The operand is in the memory, and its effective address is the sum of the contents of a base register (BX, BP) and an index register (SI, DI). The formula for calculating its effective address is shown in the formula. 
When the segment override prefix is not used, it is stipulated that: if the effective address contains BP, the default segment register is SS; otherwise, the default segment register is DS.
Example: Assume the instruction: MOV BX, [BX+SI]. When it is executed, (DS)=1000H, (BX)=2100H, (SI)=0011H, and the content of memory unit 12111H is 1234H. What is the value of BX after the instruction is executed?
Solution: According to the rules of base plus index addressing mode, when executing this instruction, the effective address EA of the source operand is:
EA=(BX)+(SI)=2100H+0011H=2111H.
The physical address of the operand should be formed by the values of DS and EA, that is:
PA=(DS)*16+EA=1000H*16+2111H=12111H.
Therefore, the execution effect of this instruction is: transfer the value of a word starting from the physical address 12111H to BX.
The execution process is shown in the figure.
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