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Intel Pentium Pro ("P6")

The Pentium Pro was introduced in 1995 as the successor to the Pentium. It introduced several unique architectural features that had never been seen in a PC processor before. The Pentium Pro was the first mainstream CPU to radically change how it executes instructions, by translating them into RISC-like microinstructions and executing these on a highly advanced internal core. (The Nexgen Nx586 processor was actually the first x86 CPU to use this design, but this chip was used in very few systems.)

The Pentium Pro achieves performance approximately 50% higher than a Pentium of the same clock speed. In addition to its new way of processing instructions, the Pentium Pro incorporates several other technical advances that contribute to this increased performance:

• Superpipelining: The Pentium Pro dramatically increases the number of execution steps, to 14, from the Pentium's 5.
• Integrated Level 2 Cache: The Pentium Pro features a dramatically higher-performance secondary cache compared to all earlier processors. Instead of using motherboard-based cache running at the speed of the memory bus, it uses an integrated level 2 cache with its own bus, running at full processor speed, typically three times the speed that the cache runs at on the Pentium. The Pentium Pro's cache is also non-blocking, which allows the processor to continue without waiting on a cache miss.
• 32-Bit Optimization: The Pentium Pro is optimized for running 32-bit code (which most modern operating systems and applications use) and so gives a greater performance improvement over the Pentium when using the latest software.
• Wider Address Bus: The address bus on the Pentium Pro is widened to 36 bits, giving it a maximum addressability of 64 GB of memory.
• Greater Multiprocessing: Quad processor configurations are supported with the Pentium Pro compared to only dual with the Pentium.
• Out of Order Completion: Instructions flowing down the execution pipelines can complete out of order.
• Superior Branch Prediction Unit: The branch target buffer is double the size of the Pentium's and its accuracy is increased.
• Register Renaming: This feature improves parallel performance of the pipelines.
• Speculative Execution: The Pro uses speculative execution to reduce pipeline stall time in its RISC core.

For a few reasons, the Pentium Pro is still, despite its age, an ideal choice for servers. First, it is a fast chip in general. Second, its integrated level 2 cache makes it ideal for multiprocessing; instead of having a single motherboard-based level 2 cache that all the processors must share, each has its own. Third, the Pentium Pro has chipsets available for it that are designed for high-end server use, moreso than the Pentium.

The most widely-publicized advanced feature of the Pentium Pro is of course the integrated level 2 cache. The Pentium Pro is shipped in a special dual cavity SPGA package that includes the chip itself and the integrated cache. It goes into a special Socket 8 interface unique to the Pentium Pro. One disadvantage of this arrangement is that the cache is not upgradable without also replacing the processor.

The integrated-cache design has been both a blessing and a curse for Intel. The blessing is that it greatly improves the performance of the chip. The curse is that it has been very difficult for Intel to manufacture the Pentium Pro at the volumes and cost levels necessary for it to become a mainstream processor. There are two main reasons for this. First, the cache itself is highly miniaturized and therefore much more expensive to produce than the typical SRAM chips used on a Pentium motherboard for level 2 cache. Second, some problems with the cache are not found until after it has been mated with the processor and installed in their shared package; when this happens the whole package (including the processor) must be thrown away, reducing yields and increasing costs. Due to the problems with its design, Intel has abandoned the integrated-cache concept and it is unlikely that any future PC processors will use it in the same way that the Pentium Pro does.

The Pentium Pro is usually found in either 180 MHz or 200 MHz versions. Older Pentium Pros ran at 150 and 166 MHz; these are far less common and the 166 MHz chip is in particular rarely seen. The 150 and 180 chips ship only with 256 KB level 2 cache, while the 200 is available with 256 KB, 512 KB or 1 MB of level 2 cache. The cost of the 200 MHz chip with 512 KB or 1 MB of cache is very high due to production costs and demand. The 166 MHz chip is unusual in that it was available with 512 KB of cache only.

Despite being almost two years old, the Pentium Pro processor is still commonly used in high-end systems, although the Pentium II is now starting to take some of this market. Until Intel comes out with a proper Pentium II chipset for servers, demand for the 200 MHz version (especially with 512 KB or 1 MB of cache) will continue to be high. In addition, multiple-Pentium-Pro servers are quite common and provide good performance at a reasonable price. The Pentium Pro often competes against non-Intel server processors such as DEC's Alpha.

Look here for an explanation of the categories in the processor summary table below, including links to more detailed explanations.

Next: Intel Pentium II ("Klamath")

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