CACHE MEMORY (RAM)
CACHE MEMORY provides the quick access to information. Main memory access by the CPU may take as long as 180ns; however, it may only take 45ns or less to access external cache memory, and even less for internal (L1) cache.

While the CPU is processing information retrieved from cache, the cache controller is refreshing cache with data and instructions from main memory or a storage device. Cache memory chips look something like this illustration:

There are two categories of cache memory: Internal and External.

A as a general rule, Internal cache is memory located inside the CPU, and external is located on the motherboard between the RAM and the CPU.

This type of memory is often called 'cache RAM'.

  • Internal Cache memory is often called "level 1"or L1 cache.

  • External cache is often called "level 2" or L2 cache. L2 Cache may be on-board (in the CPU chip) or in Static RAM (SRAM) chips on the motherboard.

L2 Cache RAM is a small block of high-speed memory, usually SRAM (Static RAM), located between the CPU and main memory. It is used to store data or instructions that are used often. It also has the quickest access for memory for the CPU. There are several types of SRAM with the fastest having a speed of 4-8 ns.

External cache acts as a quick access buffer between the RAM and L1 cache in the CPU.

Years ago, external cache was a type of buffer (set-up in regular RAM) for for data movement between devices such as disks, or CD-ROM. Our experience with a 486DX with "external" cache versus one with no external cache is this:

The two were started reading 31MB of data from a CD-ROM over the network. The one with external cache finished the job in half the time of the one without external cache.

Internal Cache
Internal cache memory usually runs from about 1KB (1024 bytes) to 64KB. The pentium II and II chips have even more.

External Cache
External cache memory usually runs from 64KB to 1 MB. Internal cache normally cannot be upgraded; however, external cache can often be upgraded.

External cache is normally a plug device - it plugs into "cache" sockets located on motherboards, and even on individual boards such as Network Interface Cards, video cards, etc.

In the newer processors, such as Pentium Pro and Pentium II, you will see new designations for cache. Some will have L1 cache (internal cache) and L2 cache (what used to be external cache). The Pentium II processor claims to have 512KB of integrated L2 cache.

These machines have both types of cache integrated into the microprocessor. This is a serious improvement - it gets the external cache, or L2, off the motherboard, and puts it where the cache can run at the full CPU speed rather than at the slower motherboard speed. A new designation "L3" is used for cache that is on the motherboard.

For a complete discussion of all types of memory, I suggest you go to http://www.kingston.com/ or http://www.centon.com/on the WEB.