What is the most important CPU that Intel ever produced?

• The Pentium? The chip was no slouch, and was also a brand that was heavily marketed by Intel, putting the firm on a map for many computer illiterate consumers.
• The Pentium Pro? It could rival the RISC chips of its time and was the core architecture of all Intel processors for around ten years.
• The 8088? It won the IBM PC market, paving the way for a bright future to its descendants.

In my opinion, there is no doubt that the crown has to go to the Intel 80386. It is indeed the first 32-bit CPU in the x86 family. But it is way more than that.
Despite being less “elegant” than its nemesis the Motorola 68030, and less powerful that the countless RISC chips that began to emerge around the same time period, I will dare to say that the 80386 is not only the most important CPU Intel ever produced, but also one of the most important CPU ever produced, period. It triggered a revolution.

Here is why.

The birth of the 80386

Designing the 80386

The i386 was not planned nor wanted by Intel.

In the late 70s, the most important project at Intel was the iAPX 432, a very ambitious CPU that was intended to be Intel’s major design for the 1980s. The intention was to build a chip very well suited to the high level languages of the time, such as ADA, by offering niceties like object-oriented programming and storage allocation in hardware. A very ambitious design that would take years to mature, so Intel launched a stop-gap design in 1976: the 8086.

The rest is history: the 8086 family won the IBM PC contract, while the iAPX 432 was introduced very late, in 1981, and with very disappointing performances. In benchmarks, it performed at ¼ the speed of the much cheaper 80286 running at the same frequency.

The IBM PC 5150. The very first PC equipped with a 4.77MHz Intel 8088

Needless to say, the 432 did not sell very well…

In 1982, Intel still did not recognize the importance of the PC platform and the binary compatibility with its software. The 80286 was another gap-filler CPU and had a slow start, so Intel was not very keen to pour more money into yet another x86.
By that time, Intel engineers finally understood that the 432 line was doomed; they began to work on a brand new 32-bit RISC architecture code-named P7.

In the meantime Bob Childs, one of the architects of the 286, worked underground to lay out some ideas of what could be a 32-bit extension to the 286. After about six months, Intel knew that it would need another iteration in the x86 family before the P7 was ready, and the work on the 386 got the green light; albeit with a very small team and on a shoestring. The team conducted extensive research about the needs of the x86 customers and it quickly appeared that everyone hated the segmented memory scheme of the 8086, and that they regretted that the 80286 was a missed opportunity to get rid of it. UNIX was also becoming a thing in more affordable workstations, and the 386 team wanted to design a chip well suited for it.
Thus, being able to address a “flat memory” space was considered top priority. In order to help keep compatibility with prior x86, the 386 would be still segmented. But as each segment could be 4GiB long, it became irrelevant. Paging and the ability to provide virtual memory was also decided around that time. The address unit was very carefully designed to play well with the pipeline so the address computation would not impact performance. This was very beneficial to the 386, as we will see later on.

Early-on, it was also decided that the 386 would be a full 32-bit CPU. In order to preserve the binary compatibility, the existing instruction set and registers were simply extended to 32-bits, instead of designing a very different instruction set behind a “mode header”. The price to pay was that the tiny number of registers, a drawback of the x86 CPUs, could not be increased.

For a time, Intel considered using the brand new bus that was designed for the P7. However, it was very different from the 286’s bus and would require an extensive redesign of the motherboards and the support chips. So it was abandoned in favor of a less ambitious 32-bit extension of the existing bus.

Around 1984, the PC market was blossoming and Intel finally understood the importance of the x86 line and the completion of the 80386 became top-priority. As stated by Jim Slager, from the original team:

[…] probably over a 12-month period we went from stepchild to king. It was amazing because the money started pouring in from the PC world and just changed everything

In order not to steal sales from the 80386, P7 project, which will give birth to the i960 in 1988, was retargeted to the embedded market.

The Intel i960

Sole sourcing the 386

The 386 was introduced in October of 1985.

Back in the time, it was common that chip designing companies such as Intel provided licenses for other companies to produce their CPUs in order to “second source” them. It was very important to win computer designs as the customer could be confident that in case of catastrophic yields from the “primary source”, they would not suffer a shortage of CPUs. Intel had a long partnership with AMD, dating back to the 8085. AMD and Intel cross-licensed their products, so each one could be the second source of the other one. Starting from 1983, IBM itself had a license to directly produce 808x and 80286 chips at the condition they still bought some CPUs from Intel and did not sell their production to third parties.

But with the 80386, everything was about to change.

In 1984, IBM introduced the model 5170 also known as the PC/AT. Its main plus-value compared to its predecessor was its 286 CPU. Intel understood that if it could impose itself as the sole source of the CPUs it could have full control of the most valuable part of the PCs.

The IBM 5170 “PC/AT”

As IBM was not much interested in the future 386 and wanted to invest massively into the 286 instead, an opportunity arose. IBM preferred to produce as many CPUs for their own usage as possible, only keeping Intel as an insurance in case of production troubles. The agreement between the two companies was renegotiated to please IBM concerning the 286, but it would not be permitted to produce 386 chips.

In 1985, without the support from IBM which was entirely focused on the 286, the prospects for the 386 were not rosy. AMD was not very interested in producing it, so the second sourcing agreement with them was not extended to the 386.

Thus, Intel became the sole source of 386 chips!

Around the same time, judges ruled in a NEC vs Intel case that the micro-code was copyrighted and that nobody could copy it without a specific license from Intel. If NEC was able to reverse the 8086’s microcode, the 386 would be nearly impossible due to its complexity.

From that point, Intel was in position to control the future of the PC market, along with Microsoft.

The Clone Wars

When IBM designed the PC, it was in a hurry. It could not wait too long before introducing its micro-computer line or it risked missing the market. Thus, instead of spending years designing every single component necessary to build it, it relied on off the shelf parts. On the plus side, the IBM PC could be commercialized very quickly. On the other hand, everyone could build a compatible machine by buying the same parts…

The most successful and ambitious of those clone companies was Compaq. At the time, it was the fastest