A Look Back - The Pentium Pro

If you take a look at the marketing structure of Intel's processors currently in production, the division among the low-end, midrange, and high-end processor solutions makes sense. Although the structure doesn't always end up being implemented as it should, such as in the case of the Celeron processors which ended up being used as overclocked midrange systems among the tweaker population in our little community, the idea of gearing certain processor lines towards specific types of users makes the efficiency of the processors' usage greater as they are more accurately targeted towards your particular needs. ppro.jpg (11404 bytes)
The theory first originated, at least in the case of Intel, approximately four years ago, with the announcement of the Pentium Pro processor. Originally appearing in engineering samples rated at 133MHz, the Pentium Pro was designed to be a high-end version of the already successful Pentium line of processors. The full-fledged debut and public implementation of the Pentium Pro actually came around in 1996 at a 150MHz-clock speed, to coincide with the rising clock speeds of the Pentium processor.

What separated the Pentium Pro from the Pentium classic (as it has been referred to since the introduction of the Pentium MMX) was the fact that the Pentium Pro had on-chip (albeit not on-die) level 2 cache memory as opposed to the level 2 cache that remained off the chip of the Pentium. As we already know, from the discussions of the Intel Celeron, the Celeron A, and even AMD's K6-3, the speed of the L2 cache is at a direct relationship to the overall performance of a processor, since the L2 cache is used as the high-speed storage area for frequently accessed data. Doesn't it make sense for the most frequently used data to be stored in the fastest memory locations in the system? It should. For a quick description of the primary function of a processor/system's cache, let's take a look at an excerpt from AnandTech's AMD K6-3 Review:

Cache is nothing more than high speed memory that is located closer to your CPU for faster access to frequently used data. The first place your CPU looks for data is in the cache, and more specifically, the cache located on the CPU itself, referred to as Level 1 or L1 cache. If the data the CPU is looking for isnt present in the L1 cache, or it fails to retrieve it in the current clock cycle, it then looks for it in the secondary cache, if present, otherwise it retrieves it from your system memory. Assuming that there is a secondary cache present (L2 cache), the processor can then retrieve it from a source slower than that of the L1 cache, yet still faster than if it had gone all the way to the system memory to retrieve the data. This process continues with however many levels of cache your system has before the processor has no other option than to retrieve the data from system memory, the slowest option out of them all.

By including the L2 cache in the physical package of the Pentium Pro (at the time, integrating L2 cache into the die of the processor itself was not a viable option due to the manufacturing process of Intel's core) Intel managed to outfit the Pentium Pro with an introductory 256KB of L2 cache running at clock speed. Once the Pentium Pro rose in clock speed, so did the amount of cache available for the truly high-end users, Intel began offering 512KB versions of the Pentium Pro at approximately twice the price of a regular Pentium Pro with 256KB of L2 cache, and sometimes in upwards of quadruple the cost of equivalently clocked Pentium processors. It came to the point, that a 200MHz Pentium Pro was made available with a full megabyte of L2 cache, at a price that, even today, still rivals the most expensive Pentium II and Pentium III processors. One could be led to believe that the Pentium Pro would be a complete failure because of its cost, but the Pentium Pro was quite the contrary, it was a highly successful processor among those users it was intended for. The Pentium Pro found its way into servers, high-end workstations, as well as the homes of graphics artists, and those users that happened to have an annoyingly large budget for computer equipment.

Today, Intel has avoided this problem of cost by integrating the L2 cache in the die of the processor, rather than packaging it beside the core of the CPU, however at the time, this seemed to be the best option. You can think of the Pentium Pro, as the Pentium II, with it's cache sitting quietly beside the processor, except instead of the black casing around the Pentium II, you have the ceramic casing around the processor and cache.

As you might be able to guess, the Pentium Pro was considerably larger than its little brother, the classic Pentium (that cache had to go somewhere). In addition to the physically larger size, the Pentium Pro required a new bus architecture, Intel hoped to improve the multiprocessor performance of their Pentium bus architecture as well as CPU to cache data transfers with the new P6 bus and they did so with the Pentium Pro. As a result, the memory and cache performance of the Pentium Pro was dramatically greater than that of the old Pentium, however the cost of the processor kept most desktop users away from the realm of the Pentium Pro. The extremely large and long forgotten interface the Pentium Pro used was dubbed Socket-8.

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