Cache and Buses
Todays PC has several layers of cache. The first level has the catchy title of Level 1 cache. This is the fastest hunk of memory in your computer. Its easy to find the general location of it, because it is inside your CPU. When your CPU makes a request, it looks at the Level 1 cache. If what the CPU needs is there, it takes it without ever leaving the CPU. Finding what it is looking for is known as a hit. Naturally, the lack of a hit is a miss. The 80486 CPUs were the first CPU with cache built-in. This explains why if you read other technical works, you may find a reference to Level 1 cache being outside of the CPU. Before the 486, the first level of cache had to be outside the CPU.
If we have a miss in the L1 cache, the next place to look is the Level 2 cache. This is where the Backside Bus comes into play. In earlier designs, the Level 2 caches had to fight for time with other buses, like the expansion bus. This became a digital version of rush hour traffic. And since the CPU was left waiting, well you already read that outcome. So, the solution became to put a Backside Bus that typically runs at full processor speed just to the L2 cache.
Before closing on cache. Yes, there can be a Level 3 cache. And typically, hard drives, CD-ROMs, almost all drives have some cache. Cache works as well as it does (typically more than 90% of all CPU request are in Level 1 or Level 2 cache) because the design engineers have developed some really slick routines, known as algorithms to determine what is really needed to keep the CPU busy.
Almost done with buses. There is one more bus that arrived only a few years ago. As you will see later in this chapter, the video sub-system has been starved for data. This isn't good, since this affects the users perceptions. Leave to Intel to address this quandary.
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