The CPU (central processing unit) is the major actor in a computer. To run a program or process data, the computer must first transfer the program or data from disk to main memory. Then, to execute an instruction, it moves the instruction from main memory into the CPU via the data channel or bus. The CPU has a small amount of very fast memory called a cache. The CPU keeps frequently used instructions in the cache. The main memory of the computer contains program instructions for application programs and a browser. It also contains instructions for the operating system. Main memory is too small to hold all of the programs and data that a user may want to process. Consequently, the CPU loads programs into memory in chunks. If the user opens another program or needs to load more data, the operating system will direct the CPU to attempt to place the new program or data into unused memory. If there is not enough memory, it will remove something, and then it will place the just-requested program or data into the vacated space. This process is called memory swapping.