Intel 80286
The Intel 80286, introduced in 1982, was a 16-bit microprocessor that extended the x86 line with protected-mode operation and the ability to address far more memory than the 8086, and it is best remembered as the processor of the IBM PC/AT. It kept compatibility with the earlier x86 software base while adding hardware features aimed at more capable multitasking operating systems, notably a protected mode that provided memory protection between programs. The 286 occupies an awkward but important place in x86 history: a meaningful step up in capability whose new features were constrained by compatibility compromises that the later 386 would resolve.
The 286's signature addition was protected mode, a hardware mechanism for memory protection and larger addressing intended to support multitasking operating systems. Beyond simply running existing programs, the 286 introduced a mode in which the processor could enforce boundaries between programs and access a much larger memory space than the 8086's one-megabyte limit. This was aimed at the needs of more sophisticated operating systems that run multiple programs and must prevent them from interfering with each other. Protected mode represented Intel moving the x86 line toward the capabilities expected of serious multi-user and multitasking systems, a significant architectural expansion over the original 16-bit design.
The 286 is historically tied to the IBM PC/AT, the more powerful successor to the original IBM PC, which made the processor widely deployed. As the CPU of the PC/AT, the 286 reached a large installed base and became a standard rung in the evolution of the PC-compatible platform. Its presence in a mainstream, successful computer meant that its capabilities and its limitations alike were widely felt, and the PC/AT established hardware conventions that carried forward. The 286's role as the AT's processor cements its place in the lineage of PC-compatible machines that grew around the x86 architecture.
The 286's protected mode was constrained by awkward compatibility compromises, which limited its practical usefulness and set up the problem the 386 would solve. A well-known shortcoming was the difficulty of cleanly switching the processor back out of protected mode into the compatible real mode, which made the new capabilities cumbersome to use in practice given the need to still run older software. This awkwardness meant the 286's protected mode was not as smoothly usable as it needed to be, and it was one of the specific problems that the following generation addressed, making the 286 a transitional step whose rough edges motivated its successor's design.
The 286 illustrates the recurring x86 tension between adding new capability and preserving compatibility with the existing software base. Intel wanted to move the architecture forward with protected mode and larger addressing while not abandoning the enormous body of software written for earlier x86 chips, and the compromises required to do both produced a processor that was more capable but awkward in its new features. This tension between progress and backward compatibility is a defining theme of the x86 line, and the 286 is a clear instance where the compromise showed, setting the stage for the cleaner solution of the 386.
Read the Intel 80286 as the awkward but important x86 step-up: a 16-bit processor that added protected mode and larger addressing for multitasking while keeping compatibility, powered the IBM PC/AT, and whose cumbersome mode-switching compromises defined exactly the problem its successor the 386 was built to resolve.