Stephen Morse: father of the 8086 processor

The engineer whose 30-year-old architecture still influences PCs today talks about how Intel took the SEX out of the pioneering CPU, and other tales from the beginning of the x86 era

The 8086 in Context

PCW: What are some of the distinguishing characteristics of the 8086 that made it stand out from other microprocessors of the day?

SM: Its most distinguishing characteristic was that it was a 16-bit microprocessor. I believe it was the first commercial 16-bitter in the microprocessor field. But the characteristics that I liked the most, and had the most fun designing and unifying, were the decimal arithmetic instructions and the string-processing instructions.

PCW: Why did Intel start making backward-compatible CPUs, and why did the company do it so well compared with other CPU manufacturers?

SE: The reason that Intel was concerned about backward-compatibility (and the reason everyone is, as well) is that you have a captured market base that you don't want to lose. If you have customers all using the 8008, when you come out with your 8080 processor you want your customers to be able to migrate their existing applications easily. If they had to rewrite all their applications, they would also be free to consider a new processor from the competition.

That's a lesson that Zilog learned the hard way. Zilog made its first splash with the Z80; that chip was compatible with Intel's 8080, so Zilog was able to steal Intel's customers easily. And it became a significant player in the marketplace. Then when the 16-bit race started, Zilog figured it had made a name for itself and could afford to do its own incompatible design for a 16-bit product, called the Z8000. But once Zilog's own customers discovered that programs could no longer be migrated from the Z80 forward, those customers became free to look around at the 16-bit marketplace, and they chose the 8086. Had Zilog gone with a 16-bit compatible upgrade of the Z80, history might have been different.

PCW: Was the 8086 designed with future backward-compatibility in mind?

SM: Backward-compatibility was certainly an issue when the 8086 was being designed. There were some instructions that were implemented and then hidden because we couldn't see a logical upgrade path for them in future processors. These instructions were actually on the chip, but we never documented them so that we would not be constrained by them in the future.

PCW: Can you share any funny, interesting, or unusual anecdotes about the 8086 that we haven't covered already?

SM: I always regret that I didn't fix up some idiosyncrasies of the 8080 when I had a chance. For example, the 8080 stores the low-order byte of a 16-bit value before the high-order byte. The reason for that goes back to the 8008, which did it that way to mimic the behavior of a bit-serial processor designed by Datapoint (a bit-serial processor needs to see the least significant bits first so that it can correctly handle carries when doing additions). Now there was no reason for me to continue this idiocy, except for some obsessive desire to maintain strict 8080 compatibility. But if I had made the break with the past and stored the bytes more logically, nobody would have objected. And today we wouldn't be dealing with issues involving big-endian and little-endian--the concepts just wouldn't exist.

Another thing I regret is that some of my well-chosen instruction mnemonics were renamed when the instruction set was published. I still think it's catchier to call the instruction SIGN-EXTEND, having the mnemonic of SEX, than to call it CONVERT-BYTE-TO-WORD with the boring mnemonic CBW.

Join the Good Gear Guide newsletter!

Error: Please check your email address.

Tags intel

Our Back to Business guide highlights the best products for you to boost your productivity at home, on the road, at the office, or in the classroom.

Keep up with the latest tech news, reviews and previews by subscribing to the Good Gear Guide newsletter.

Benj Edwards

PC World (US online)
Show Comments

Essentials

Microsoft L5V-00027 Sculpt Ergonomic Keyboard Desktop

Learn more >

Lexar® JumpDrive® S57 USB 3.0 flash drive

Learn more >

Mobile

Lexar® JumpDrive® S45 USB 3.0 flash drive 

Learn more >

Exec

Audio-Technica ATH-ANC70 Noise Cancelling Headphones

Learn more >

Lexar® Professional 1800x microSDHC™/microSDXC™ UHS-II cards 

Learn more >

HD Pan/Tilt Wi-Fi Camera with Night Vision NC450

Learn more >

Lexar® JumpDrive® C20c USB Type-C flash drive 

Learn more >

Budget

Back To Business Guide

Click for more ›

Most Popular Reviews

Latest News Articles

Resources

PCW Evaluation Team

Michael Hargreaves

Windows 10 for Business / Dell XPS 13

I’d happily recommend this touchscreen laptop and Windows 10 as a great way to get serious work done at a desk or on the road.

Aysha Strobbe

Windows 10 / HP Spectre x360

Ultimately, I think the Windows 10 environment is excellent for me as it caters for so many different uses. The inclusion of the Xbox app is also great for when you need some downtime too!

Mark Escubio

Windows 10 / Lenovo Yoga 910

For me, the Xbox Play Anywhere is a great new feature as it allows you to play your current Xbox games with higher resolutions and better graphics without forking out extra cash for another copy. Although available titles are still scarce, but I’m sure it will grow in time.

Kathy Cassidy

STYLISTIC Q702

First impression on unpacking the Q702 test unit was the solid feel and clean, minimalist styling.

Anthony Grifoni

STYLISTIC Q572

For work use, Microsoft Word and Excel programs pre-installed on the device are adequate for preparing short documents.

Featured Content

Latest Jobs

Don’t have an account? Sign up here

Don't have an account? Sign up now

Forgot password?