How the Intel 8088 got its bus – EEJournal

The IBM PC (aka the IBM 5150) had such a massive impact on the history of computing that origin stories about the machine are legion. Many books have been written to tell the stories of a motley group of engineers isolated in IBM’s Entry Systems business in Boca Raton, Florida. William C. Lowe first led the group. He got the job after telling IBM’s Corporate Management Committee that IBM couldn’t create a microcomputer from scratch within the company because of a corporate thinking and culture. fossilized businesses. IBM’s only hope, Lowe said, was to acquire a smaller, more nimble company to create such a machine. So naturally, IBM gave him the job of doing exactly what he said he couldn’t do. He led the team that developed a proof-of-concept prototype in just 40 days. Don Estridge was appointed director of entry-level systems – small systems – in 1980 and led the team of engineers that brought the IBM PC to market in a remarkably short 12 months. IBM announced the IBM PC on August 12, 1981, and swung the computing world on its axis.

However, that is not today’s story. Instead, I’ll explain how and why IBM’s entry-level systems division chose the Intel 8088 microprocessor and put Intel on the path to becoming the world’s largest semiconductor manufacturer. It happened because of a series of seemingly unrelated intertwined events.

David House joined Intel in 1974 and became general manager of the company’s Microcomputer Components Division in 1978. He immediately inherited a huge problem. After introducing the world’s first commercial microprocessor, the 4004, quickly followed by the 8008 and 8080 8-bit microprocessors, Intel’s leadership in processors began to erode. Federico Faggin had left Intel, launched Zilog, and introduced the unique 8-bit Z80 microprocessor, compatible with the 8080 object code. Even the 8080’s successor, the Intel 8085, could not harm the Z-80. mastodon. As House says of Zilog and the Z-80 in an oral history taken from the Computer History Museum, “…they were kicking our ass in the marketplace.”

During this time, Intel’s microprocessor focus was firmly on an object-oriented processor architecture that came to be known as the iAPX 432. When Intel realized that the iAPX 432 would be late to market, very late indeed, the company started an interim 16-bit processor project that would become the Intel 8086 microprocessor, which the company later announced in mid-1978. Although the 8086 register set looks like an extended 16-bit version of the 8080 register set, the 8086 ISA is completely different and not at all compatible with the 8080 ISA. (Intel has resolved this incompatibility with a source code translator.)

In 1980, when IBM’s Entry Level Systems division was looking for an off-the-shelf 16-bit microprocessor on which to base the IBM PC, there were several alternatives. Texas Instruments offered the TMS 9900, a microprocessor-based implementation of the company’s successful 990 minicomputer architecture. However, the TMS 9900 had a 16-bit address space, which was no better than the address space of any 8-bit microprocessor on the market. As Wally Rhines wrote in his book, “From the Wild West to Modern Life: Evolution of the Semiconductor Industry”:

“TI had attempted to overtake the microprocessor market by introducing the TMS 9900 16-bit microprocessor in 1976. But the TMS 9900 only had 16 bits of logical address space and the industry needed a 16-bit microprocessor for address space rather than performance.”

IBM’s Entry Level Systems division did not choose the TMS 9900 as the microprocessor for the IBM PC.

Motorola Semiconductor introduced the 16/32-bit MC68000 microprocessor in 1979. This processor had a 32-bit instruction set, 32-bit data and address registers, and a 24-bit external address bus, giving it a 16 MB address space. At the time, attaching 16 MB of RAM to a microprocessor was unthinkable, so the MC68000’s 24-bit address space was more than enough. The MC68000’s 32-bit address and data registers allowed the processor to offer excellent performance over competing 8- and 16-bit processors of the time, and the expansion of the processor’s 24-bit addressing to The original full 32-bit simply required more pins on the package.

The MC68000 would have been IBM’s choice of logic processor, except that Motorola was only beginning to sample the device and IBM wanted a processor already in full production, with a second source. IBM also wanted a low-cost microprocessor, and the original 64-pin ceramic package of the MC68000 (nicknamed “The Aircraft Carrier”) was anything but cheap.

This is why IBM’s Entry Level Systems division did not choose the Motorola MC68000 as the microprocessor for the IBM PC.

Zilog had also developed a 16-bit microprocessor, the Z8000, but IBM does not seem to have seriously considered this microprocessor for the IBM PC. According to Federico Faggin, this is because Exxon Enterprises owned Zilog at the time and IBM viewed Zilog’s parent company as a potential competitor.

This left the Intel 8086, the interim microprocessor that keeps Intel’s microprocessor dynasty alive until the magnificent iAPX 432 can be completed. The Intel 8086 had a smaller address space than the MC68000 or the Zilog Z8000. It also offered less performance. But it was really IBM’s only choice for a 16-bit processor, given the design constraints of the IBM PC.

There are many stories claiming to explain why IBM chose a 16-bit Intel processor. Bill Gates and Paul Allen claim that Microsoft convinced IBM to choose Intel’s microprocessor. Some people have said that Regis McKenna’s CRUSH project deserved credit for the win, but in his oral history, House says he doesn’t think there’s a direct connection.

David Bradley – designer of the IBM System/23 DataMaster, which was based on the Intel 8085 – joined the IBM PC design team in August 1980. In 1990 he published an article titled “The creation of the IBM PC” in Byte magazine that said:

“There were a number of reasons why we chose the Intel 8088 as the central processor for the IBM PC.

“1. The 64K address limit had to be exceeded. This requirement meant that we had to use a 16-bit microprocessor.

“2. The processor and its peripherals had to be available immediately. There was no time for the development of new LSI chips, and manufacturing delays meant that quantities had to be available immediately.

“3. We couldn’t afford a long learning curve; we had to use the technology we knew. And we needed a rich set of supporting chips – we wanted a system with a DMA controller, a interrupt controller, timers and parallel ports.

“4. There had to be both an operating system and application software available for the processor.

“We limited our decision to the Intel 8086 or 8088.”

However, IBM’s Entry Level Systems division did not choose the 8086 microprocessor for the IBM PC because IBM wanted to pay $5 for the microprocessor and Intel could not meet that price target due to existing 8086 sales contracts with other customers. Intel really wanted to win a high-volume personal computer design and the IBM PC looked like the thing, but those existing 8086 contracts prevented Intel from giving IBM a bargain price on that microprocessor. A five dollar 8086 was out of the question. Intel could not put this choice on the negotiating table. The IBM PC processor pricing problem fell on Dave House to solve it.

By a happy coincidence, Intel had sent the original design of the 8086 microprocessor to its relatively new design facility in Haifa, Israel, for cost-saving die shrinking in 1979. Along with this effort, two engineers in Haifa, Rafi Retter and Daniel (Dani) Star, converted the 8086’s 16-bit data bus to an 8-bit data bus by changing some circuitry in the bus interface unit and modifying some of the processor’s microcode. Since the 8086 already used a multiplexed address/data bus, the pinout of the new processor did not need to change from the original 8086 pinout. The microprocessor address/data pins AD0 through AD7 remained the same for both processors, while address/data pins AD8 through AD15 on the 8086 simply became address pins A8 through A15 on the 8088 .

You will find Rafi Retter’s initials in the center of die 8088:

The Intel 8088 microprocessor was designed by Rafi Retter and Daniel Star. Retter’s initials appear in the center of the chip. Photo credit: Ken Shirriff.

Here is House’s explanation of what happened, taken from his oral history:

“So I try to get that [IBM] design wins, and I’ll lose on architecture to 68K [Motorola’s MC68000] or the [Zilog] Z8000. So I said, where is this 8-bit bus version?

“…we went to sell it to IBM.”

To be clear, because the 8088 was not an 8086, because it was a “completely” different microprocessor with a different part number, Intel was no longer bound by contractual price constraints on the 8086. It used the same ISA and a very similar semiconductor chip, but it was a different microprocessor. Sure, the 8088’s narrower data bus limited system performance a bit, but IBM didn’t care that much. After all, the PC was not going to replace IBM mainframes. Price was the primary consideration for the IBM PC.

In the final analysis, there is clearly no significant manufacturing cost difference between an 8086 and an 8088. They have very similar transistor counts and die sizes, and are both packaged in the same 40-pin DIP . However, there is a price difference, and that difference won the socket. IBM didn’t particularly want a 16-bit processor with an 8-bit data bus, but if that’s what it took to bring the unit price down to $5 in volume, so be it. (Also, IBM wanted a second source, so Intel hired AMD as a second source. This AMD deal created its own parallel universe of fascinating stories, too many to list in this article.)

In addition to the microprocessor’s low price, adoption of the Intel 8088 gave the IBM PC design team direct compatibility with Intel’s line of low-cost 8-bit peripheral chips (designed in origin for Intel’s 8-bit microprocessors) and many of these peripheral chips have come to an end. also in the design of the IBM PC system. These 8-bit peripheral chips also had alternative sources, including AMD, so the competition drove their prices down as well, which made IBM happy and further cemented the deal for the 8088. The IBM PC did its debut on August 12, 1981, after twelve months of development. The elephant named IBM had learned to dance.

In his oral history, House summarizes the significance of IBM PC’s introduction to Intel:

“So the IBM PC is announced, and it’s starting to take off, and it’s exceeding everyone’s expectations… And like, wow! I mean, this turns out to be the watershed moment, really, for Intel.


Dave House Oral HistoryComputing History Museum, 2004-04-20.

Wally Rhines Oral HistoryComputer History Museum, 08/10/2012.

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