The Memory System
It's important to think of storage in the context of our theme of making computers faster. If you look at Table 2.1, you'll see that there is quite a lot of bad news. Viewing time in terms of CPU clock cycles, there isn't much of a penalty when the CPU has to wait to get something from a register, or from the cache. But regular memory is notably slower, and disk storage is vastly slower. Computer designers use the term wait state for intervals when the CPU babbles to itself and no useful work occurs because needed information has to be retrieved from the outside world. Some proportion of wait states is obviously unavoidable, but if they begin to mount up, they greatly diminish the effective speed of the CPU.