x86 is an instruction-set architecture: the contract between hardware and software that defines what instructions a processor understands, what registers it has, and how it manages memory. It takes its name from a line of Intel processors whose model numbers ended in 86, beginning with the 8086 of 1978 and continuing through the 80286, 80386, and 80486. Intel’s authoritative reference for the architecture is the “Intel 64 and IA-32 Architectures Software Developer’s Manual,” published and maintained by Intel.
The architecture grew by accretion rather than redesign. The 8086 defined a 16-bit instruction set with segmented memory. The 80386 extended this to 32 bits, a generation Intel’s manuals call IA-32, adding flat 32-bit addressing and protected-mode features while keeping the older 16-bit modes intact. Later, the 64-bit extension known as x86-64 or Intel 64 widened registers and addresses again. Each step added capability without discarding what came before.
That insistence on backward compatibility is the defining characteristic of x86. A modern processor can still execute, in principle, code written for the original 8086, because each new generation kept the older instruction encodings and operating modes available. Intel’s Software Developer’s Manual documents this layered structure, describing the real-address, protected, and 64-bit modes side by side along with the full instruction set. This continuity is what let decades of software accumulate on the platform without being rewritten.
x86 is usually classified as a complex instruction-set computer, or CISC, design: its instructions vary in length and many perform elaborate operations, in contrast to the fixed-length, simpler instructions of RISC architectures. Modern x86 processors reconcile this by translating those complex instructions internally into simpler micro-operations, gaining RISC-like execution efficiency while preserving the CISC programming interface.
The combination of the IBM PC’s success and this unbroken compatibility made x86 the dominant architecture for desktop and server computing for a generation. Even as competing architectures rose in mobile devices, x86, documented in Intel’s developer manuals and implemented by both Intel and AMD, remained the standard that an enormous body of software was written against.