MCU can be divided into Harvard structure and Feng? Von Neumann structure. Nowadays, most single-chip computers are based on von Neumann structure, which clearly defines four basic parts necessary for embedded systems: a central processing unit core, program memory (read-only memory or flash memory), data memory (random access memory), one or more timers, and input/output ports for communicating with peripheral devices and extended resources, all of which are integrated on a single integrated circuit chip. A microprocessor can intelligently perform a certain task. Besides their powerful hardware, it also needs the software they run. In fact, microprocessors are not smart. They just execute them completely according to people's pre-written programs. Then the program written by the designer is stored in the program memory of the microprocessor, commonly known as read-only program memory (ROM). A program is equivalent to a series of commands for a microprocessor to deal with problems. In fact, programs, like data, are all code strings composed of machine code. Only the program code is stored in the program memory. It has an addressing space of 64kB program memory, which is used to store information such as user programs, data and tables. (For the 831 single-chip microcomputer without ROM inside, its program memory must be externally connected, and the space address is 64kB. At this time, the EA end of the single-chip microcomputer must be grounded. Force the CPU to read the program from the external program memory. ) For single-chip microcomputer 851 with ROM inside, during normal operation, it needs to be connected with high level, so that the CPU can read the program from the internal program storage first, and when the PC value exceeds the capacity of the internal ROM, it will turn to the external program storage to read the program.