170、The principal function of main memory is to act as an intermediary between the central processing unit ( ) and the rest of the computer system components.It functions as a sort of desktop on which you place the things with which you are about to begin to work.The CPU can utilize only those software instructions and data that are stored in main memory.As you know, main memory is random access memory, or RAM.The name derives from the fact that data can be stored in and retrieved at random from anywhere in the electronic main memory, chips in approximately the same amount of time, no matter where the data is.Main memory is in an electronic, or volatile state.When the computer is off, main memory is empty; when it is on, main memory is capable of receiving and holding a copy of the software instructions and data necessary processing.Because main memory is a volatile form of storage that depends on electric power and the power can go off during processing, users often save their work frequently onto nonvolatile secondary storage devices such as diskettes or hard disks.In general, main memory is used for the following purposes: A copy of the main software program stored in the computer that controls the general operation of the computer.This copy is loaded into main memory when the computer is turned on, and it stays there as long as the computer is on.(1) Temporary storage of a copy of application program instructions ( ) to be retrieved by the CPU for interpretation and execution.(2) Temporary storage of data that has been input from the keyboard or other input device until instructions call for the data to be transferred into the CPU for processing.(3) Temporary storage of data that has been produced as a result of processing until instructions call for the data to be used again in subsequent processing or to be transferred to an output device such as the screen, a printer, or a disk storage device.(1)、1.What is the passage mainly about?