Pentium Pro

The Pentium Pro is an x86 architecture microprocessor by Intel originally intended to replace the Pentium classic in a full range of applications, but later reduced to a more narrow role as a server and high-end desktop chip. It was introduced using an enormous rectangular Socket 8 form factor in November 1995. Intel has since discontinued it in favour of the newer high-end Xeon[?] processor lines.

Despite the name, the Pentium Pro is architecturely quite different from Intel's earlier Pentium processor, being based on the then-new P6 core (which in a modified form would later be used for the Pentium II and Pentium III). The P6 core features out-of-order execution, speculative execution and an additional pipe for simple instructions. Speculative execution - the provisional execution of code in a branch that is not yet certain to be taken - considerably increases the penalty for mispredicting a branch, and the Pentium Pro therefore uses a more sophisticated branch prediction algorithm than the Pentium. For the same reason, the Pentium Pro also introduced a conditional move (cmov) instruction which in some cases can be used to avoid the need for a branch instruction altogether. Performance with 32-bit code was excellent, however there was no performance advantage over a Pentium classic with 16-bit code and operating systems.

The Pentium Pro had a 256K-512K on-package cache at introduction, with a 1 MB version introduced later. All versions were expensive, those with more than 256K particularly so. The Pro's "on-package cache" arrangement was unique. The processor and the cache were on separate dies in the same package and connected closely by a full-speed bus. The two dies - both very large by the standards of the day - had to be bonded together early in the production process, before QC testing was possible. This meant that a single tiny flaw in either die made it necessary to discard the entire assembly, which was one of the reasons for the Pros relatively low production yield and high cost.

Following discussions of future product planning with Microsoft, Intel made the design decision to optimise the Pentium Pro for 32-bit code. In consequence it performed poorly on 16-bit code, and when Windows 95 unexpectedly turned out to be largely 16-bit, the Pro was not well positioned to compete in the desktop market. Introduced as a high-end server and workstation chip, it was now destined to stay within that market rather than migrate onto the desktop. Intel filled the gap with an improvement to the older Pentium Classic design called the Pentium MMX. Pentium Pro clock speeds were 150, 166, 180 or 200 MHz with a 60 or 66 MHz external bus clock.

see also: List of Intel microprocessors