Seven addressing modes (register relative addressing mode)

The operand is in memory and its effective address is a base register (BX, BP) or an index register (SI, D

I) and the 8-bit/16-bit offset in the instruction. The calculation formula for its effective address is shown in the formula.

Without using the segment override prefix, 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.
 
The 8-bit/16-bit offset given in the instruction is represented by a complement code. When calculating the effective address, if the offset is 8 bits, it is sign-extended to 16 bits. When the effective address obtained exceeds 0FFFFH, its 64K modulus is taken.

Example: Assume the instruction: MOV BX, [SI+100H]. When it is executed, (DS)=1000H, (SI)=2345H, and the content of memory unit 12445H is 2715H. What is the value of BX after the instruction is executed?

Solution: According to the rules of register relative addressing mode, when executing this instruction, the effective address EA of the source operand is:
 
EA=(SI)+100H=2345H+100H=2445H.
 
The physical address of the operand should be formed by the values ​​of DS and EA, that is:
 
PA=(DS)*16+EA=1000H*16+2445H=12445H.

Therefore, the execution effect of this instruction is: the value of a word starting from the physical address 12445H is transferred to BX.

The execution process is shown in the figure.