The calculation method of the physical address of the register 
indirect addressing mode is as follows: The principle of reading the storage unit in the register indirect addressing mode is shown in the figure. 
When the segment override prefix is not used, the following provisions apply:
If the effective address is specified by one of SI, DI and BX, the default segment register is DS;
if the effective address is specified by BP, the default segment register is SS (i.e., stack segment).
Example: Assume that there is an instruction: MOV BX, [DI]. When it is executed, (DS) = 1000H, (DI) = 2345H, and the content of the storage unit 12345H is 4354H. What is the value of BX after the instruction is executed?
Solution: According to the rules of the register indirect addressing mode, when executing this instruction, the value of register DI is not the operand, but the address of the operand. The physical address of the operand should be formed by the values of DS and DI, that is:
PA = (DS) * 16 + DI = 1000H * 16 + 2345H = 12345H.
Therefore, the execution effect of this instruction is: transfer the value of a word starting from the physical address 12345H to BX.
Its execution process is shown in the figure.
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