The HP PA-8000 RISC CPU

The PA-8000 RISC CPU is the first of a new generation of Hewlett-Packard microprocessors. Designed for high-end systems, it is among the world's most powerful and fastest microprocessors. It features an aggressive, four-way, superscalar implementation, combining speculative execution with on-the-fly instruction reordering. The heart of the machine, the instruction reorder buffer, provides out-of-order execution capability. Our primary design objective for the PA-8000 was to attain industry-leading performance in a broad range of applications. In addition, we wanted to provide full support for 64-bit applications. To make the PA-8000 truly useful, we needed to ensure that the processor would not only achieve high benchmark performance but would sustain such performance in large, real-world applications. To achieve this goal, we designed large, external primary caches with the ability to hide memory latency in hardware. We also implemented dynamic instruction reordering in hardware to maximize instruction-level parallelism available to the execution units. The PA-8000 connects to a high-bandwidth Runway system bus, a 768-Mbyte/s split-transaction bus that allows each processor to generate multiple outstanding memory requests. The processor also provides glueless support for up to four-way multiprocessing via the Runway bus. The PA-8000 implements the new PA (Precision Architecture) 2.0, a binary-compatible extension of the previous PA-RISC architecture. All previous code executes on the PA-8000 without recompilation or translation.