How does the `asm()` function works in C language?

Question

I am learning Operating System Development and a Beginner of course. I would like to build my system in real mode environment which is a 16 bit environment using C language.

In C, I used a function asm() to convert the codes to 16 bit as follows:

asm(".code16")

which in GCC's language to generate 16 bit executables(not exactly though).

Question:

Suppose I have two header files head1.h and head2.h and a main.c file. The contents of main.c file are as follows:

asm(".code16");
#include<head1.h>
#include<head2.h>
int main(){
  return 0;
}

Now, Since I started my code with the command to generate 16 bit executable file and then included head1.h and head2.h, will I need to do the same in all header files that I am to create? (or) Is it sufficient to add the line asm(".code16"); once?

OS: Ubuntu

Compiler: Gnu CC

Answer 1

To answer your question: It suffices for the asm block to be present at the beginning of the translation unit.
So putting it once at the beginning will do.

But you can do better: you can avoid it altogether and use the -m16 command line option (available from 5.2.0) instead.

But you can do better: you can avoid it altogether.

---

The effect of -m16 and .code16 is to make 32-bit code executable in real mode, it is not to produce real mode code.

Look

16.c

int main()
{
   return 4;
}

Extracting the raw .text segment

>gcc -c -m16 16.c
>objcopy -j .text -O binary 16.o 16.bin
>ndisasm 16.bin

we get

00000000  6655              push ebp
00000002  6689E5            mov ebp,esp
00000005  6683E4F0          and esp,byte -0x10
00000009  66E800000000      call dword 0xf
0000000F  66B804000000      mov eax,0x4
00000015  66C9              o32 leave
00000017  66C3              o32 ret

Which is just 32-bit code filled with operand size prefixes.
On a real pre-386 machine this won't work as the 66h opcode is UD.

---

There are old 16-bit compilers, like Turbo C¹, that address the problematic of the real-mode applications properly.

Alternatively, switch in protected mode as soon as possible or consider using UEFI.

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¹ It is available online. This compiler is as old as me!