If you have learned Computer architecture and organization you would get it. But anyway, different processors have different architecture. Processors main components are ALU, CU, Registers. Registers are important information to process in the ALU. The registers are further divided to their functionalities like data, flag, address and others. So assembly languages depend on the registers to process data. For example, 8086 and any normal *commercial* computer depend on the MOV ax bx format. There are other formats to be followed for mainframe computers. Plus still on commercial computers, it depends on OS architecture and processor vendor. There are different assemblers for different OS. There are many assemblers for intel processors, and for amd processors You can just read more on google