Memory structure inside the microcontroller

　　Memory is one of the important hardware of computer. There are two types of memory structure of  single chip microcomputer
: one is that program memory and data memory are uniformly addressed, which belongs to Princeton structure, and the other is Harvard structure in which program memory and data memory are separately addressed. MCS-51 adopts Harvard structure. 　　1. Characteristics of 
　　memory Program memory and data memory are completely separated, each with its own addressing system, control signal and specific function. Program memory only stores programs and constants that must be retained at all times, and data memory is usually used to store a large amount of data required for program operation. 
The names related to memory in single chip microcomputer are: program memory and data memory, internal memory and external memory, byte address and bit address. The memory has a certain capacity, and an 8-bit binary number is often used as the basic unit, which is called a byte. The memory has many byte units, which are also identified by binary numbers, called addresses. Most of the addresses of these memory spaces are addressed from zero. 8-bit address 00H ~ FFH, 16-bit address 0000H ~ FFFFH. Since each hexadecimal digit can be directly converted into a four-bit binary digit (such as 0H-0000B, 9H-1001B, FH-1111B, where H represents the hexadecimal digit identifier and B represents the binary digit identifier), we will also refer to two hexadecimal digits as 8-bit binary digits in the future. 
　　The working register is composed of RAM, and the I/O interface also works in memory mode. The working register, I/O port latch and data memory RAM are uniformly addressed in the microcontroller. 
　　It has a powerful Boolean processor with 256 addressable bits.
　　2. The memory addressing space of MCS-51 
　　The register structure of MCS-51 microcontroller is shown in the figure. There are six addressing spaces and four physical memory spaces:
　　① The on-chip 4K (0000H-0FFFH) program memory with PC as the address pointer. 
　　② The off-chip 4K+60K (0000H-FFFFH) program memory. 
　　③ 64K data memory outside the chip with data pointer as address. 
　　④ 128 bytes RAM (00H-7FH) and special function register (80H-FFH) with 8-bit address inside the chip. 
　　Program memory, 4K program memory space inside the chip, its address is 0000H ~ 0FFFH, and the external EPROM is also addressed from 0000H. In the address range of 0000H ~ 0FFFH, the address overlaps, and the selection of internal and external program memory is controlled by the EA pin signal. 
　　When EA = 0, regardless of the size of the PC value, the CPU always accesses the external program memory. For the 8031 ​​chip, there is no program memory inside, and an external EPROM must be connected, so EA must be grounded, that is, when EA = 0. The external program memory is addressed from 0000H, and the addressing range is 64K. When EA = 1, the internal 4K program is executed first, and then the external program is executed after 4K is full. 
The program memory uses the 16-bit program counter PC as the address pointer to address a 64K byte space range. PSEN is used as the read select signal of the program memory. 
　　The program memory also stores constants required by the program. The microcontroller uses different instruction forms to distinguish whether it is accessing the program memory or the data memory. Whenever data is taken from the constant table of the program memory, the table lookup instruction MOVC form must be used.