The PA-8000 RISC CPU is the first of a new generation of Hewlett-Packard microprocessors. Designed for highend 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 splittransaction
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.