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assemblyx86masmmasm32addressing-mode

Assembly Addressing Modes

Can someone please explain what the difference between the following two are? I'm finding it a little difficult to understand the concepts behind addressing modes

mov ax, [bx + di + 10]
mov ax, [bx + di] + 10

Thanks so much!

Solution

  • You labelled this MASM32 but neither instruction is legitimate for x86. Unless you are doing 16 bit programming, in which case you should make that clear.

    mov ax, [bx+di+10]

    Is not legal in x86 because it uses 16 bit addressing. The following is allowed, however:

    mov ax, [ebx+edi+10]

    Which means take the value of ebx, add it to the value of edi, and add 10 to that value. Then treat the final value as a pointer. Take the word (2 bytes) pointed to by that address and assign the value to ax.

    mov ax, [bx+di]+10

    Is not legal similarly (16 bit addressing). If you were to do:

    mov ax, [ebx+edi]+10

    That is also not allowed since mov does not allow an extra input after [ebx+edi]