I386

The Intel 386, originally released as the 80386 and later renamed i386, is the first 32-bit processor in the line, marking it a significant evolution in the x86 microarchitecture. It is the third-generation x86 architecture microprocessor developed jointly by AMD, IBM and Intel. Pre-production samples of the 386 were released to select developers in 1985, while mass production commenced in 1986. It implements the IA-32 microarchitecture, and is the first CPU to do so. It was the central processing unit (CPU) of many workstations and high-end personal computers of the time. It began to fall out of public use starting with the release of the i486 processor in 1989, while in embedded systems the 386 remained in widespread use until Intel finally discontinued it in 2007.

Compared to its predecessor the Intel 80286 ("286"), the 80386 added a three-stage instruction pipeline which it brings up to total of 6-stage instruction pipeline, extended the architecture from 16-bits to 32-bits, and added an on-chip memory management unit. This paging translation unit made it much easier to implement operating systems that used virtual memory. It also offered support for register debugging. The 386 featured three operating modes: real mode, protected mode and virtual mode. The protected mode, which debuted in the 286, was extended to allow the 386 to address up to 4 GB of memory. With the addition of segmented addressing system, it can expand up to 64 terabytes of virtual memory. The all new virtual 8086 mode (or VM86) made it possible to run one or more real mode programs in a protected environment, although some programs were not compatible.

The 32-bit i386 can correctly execute most code intended for the earlier 16-bit processors such as 8086 and 80286 that were ubiquitous in early PCs. As the original implementation of the 32-bit extension of the 80286 architecture, the i386 instruction set, programming model, and binary encodings are still the common denominator for all 32-bit x86 processors, which is termed the i386 architecture, x86, or IA-32, depending on context. Over the years, successively newer implementations of the same architecture have become several hundreds of times faster than the original 80386 (and thousands of times faster than the 8086).

In the early 1980s Intel, the creator of 80286, was aware of that microprocessor's poor reputation. The company's own engineers believed that Motorola 68000 was superior to their "ugly duckling". Bill Gates called 80286 "brain dead", and important customer IBM thought that its architecture was a flawed dead end. While the company expected that Intel i432 would be its future architecture, i432 was very slow and many also believed unsuitable. Groups worked on various successors, including a completely new architecture ("P4") from i432 designer Glen Myers that resembled DEC VAX, and another ("P7") intended to combine Myers's work and i432 technology.

Although many in the company believed that a 32-bit successor to 80286 was nonviable, Gene Hill and 80286 co-designer Robert Childs secretly worked on the "stepchild" project and persuaded others of its potential over Myers's plan, which people such as John Crawford compared to the events at Data General in The Soul of a New Machine. Binary compatibility with the Intel 8086 architecture the recently introduced IBM PC used was at first not seen as important, and many disliked the older CPUs' segmented memory model. A greater priority was a 32-bit flat memory model so 80386 can, like 68000, run Unix well.

80386 development began in 1982 under the internal name of P3. Intel previously used NMOS logic but 80386 was its first CMOS product, consistent with the industry trend. The rapidly growing IBM PC installed base made supporting its software library more important, and Intel salespeople told customers that their 286 software would run on 386. The 386 designers thus supported both flat and segmented memory models, what Crawford described as "the best of both worlds". Pat Gelsinger led the port of Amdahl UTS to the CPU to confirm Unix's viability. The limited die size made difficult incorporating, for marketing purposes, a CPU cache twice as large as the 68020's. The team's Jim Slager later described both CPUs' caches as useless, but he and his colleagues succeeded.

The tape-out of the 80386 development was finalized in July 1985. The 80386 was introduced as pre-production samples for software development workstations in October 1985. Intel had exited the DRAM market to focus on microprocessors, so the former "stepchild" was vital to its future; the company moved memory engineers to the 80386 project, improving the die shrink. The forthcoming product persuaded customers that the 80286 was not a dead end, increasing the latter's sales.

80386 manufacturing in volume began in June 1986, along with the first plug-in device that allowed existing 80286-based computers to be upgraded to the 386, the Translator 386 by American Computer and Peripheral. The 80386 being sole sourced made the CPU very expensive, but it was very successful. Hill recalled representing the design team at a PC Magazine awards ceremony: