Appendix D. Itanium Registers and Their Uses

The nature, number, and uses of CPU registers characterize any given computer architecture and distinguish it from all others. As this book discusses only a carefully chosen subset of Itanium instructions, it does not touch upon every aspect of the architecture. Accordingly, the main text did not seem the appropriate place for charts of all the Itanium registers.

Principles of design for system software, such as operating systems and shared libraries, have been converging toward a model where conventions apply across many different operating systems and programming languages. Standardized assignments of certain processor registers to particular roles make possible, among other things, consistent calling conventions. Shared functions can thus be invoked from programs written in any language, and software development efforts will not be confronted with arbitrary differences and impediments to program design from one operating system or programming tool to another.

The Itanium architecture includes more processor registers than any previous architecture, with much variety in their widths, features, and applications. This appendix covers each type of register, including a short narrative, a diagram if bit fields in a register have special purposes, and a summary of major conventions on their use in software.