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assembly x86 x86-16 nasm addressing-mode

NASM x86 16-bit addressing modes

发布于 2012-09-18 09:21:43

I am having trouble with pointing to a address and write in my case a variable of byte in size. This gives me the error "error: invalid effective address":

mov byte[AX], byte 0x0

After some trail and error i tested the same but with EAX. This compiles just fine:

mov byte[EAX], byte 0x0

What am I missing here?

Questioner
Michael
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237k 2017-10-21 19:31:48

[AX] is an invalid memory operand specification.

The valid 16-bit ones are:

[constant]  
[BX]  
[SI]  
[DI]  
[BX+constant]  
[BP+constant]  
[SI+constant]  
[DI+constant]  
[BX+SI]  
[BX+DI]  
[BP+SI]  
[BP+DI]  
[BX+SI+constant]  
[BX+DI+constant]  
[BP+SI+constant]  
[BP+DI+constant]

[BP] is formally invalid, but many assemblers will quietly convert it into [BP+0].

See the CPU manual for memory operand encodings and the ModR/M and SIB bytes.

nrz 2012-09-18 11:15:04

When using [bp+constant], [bp+si+constant] or [bp+di+constant], it's good to remember that the default segment for all these addressing modes with bp is ss (stack segment), not ds (data segment), as it is for all other addressing modes listed above.

Peter Cordes 2017-11-23 10:32:35

Note that 16-bit addressing modes can't use a SIB byte, only ModR/M, which is why they're limited to (BP|BX) + (DI|SI) + disp0/8/16

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