CPU Wait State

It should also be noted that RAM, which operates in nanoseconds, is faster than most CPUs. This suggests a problem, but the CPU works through wait states, which are intervals of a set number of cycles between CPU actions, such as data requests, reads, writes, moves, etc., to allow the requests to be carried out. To read data from memory, the CPU may use three wait states, as illustrated in the below figure.

The CPU issues the request for data along with an address. Receiving the address and transferring it to the memory controller uses about one wait state. Finding the data in memory also takes about one wait state. Transferring the data to the CPU’s storage areas (called registers) uses a third wait state. Even if each wait state only took about 1/400 millionth of a second (based on a 400MHz CPU), RAM only requires perhaps 50 to 60ns to do its part. The significance here is that the closer the RAM’s speed is matched to that of the data bus and CPU clock, the more data will be transferred from the ram to the CPU and other components of the PC on each cycle.