| Before we dive into HP-UX itself, we should have a look at the underlying system architecture. The very nature of an operating system requires that it be aware of the architecture it's running on. The operating system's kernel insulates the user applications from the hardware, making the architecture mostly transparent to the applications. But the kernel itself has to work directly with the hardware and so has to be designed to run on a particular type of hardware. HP-UX has been implemented on a variety of platforms, among them the Motorola 68000 family, HP's PA-RISC family, and the new Itanium family. In this book we concentrate on the PA-RISC architecture. The 68000 versions of HP-UX are long obsolete, and the Itanium versions are new enough that there are few implementations in use. The vast majority of HP-UX systems are running on PA-RISC hardware. Why study architecture at all in a software book? The kernel has architecture-specific code for handling interruptions, dealing with I/O devices, managing multiprocessor issues, and for many other tasks. In fact, parts of the kernel are written in assembly language for performance reasons. Before we get into how the kernel works, it's important to have at least a general idea of how the hardware works. We won't get into every detail of the architecture. That's covered thoroughly in PA-RISC 2.0 Architecture by Gerry Kane.[1] In this book, we cover just those implementation details that are critical to an understanding of the internals of HP-UX. In particular, we look at the general hardware building blocks, the registers that are available and how they are used, provisions for virtual memory addressing, and some of the assembly instructions that are important to the implementation of the kernel. [1] Gerry Kane, PA-RISC 2.0 Architecture (Pearson Education POD, 1995).
We also look a little at the differences between the PA-RISC 2.0 and PA-RISC 1.1 architectures. PA-RISC 2.0 expanded the PA-RISC architecture to a 64-bit model and added many enhancements for improving system performance. Systems built on the PA-RISC 1.1 architecture run 32-bit kernels. A PA-RISC 2.0 system can run either a 32-bit or a 64-bit kernel. A 64-bit kernel in turn can run either 32-bit or 64-bit user applications. | 