PA-RISC 2.0 Architecture

Hewlett-Packard shall not be liable for errors contained herein or for incidental or consequential damages in connection with furnishing, performance, or use of this material. Hewlett-Packard assumes no responsibility for the use or reliability of its software on equipment that is not furnished by Hewlett-Packard. This document contains proprietary information which is protected by copyright. All rights are reserved. No part of this document may be photocopied, reproduced, or translated to another language without the prior written consent of Hewlett-Packard Company. Acknowledgements Special thanks to Martin Whittaker who was the impetus behind this book and provided leadership and direction at every turn and to Dale Morris and Jim Hall who contributed key sections. Many other folks at Hewlett-Packard provided critical information: among them Ruby Lee, and Jerry Huck. iii PA-RISC 2.0 Architecture Foreword " Everything should be made as simple as possible, but not simpler. " A. Einstein When the first PA-RISC systems were shipped in 1986, the architecture was clearly recognized as a break with the past, with regular, hardware-inspired instructions rather than variable, interpretive forms. But its simple instructions were somewhat richer than other RISC designs, providing basic support for operations on strings and other data types prevalent in commercial applications. This semantic richness, unusual in the RISC designs of the time, was a direct result of the breadth of markets for HP computers and the decision to optimize PA-RISC for the full range of technical and commercial applications. In the intervening years, PA-RISC has become the basis of a large family of computer systems, currently spanning a capacity range of over two orders of magnitude. As the product family has grown, the range of applications has also expanded geometrically. PA-RISC workstations now host applications which were once the province of supercomputers. Database servers now supply realtime streams of compressed video and audio. And PA-RISC has evolved to meet the demands for leadership performance in these emerging application domains. The purpose of a processor architecture is to define a stable interface which can efficiently couple multiple generations of software investment to successive generations of hardware technology. Stability and efficiency are the goals, and the range of software and hardware technologies expected during the architecture's life determine the scope for which the goals must be achieved. The desired stability does not rule out change, but it does require that any evolution of the architecture contain the prior definition as …