ORG 0030H

ORG is a pseudo-instruction, telling the compiler to store the program from 0000 in ROM, but AJMP MAIN is an unconditional jump instruction, that is, after the microcontroller is powered on, it will first start executing the program from 0000, but AJMP will directly jump the program to the storage area pointed to by the MAIN label to start fetching instructions and executing them. Generally, the MAIN label is placed after the statement ORG 0030, that is, the main program is stored from the address 0030;

This is because there are several interrupt sources inside the microcontroller. The default entry for internal interrupts (including several timer interrupts and external interrupts) is between 0000-0030. Each interrupt takes up about two or four bytes, I can't remember clearly. Once these internal interrupts occur, the pointer must immediately point to between 0004-0030. If your program has statements placed in this address, the program will definitely run away!!!

Therefore, in the design of the 8051 MCU, 0004-0030 has been allocated for internal use. Therefore, your program can only be stored from any storage area after 0030. AJMP MAIN is to know which address in the ROM to start fetching instructions from when the MCU is powered on.

If you use interrupts, you must also write AJMP at the address 0004--0030 that has been defined as the interrupt entry, so that you can jump to the corresponding interrupt program you wrote.