The Pentium III - "Revolutionary" or "Revolving" Technology?
Imagine a Pentium II, except outfitted with 64KB of L1 cache, operating at clock speeds derived by the 133MHz Front Side Bus (FSB), and combined with the release of a killer chipset that would boast AGP 4X acceleration as well as the introduction of Rambus DRAM. The processor you're imagining is what the majority of the market envisioned the Pentium III's release as being, and as you can probably conclude by now, the vision we all shared of the Pentium III was a bit on the fantasy side.
The fact of the matter is this, the Pentium III, in spite of its dramatic name change, is nothing more than what the Pentium MMX was to the Pentium classic. The Pentium III offers the same basic features the Pentium II brought to the table, with a major difference, the introduction of the 70 new SSE instructions as previously mentioned. While the Pentium MMX doubled the amount of L1 cache of its predecessor, the Pentium III remains virtually unchanged from the original Pentium II, leaving many critics disappointed with Intel's lack of originality with the Pentium III processor.
For those of you that aren't already familiar with the Pentium II, it features an internal 32KB Level 1 cache split evenly between data and instruction set caches, and support for an external, closely-coupled, on-package Level 2 cache operating at 1/2 clock speed. The Pentium III's L2 cache running at 1/2 clock speed, as with the Pentium II, is located on the processor's Slot-1 cartridge in the form of two 256KB modules placed adjacent to the processor core. Since the speed of the L2 cache is derived from the clock speed of the processor and not the FSB speed of the system, future Pentium III processors will continue to offer almost linear raw performance increases as a large percentage of system performance is courtesy of a fast L2 cache. As described in AnandTech's AMD K6-3 review, the function of all system level cache in a computer is critical to performance:
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.
Unlike AMD's K6-2, whose L2 cache was generally stationary at the supported 100MHz FSB frequency, the Pentium III's L2 cache speed scales with the processor's clock speed. At the same time, unlike the Celeron A and AMD K6-3 processors, the Pentium III does not offer the integrated L2 cache running at clock speed that other newer 6th generation processors offer, including those from Intel themselves. The Pentium III, like its predecessor, is still outfitted with 512KB of L2 cache and as the Celeron A and K6-3 processors have already shown us, the performance of a system with 256Kb of L2 cache running at clock speed is approximately equal to that of a system with 512KB of L2 cache running at 1/2 clock speed in most cases (although AnandTech will get into the performance differences later on).