L2 cache … HUH?

Question: Can you explain what L2 cache is and, also, what does it do? – A.P.

Answer: Let’s start with the basics. In the computer world, cache is a place where stuff is processed or reserved separately from other processes. A web browser cache is a store of images from web pages, kept so they don’t have to be downloaded each time a visitor returns to a page. A disk cache is an area of the hard drive reserved to process data. The computer often uses it when it runs out of main memory.

Memory cache is a kind of memory made of high-speed static RAM or SRAM instead of the slower and cheaper dynamic RAM or DRAM, which most of us usually just refer to as RAM or random access memory.

Memory caching happens because many programs use the same data over and over again. Cache is where microprocessors keep instructions that are reused. By keeping frequently used information in SRAM, the computer avoids accessing the slower main memory (DRAM).

When you hear people refer to a computer’s cache, it usually means they are referring to Level 2, or “L2”, cache. It is cache memory that is outside the processor chip.

Some memory caches are built into the architecture of microprocessor chips. The 486 chip, for example, contains 8K of memory cache, and the Pentium has a 16K of cache. Those internal caches are often called Level 1 or L1 caches.

The Celeron chip is a cheaper chip because it has 128K of L2 cache built into the chip. The Pentium II and Pentium III cache is 512K in size and is external to the chip itself.

The Pentium Pro, however, has an L2 cache on the same chip as the microprocessor.

Data is transmitted from the processor chip into main memory through the L2 cache. This stops a data bottleneck. L2 cache that’s 256K in size can handle the cache functionality for up to 64Mb of DRAM. 512K of L2 cache handles caching for up to 128Mb of RAM.