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A Tour Of The Pentium Pro Processor Microarchitecture

A Tour of the Pentium Pro Processor Microarchitecture

Introduction

One of the Pentium Pro processor's primary goals was to significannot
ly exceed the
performance of the 100MHz Pentium processor while being manufactured on the same
semiconductor process. Using the same process as a volume production processor
practically assured that the Pentium Pro processor would be manufacturable, but
it meant that Intel had to focus on an improved microarchitecture for ALL of the
performance gains. This guided tour describes how multiple architectural
techniques - some proven in mainframe computers, some proposed in academia and
some we innovated ourselves - were carefully interwoven, modified, enhanced,
tuned and implemented to produce the Pentium Pro microprocessor. This unique
combination of architectural features, which Intel describes as Dynamic
Execution, enabled the first Pentium Pro processor silicon to exceed the
original performance goal.

Building from an already high platform

The Pentium processor set an impressive performance standard with its pipelined,
superscalar microarchitecture. The Pentium processor's pipelined implementation
uses five stages to extract high throughput from the silicon - the Pentium Pro
processor moves to a decoupled, 12-stage, superpipelined implementation, trading
less work per pipestage for more stages. The Pentium Pro processor reduced its
pipestage time by 33 percent, compared with a Pentium processor, which means the
Pentium Pro processor can have a 33% higher clock speed than a Pentium processor
and still be equally easy to produce from a semiconductor manufacturing process
(i.e., transistor speed) perspective.

The Pentium processor's superscalar microarchitecture, with its ability to
execute two instructions per clock, would be difficult to exceed without a new
approach. The new approach used by the Pentium Pro processor removes the
constraint of linear instruction sequencing between the traditional "fetch" and
"execute" phases,...