The NexGen Nx586 CPU required special motherboards, largely because it used a separate bus for its L2 cache. Like the cache of Pentium Pro and later Intel CPUs, the Nx586 cache ran at the CPU's internal clock rate, rather than the motherboard bus speed. The Nx586's L2 cache was physically located on the motherboard, however.
Pentium-class CPUs, including faster Socket 7 models from Cyrix and AMD, place the L2 cache on the motherboard. Most 80486 and Pentium-class motherboards include a certain amount of cache soldered to the board, and some allow you to increase the motherboard's cache by adding chips or special cache memory modules. There is no standardized form for such modules, although a number of attempts have been made to create such a standard. In the end, you should consult your motherboard's documentation for details.
The AMD K6-III CPU includes an L2 cache in the CPU's package, but it's used on Socket 7 motherboards that contain L2 caches themselves. When used with a K6-III CPU, the motherboard's cache becomes a Level 3 (L3) cache. Its importance is reduced relative to its importance when used with other CPUs, but the presence of three levels of cache does improve performance marginally. In theory, an L3 cache could be added to a Slot 1, Slot 2, Socket 370, or Slot A motherboard, but few such motherboards include a cache because the benefit is relatively minor.
When considering Socket 7 motherboards, it's quite important that you consider the maximum cacheable RAM supported by the motherboard. With many products, it's possible to install more RAM than can be cached. The end result is frequently reduced overall system performance. Suppose, for example, that a motherboard can cache up to 64MB of RAM, but that you install 128MB on that motherboard. In this scenario, only half the memory accesses can benefit from the cache. If you happen to run some important piece of code (such as the Linux kernel or a heavily-used program) from the uncached 64MB of RAM, that program performs worse than it would if run from the cached 64MB of RAM—or if only 64MB were installed in the computer. To be sure, adding RAM can still improve matters if you routinely run so many programs that you would otherwise make heavy use of swap space, but, in general, it's wise not to extend your RAM beyond that which can be cached by the motherboard. If necessary, buy a new motherboard that can support more cacheable RAM.
Tables 2.1 through 2.4 list the total cacheable RAM for Pentium-class motherboard chipsets. The actual cacheable RAM on any given motherboard, however, depends in part on how large a cache is installed in the motherboard. The details of how large a cache is needed to support a given amount of RAM vary with the motherboard chipset and the type of cache it uses, so you should check your motherboard's manual, or contact the motherboard's manufacturer, for details.
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