Fast powerful CPUs need quick and easy access to large amounts of data in order to maximize their performance. If the CPU cannot get to the data it needs, it literally stops and waits for it.
Hard disks provide large quantities of inexpensive, permanent storage.
Because storage space on a hard disk is so cheap and plentiful, it forms the final stage of a CPUs memory hierarchy, called virtual memory . We discuss RAM in detail in another article in this series, but several points about RAM are important here. The bit size of a CPU tells you how many bytes of information it can access from RAM at the same time. The goal of the memory system is to meet those requirements. However, the faster RAM is, the better.