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Asked: 2026-06-11T14:17:52+00:00

I am having trouble with pointing to a address and write in my case

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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?

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1 Answer

  1. Editorial Team

    [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.

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