsoft Windows, many distributions of Linux, and BSD variants. The DOS family, including MS-DOS and DR-DOS, relied on its real mode. Compaq and Phoenix Technologies played key roles in establishing BIOS compatibility standards. This software legacy, combined with the dominance of the IBM PC compatible platform, has made it the most successful instruction set architecture in computing history.

Category:Instruction set architectures Category:Intel microprocessors Category:AMD microprocessors Category:1978 introductions