VS embed string in assembly opcodes

Question

I'm trying to write a hello world program in assembly in visual studio. I would like to have the string saved as opcodes between instructions like this

call label1
    "hello world"
label1:
    pop esi
    push esi
    call print

How can I do this in Visual Studio?

Answer 1

With inline assembly you can use the _emit pseudo instruction, like this (here for 32-bit code):

auto foo()
    -> char const*
{
    __asm
    {
        mov eax, offset my_data
        jmp epilogue
    my_data:
        _emit 'H'
        _emit 'e'
        _emit 'l'
        _emit 'l'
        _emit 'o'
        _emit '!'
        _emit 0
    }
    epilogue: ;
}

#include <iostream>
using namespace std;
auto main() -> int
{
    wcout << foo() << endl;
}

I don't know of any way to write the strings as strings with inline assembly.

I recommend using proper full assembler instead. If you're using Visual Studio then you already have it installed, it's ml.exe .

Answer 2

In inline assembler in Microsofts compiler, you can't do the obvious solution of

 db "hello world"

so you have to actually generate the instruction sequence that produces the right bytes:

From my hand-disassembly, this should do it - I have not CHECKED that this gives the right sequence

[It could perhaps be possible to do:

 __asm 
  {
    call label1
    _emit(0x68)
    _emit(0x6f) 
    ... // rest of "hello world"
  label1: 
      pop esi
      push esi
      call print
  }

I haven't got a Visual studio compiler to try it on. Not sure if it's happy to jump to a label in a different __asm section tho.]

PUSH 6f6c6c65h     ; push = 'h', 6f6c6c64 'ello'
AND  al,al         ; space
JA   6fh           ; ja = 77 = 'w', 6f = 'o' 
JB   6ch           ; JB = 72 = 'r', 6c = 'l'
FS: ADD AL,AL      ; fs = 64 = d, ADD AL, AL = 0

This is obviously very unpractical, and I would not do this for anything that needs maintenance... [And I have probably got something wrong, and I expect that JA constant and JB constant isn't valid in inline assembler, you have to jump to a label... Which means you need to have a label about 100-110 bytes forwards to make 'l' and 'd'