An index register is used in brackets and the actual address accessed depends on the value contained in that register. For example “mov [si], ax” moves the contents of the AX register to the word starting at address contained in SI in the current data segment. The instruction “mov [di], ax” moves the word contained in AX to the offset stored in DI in the current data segment.
Based Register Indirect + Offset
A base register is used with a constant offset in this addressing mode. The value contained in the base register is added with the constant offset to get the effective address. For example “mov [bx+300], ax” stores the word contained in AX at the offset attained by adding 300 to BX in the current data segment. The instruction “mov [bp+300], ax” stores the word in AX to the offset attained by adding 300 to BP in the current stack segment.
Indexed Register Indirect + Offset
An index register is used with a constant offset in this addressing mode. The value contained in the index register is added with the constant offset to get the effective address. For example “mov [si+300], ax” moves the word contained in AX to the offset attained by adding 300 to SI in the current data segment and the instruction “mov [di+300], al” moves the byte contained in AL to the offset attained by adding 300 to DI in the current data segment. 
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