Error in bootloader: Help in assembly

Question

I am trying to create a simple operating system, yet when I compile the bootloader, I get the error:

error: binary output format does not support external references

I know what this error means, but is it possible to make external references when compiling to binary format? Here is my complete code:

;Bootloader.s
[BITS 16]     
[ORG 0x7C00]
global loader
extern kmain

loader:
call kmain
times 510-($-$$) db 0
dw 0xAA55

And my kernel:

/*kernel.c*/
void kmain()
{
    unsigned char *vidmem = (unsigned char *) 0xb8000;
    int i
    for(i=1;i<=11;i+=2)
    {
        vidmem[i]=0x07;
    }
    vidmem[0]='H';
    vidmem[2]='e';
    vidmem[4],vidmem[6]='l';
    vidmem[8]='o';
    videmem[10]='!';
}

How I compiled:

nasm -o '/home/myusername/Cubed OS/Bootloader.o' '/home/myusername/Cubed OS/Bootloader.s'

Mat · Accepted Answer · 2011-08-20 13:31:47Z

3

You can only have external references in object file formats that are linkable. A bare binary is not, so you can't do that directly.

(And you need to set up a stack before you can call a C function AFAIK.)

I see three ways of going about this:

Use a bootloader that has Multiboot, and write your kernel as an ELF image. See the OSDev Bare bones for an example of that.
Write the whole thing in assembly. See for example How to write a simple operating system.
Write it all in C and use linker tricks to get things aligned where they need to be. An example of this with the GCC toolchain can be found here: Real mode in C with gcc : writing a bootloader

answered Aug 20, 2011 at 13:31

Mat

208k41 gold badges407 silver badges423 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

smilinggoomba Over a year ago

What format can you have external references and use the "ORG" directive?

Collectives™ on Stack Overflow

Error in bootloader: Help in assembly

1 Answer 1

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related