Dont Look Back

By Jim Turley  |  Posted 2002-02-11 Print this article Print

: How Itanium Handles x86 Code"> Yes, Virginia, there is an x86-compatibility mode in Itanium. Its awkward and unnatural, but we know how attached you are to your old binaries. IA-64 does not normally support older x86 binaries, and its entirely possible that some future IA-64 implementation might drop this feature or water it down, but for now your old Lotus 1-2-3 diskettes are safe. Itanium supports all x86 instructions in one way or another, even MMX, SSE (not SSE2), Protected, Virtual 8086, and Real mode features. You can even run entire operating systems in x86 mode, or just run the applications under a new IA-64 OS. All the x86 registers map onto Itaniums own general-purpose registers, but some of the less orthogonal x86 registers appear in Itaniums "application registers" AR24 through AR31.
x86 mapping onto IA-64 registers
Switching modes appears trivial but isnt. Theres one IA-64 instruction that switches the processor to x86 mode and another (newly defined) x86 instruction, JMPE, that switches to IA-64 mode. If the programmer so wishes, interrupts can switch automatically to IA-64 mode or the machine can stay in x86 mode. In the latter case, you can reuse your x86 interrupt handlers. Switching to x86 mode is a lot like booting a 386 because you have to set up memory segment descriptors, status registers, and flags. Also, x86 code likes to have its way with all the resources of the processor, either overwriting or ignoring many of Itaniums state bits and registers. Its also likely to upset your cache contents. In general, its best to save the entire state of the processor before switching to x86 mode. Its awkward enough that you probably dont want to switch modes willy-nilly. Save it for dramatic changes, such as executing entire x86 applications. Not that anyone was asking, but PA-RISC compatibility is handled offline through a software translator. IA-64 instructions dont directly support PA-RISC instructions, but they do map fairly closely (hey, RISC is RISC). The fact that x86 binaries are emulated in minute detail with enormous helpings of hardware while PA-RISC code is relegated to a translator before it has any hope of running says a lot about the relative importance of these two installed bases. It may also tell us something about the "equal" relationship between the HP and Intel engineers designing IA-64.

Jim Turley is a semiconductor industry analyst, editor, and presenter working in Silicon Valley. Focus technologies are 32-bit microprocessors and semiconductor intellectual property (SIP).

Most recently Jim was the Senior Vice President of Strategy & Technology at ARC International plc (LSE:ARK), where he set the Company's strategic direction and guided its technical developments at five locations worldwide. With headquarters in London (UK) and development centers in New Hampshire, Canada, and California, ARC International is an innovative leader in the semiconductor IP (intellectual property) industry.

Previously, Jim was senior analyst for MicroDesign Resources (a unit of Cahners/Reed Elsevier) as well as the Senior Editor of the prestigious industry journal Microprocessor Report (a three-time winner of the Computer Press Award), and Editor-in-Chief of Embedded Processor Watch. He also hosted and directed the yearly Embedded Processor Forum conference, the industry's annual showcase for new microprocessors. As an analyst and editor, Turley consulting with leading semiconductor firms, providing informed advice on technology trends and market requirements, and was often called on to participate in new product reviews, strategy sessions, and technology development for large semiconductor companies.

Turley is the author of six popular books including Advanced 80386 Programming Techniques, the best-selling PCs Made Easy and others published by McGraw-Hill and Academic Press. He's served as technical editor for several of McGraw-Hill's computer and programming books. In addition, he was a regular technology columnist for Embedded System Programming, Computer Design, and Supermicro magazines, and contributed articles to dozens more. Earlier in his career, Turley held engineering or marketing positions at Adept Technology, Force Computers, TeleVideo, and other high-technology firms in Europe and the United States.

Turley has created and presented numerous seminars and training sessions around the world covering technology trends and the competitive microprocessor market. He is also a well-known speaker at industry events such as the Embedded Systems Conference and Microprocessor Forum, is frequently quoted in the Wall Street Journal, New York Times, USA Today, San Francisco Chronicle, and San Jose Mercury News, and has appeared frequently on television, radio, and Internet broadcasts. Jim volunteers for Recording for the Blind and recently earned his amateur auto-racing license. He has a talented and stunningly attractive wife, two overachieving children, an apparently brain-damaged dog, and an opossum living under the house.

Jim can be contacted at or by calling (408) 226-8086.

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