256KB L2

The most noticeable difference between the Pentium III E and all previous Pentium III processors is that the Pentium III E features a full 256KB of on-die L2 cache operating at clock speed. This not only helps the platform scale better as clock speed increases but it also improves performance across the board.

For those of you that aren’t familiar with the function of L2 cache, it is best summarized as a small amount of high speed memory. When storing data, the first place the CPU would like to have it stored would be on the chip itself, so that the CPU does not have to cross any slower busses to retrieve the data. The data can’t be stored just anywhere on the physical chip, it needs to be stored in some sort of memory location; this is cache.

Let’s split the cache into two parts, the first, a small but extremely fast cache, and the second, a cache that’s larger but slower, but still faster than the system memory that the CPU would otherwise have to use to storeand retrieve its data from. Let’s call these two caches L1 and L2 respectively.

The Pentium III processor (not the Pentium III E) features 32KB of L1 cache and 512KB of L2 cache. The L1 cache operates at the clock speed of the CPU (i.e. 500MHz) and the L2 cache operates at half the clock speed of the CPU.

As you can see, 32KB is not a lot of space to store data, so only a very small amount of information can be stored in the Pentium III’s L1 cache. What doesn’t fit into its L1 can potentially go to the L2. While the L2 cache is obviously slower (in this case, 250MHz vs 500MHz) than the L1 cache, it is still faster than the system memory, which runs at anywhere from 66 – 133MHz depending on your system; in this case it would be running at 100MHz.

The Celeron processor features 32KB of L1 cache and 128KB of L2 cache. In this case, both the L1 and L2 cache operate at the clock speed of the CPU (i.e. 500MHz).

With these two processors, applications that can fit within the 128KB L2 cache of the Celeron will run faster on the Celeron 500 than they would on the Pentium III 500, because the L2 cache of the Celeron is running at twice the speed of the L2 cache of the Pentium III (500MHz vs 250MHz).

At the same time, if you had an application that could not fit within the 128KB L2 of the Celeron but it could fit within the 512KB L2 of the Pentium III, then the application would run faster on the Pentium III because the Celeron would have to go all the way to the system memory in order to access the application.

From a cost perspective, it is cheaper to integrate L2 cache onto the die (Celeron) than to mount the chips externally on a processor card (Pentium III). This is where the Pentium III E comes in.

The Pentium III E takes the best of both worlds; it features half of the L2 cache of the Pentium III (512KB / 2 = 256KB) but they moved the L2 cache onto the die of the processor and thus it operates at the core clock speed of the processor.

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