Pentium Pro

The Pentium Pro is a sixth-generation x86-based microprocessor developed and manufactured by Intel introduced in November 1995. It introduced the (P6 microarchitecture) and was originally intended to replace the original Pentium in a full range of applications. While the Pentium and Pentium MMX had 3.1 and 4.5 million transistors, respectively, the Pentium Pro contained 5.5 million transistors. Later, it was reduced to a more narrow role as a server and high-end desktop chip. The Pentium Pro was capable of both dual- and quad-processor configurations. It only came in one form factor, the relatively large rectangular Socket 8.

In 1997, the Pentium Pro was succeeded by the Pentium II processor, which was essentially a cost-reduced and re-branded Pentium Pro with the addition of MMX and enhanced 16-bit code performance. Costs were reduced by using standard SRAM cache chips running at half-speed, which increased production yields. The next year, in 1998, Intel split the market into three segments: budget workstations and home users, higher-end workstations and power users, and multi-processor capable servers. Those segments were served by the Celeron, the Pentium II, and the Pentium II Xeon, respectively.

The Pentium Pro (given the Intel product code 80521), was the first generation of the P6 architecture, which would carry Intel well into the next decade. The design would scale from its initial 150 MHz start, all the way up to 1.4 GHz with the "Tualatin" Pentium III. The Pentium Pro had a theoretical performance of 200 MFLOPS. The core's various traits would continue after that in the derivative core called "Banias" in Pentium M and Intel Core (Yonah), which itself would evolve into Core architecture (Core 2 processor) in 2006 and onward.

Microarchitecture and performance


Belying its name, the Pentium Pro had a completely new microarchitecture, a departure from the Pentium rather than an extension of it. The Pentium Pro (P6) featured many advanced concepts not found in the Pentium, although it wasn't the first or only x86 processor that did (see NexGen Nx586 or Cyrix 6x86). The Pentium Pro pipeline employed extra decoding steps to dynamically translate IA-32 instructions into buffered micro-operation sequences which could then be analysed, reordered, and renamed in order to detect parallelizable operations that may feed more than one execution unit at once. The Pentium Pro thus featured out of order execution, including speculative execution via register renaming. It also had a wider 36-bit address bus (usable by PAE).

Performance with 32-bit code was excellent and well ahead of the older Pentium at the time, by 25-35%; however, the Pentium Pro's 16-bit performance was approximately only 20% faster than a Pentium at running 16-bit code due to the fact that register renaming was done on full 32-bit registers only (this was fixed in the Pentium-II).

It was this, along with the Pentium Pro's high price, that caused the rather lackluster reception among PC enthusiasts, given the dominance at the time of the 16-bit MS-DOS, 16/32-bit Windows 3.1x, and 32/16-bit Windows 95 (parts of Windows 95, such as USER.exe, were still mostly 16-bit). To gain the full advantages of Pentium Pro's microarchitecture, one needed to run a fully 32-bit OS such as Windows NT 3.51, Unix, Linux or OS/2.

After the microprocessor was released a bug was discovered in the floating point unit, commonly called the "Pentium Pro and Pentium II FPU bug" and by Intel as the "flag erratum". The bug occurs under some circumstances during floating-point to integer conversion when the floating-point number won't fit into the smaller integer format causing the FPU to deviate from its documented behaviour. The bug is considered to be minor and occurs under such special circumstances that very few, if any, software programs are affected.