Question

Let’s say that I want to read from absolute address gs:$30 in 64bit mode, so the asm code looks something like:

asm
  mov   rax, gs:[$30]
end;

…and compiler translate this code to…

  65 48 8B 05 30 00 00 00       mov rax,gs:[rel $00000030]

But I don’t want to use relative address (rip + $30). I want the compiler to use absolute address and compile in this way:

  65 48 8B 04 25 30 00 00 00    mov rax,gs:[+$0030]

(It is the same, if I use gs: prefix or not!)

How do I do this?

EDIT:

I know for work-around. I ask if exist any comand to tell compiler to address location as absolute instead relative.

EDIT

So far so good… 🙂

drhirsch helped me to find the command, and now the compiler translates:

mov   rax, gs:[abs qword ptr $30]
or
mov   rax, gs:[abs $30]

to this:

6548A13000000000000000 mov rax,[qword $0000000000000030]

Which is almost ok 🙂 Because I want short 32bit opcode (look upper opcodes) instlonger long 64bit opcode.

Is there any way to tell compiler to use short 32 bit address opcode instead long?

Answer 1

You need to use the movabs instruction.

movabs  rax, gs:[$30]

Edit: rip relative addressing is the default mode, on some assemblers you may be able to force 32 bit absolute addressing with

mov rax, gs:[dword $30]  #nasm, tasm
mov rax, gs:[abs $30]    #yasm