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!
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There is no difference!
You can check with debugger...
mov ax, [bx + di + 10]
mov ax, [bx + di] + 10
Compiler will compile boath instructions to: 8B443B0A
So, ax should load the 16 bit value from address: bx + di + 10
GJ.
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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]
Mike Kwan
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Suppose bx=10 , di = 10.
In case 1,
mov ax, [30]
The value at memory location 30 will be copied to AX register
In case 2,
mov ax, [20]+10
The value at memory location 20, lets say X, add 10h to it X+10h, will be copied to AX register.
Madhur Ahuja
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7Are you *sure* the `MOV` instruction can do addition? – Lasse V. Karlsen Sep 03 '11 at 08:28
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Thanks! So, for the second one, the 10 is added to the *value* of `[bi+di]`? And for the first, the location from which the value is copied is offset by +10? Ss this correct? – Yuki Sep 03 '11 at 08:28
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Also, what does `mov ax, disp[bx][di]` mean? – Yuki Sep 03 '11 at 08:29
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@alan - It all means the same, add two registers and an offset (disp) to get an address. Then load the value from that address. – Bo Persson Sep 03 '11 at 10:07
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Don't think the answer is that bad that it needs to be modded down. His example makes it more visually insightful +1. – Marco van de Voort Sep 03 '11 at 11:24
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@MarcovandeVoort, it's probably downvoted because 'case 2' is not a legitimate assembler instruction. – Mike Kwan Sep 23 '11 at 12:33