Pentium Pro

Pentium Pro. The Pentium Pro was a x86 microprocessor introduced by Intel in November 1995. Representing a radical departure from the preceding Pentium design, it was the first processor to implement the new P6 microarchitecture. Primarily aimed at the server and high-end workstation markets, it featured an innovative on-package L2 cache and was optimized for running 32-bit code on operating systems like Windows NT.

Overview

The development of the Pentium Pro, led by architects like Robert P. Colwell, was a direct response to the competitive pressures from companies like Advanced Micro Devices and the rising demands of enterprise computing. It was formally launched at the 1995 Fall Comdex trade show, signaling Intel's strategic push into the lucrative server segment. Unlike its predecessor, the Pentium Pro was not intended for the mainstream consumer market but was instead positioned against RISC-based processors from Sun Microsystems and Silicon Graphics. Its release coincided with the growing adoption of 32-bit operating systems, particularly Microsoft's Windows NT and various flavors of Unix.

Architecture

The core of the Pentium Pro was its new P6 microarchitecture, which introduced several groundbreaking features for the x86 lineage. A key innovation was its dynamic execution capability, which combined speculative execution, Out-of-order execution, and a sophisticated branch prediction unit to improve instruction throughput. The processor utilized a decoupled, 14-stage superpipelined design. Its most distinctive physical characteristic was the use of a Multi-Chip Module (MCM) package, which housed the CPU die alongside a separate, high-speed SRAM die for the integrated L2 cache, connected via a full-speed Back-side bus.

This cache, ranging from 256 KB to 1 MB, was a major differentiator. The memory subsystem was further enhanced by an integrated memory controller and support for multiprocessing via a dedicated bus, allowing configurations of up to four processors in systems like the 450NX chipset platform. However, the architecture's focus on 32-bit performance came at a cost; its handling of 16-bit code, common in older DOS and Windows 95 applications, was often slower than on a standard Pentium due to the inefficiencies of its microcode sequencer and register renaming logic when dealing with segment registers.

Performance and reception

In its intended domain of 32-bit software, the Pentium Pro's performance was exceptional, often surpassing contemporary RISC chips in benchmarks for SPECint and database transactions running on Oracle or Microsoft SQL Server. It became the foundation for powerful workstations from Compaq and IBM and was widely adopted in servers from Dell and Hewlett-Packard. The processor's strengths were showcased in the famous Deep Blue chess computer, which used a cluster of Pentium Pro chips to defeat Garry Kasparov in 1997. Critical reception from publications like InfoWorld and PC Magazine praised its server-class capabilities but noted its high cost and poor performance with legacy 16-bit applications.

This weakness, particularly in the context of the still-popular Windows 95 operating system, limited its appeal in the broader PC market. Furthermore, the complexity of its MCM packaging made it expensive to manufacture. While it achieved design wins in major corporate and government installations, its market share remained niche compared to the standard Pentium and the rising AMD K6. The architectural trade-offs made for 32-bit optimization defined its legacy as a pioneering but specialized product.

Variants and successors

The Pentium Pro line included variants with different L2 cache sizes, culminating in a 1 MB cache version. A derivative, code-named P6T, was later marketed as the Pentium II OverDrive processor for upgrading existing Socket 8 systems. The core P6 microarchitecture, however, lived on and was directly evolved into the immensely successful Pentium II, which addressed the Pentium Pro's commercial shortcomings by moving the L2 cache off-package onto a SEC cartridge and significantly improving 16-bit performance. The Pentium II was followed by the Pentium III and Pentium M, all descendants of the P6 design. The technology and lessons from the Pentium Pro also heavily influenced the development of the Xeon brand, with the first Xeon processors being a cache-enhanced variant of the Pentium II core, cementing the P6 lineage's long dominance in both enterprise and mobile computing. Category:Intel microprocessors Category:Computer-related introductions in 1995