CODEWITHSUNDEEP
assemblytasm
Maksim
Maksim

Reputation: 55

Getting offset of STRUC member in assembly

So, I have such structure:

String STRUC
   max_length       db  254
   real_length      db  ?
   buffer           db  255  dup(?)
String ENDS

In data segment I define variable of my «String» type:

source_str          String <>

And in code segment I'm trying to get the offset of structure member «buffer»:

mov     bx, offset source_str           ; in BX we have the offset of structure
lea     dx, [bx].buffer                 ; OK, the right offset in DX
mov     ax, offset [source_str].buffer  ; this works fine too
mov     dx, offset [bx].buffer          ; but this gets _wrong_ offset, according to td

I'm beginner and this is the way I think: we have an offset (in register or as label) - address, place it between [] and get the value - it's like pointer dereferencing in C/C++.

And my question is: why the last command doesn't work as I think it should? Is there a way to do this using "mov" and offset in BX register?

Upvotes: 1

Views: 900

Answers (1)

rcgldr
rcgldr

Reputation: 28921

For masm 6.11 the struc name needs to be included:

        lea     dx, (String ptr [bx]).buffer

For the other problem, you can't use offset with a base register:

        mov     dx, offset (String ptr [bx]).buffer  ; invalid

You could use add, but lea does the same thing with one instruction:

        mov     dx, bx
        add     dx, offset (String ptr ds:0).buffer

Upvotes: 3

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