Usually, primary cache is internal to the CPU, and secondary cache is external. Some early-model personal computershave CPU chips that don't contain internal cache. Earlier we used the analogy of a room with a work table and a set of file cabinets to understand the relationship between main memory and a computers hard disk. If memory is like the work table that holds the files you're working on making them easy to reach, cache memory is like a bulletin board that holds the papers you refer to most often. When you need the information on the bulletin board you simply glance up and there it is.
Memory is like a work table that makes immediate work easily accessible.
You can also think of cache memory as a worker's tool belt that holds the tools and parts needed most often. In this analogy, main memory is similar to a portable tool box and the hard disk is like a large utility truck or a workshop. When a cache memory controller retrieves an instruction from main memory, it also takes back the next several instructions to cache. This occurs because there is a high likelihood that the adjacent instruction will also be needed. This increases the chance that the CPU will find the instruction it needs in cache memory, thereby enabling the computer to run faster.