how to link binary bootcode with C object file

[drnono]

how do I link a binary format assembled with nasm with the object code from C.

I've compiled the i686-elf-gcc cross compiler.

1. nasm boot.asm

2. i686-elf-gcc -c kernel.c -o kernel.o -std=gnu99 -ffreestanding -O2 -Wall -Wextra

3. i686-elf-gcc -T linker.ld -o myos -ffreestanding -O2 -nostdlib boot.o kernel.o -lgcc

When I try to link I get:
boot.o: file not recognized: File format not recognized

linker.ld:

Code: Select all

ENTRY(kmain)

STARTUP(boot.o)
OUTPUT_FORMAT("binary")

SECTIONS
{
        . = 1M;
        .text BLOCK(4K) : ALIGN(4K)
        {
                *(.text)
        }
        .rodata BLOCK(4K) : ALIGN(4K)
        {
                *(.rodata)
        }
        .data BLOCK(4K) : ALIGN(4K)
        {
                *(.data)
        }
        .bss BLOCK(4K) : ALIGN(4K)
        {
                *(.COMMON)
                *(.bss)
        }
}

[kzinti]

Code: Select all

STARTUP(boot.o)

This can't be right (but I don't think that's the problem you are asking about).

In STARTUP(), you want to specify the entry point of your program. This is usually a symbol (name) of the function. Typically one uses _start.

[alexfru]

1. nasm boot.asm

I don't think this produces an object file. Most likely a raw binary, which you can't link with anything in a normal way.

[drnono]

1. nasm boot.asm

I don't think this produces an object file. Most likely a raw binary, which you can't link with anything in a normal way.

It is a raw binary. I tried nasm -felf boot.asm but it doesn't work. So, I'm thinking I have to get them to link or try to get gcc to compile to raw binary either.

[MichaelPetch]

Are you building a mulitboot executable to be used with GRUB or are you building a bootloader that runs in real mode that loads your kernel? I ask because your linker script outputs binary but the contents suggest you might be doing multiboot. Your line nasm boot.asm would build a BIN file. Maybe you meant nasm -f elf32 boot.asm -o boot.o. If doing multiboot you don't want to output as binary in your linker script either. If we saw your boot.asm we might be able to tell what your end goal is.

[drnono]

I'm not doing anything with multiboot.
I want to have the kernel on second sector of a disk loaded by the bootloader and then jump or pass to running C code.

I've done this before a while ago but I can't remember how exactly. I'm pretty sure I used the sample linker scripts, but it seems to me now that it would be simpler to just have raw C compiled pasted instead.

[MichaelPetch]

The linker script you are using appears to be for multiboot usage. Maybe what you did previous was build your boot.asm into a bootsector (binary file) and built your kernel separately into a separate binary and then placed them into a disk image with something like DD?

[MichaelPetch]

Linking them together isn't straight forward, so not likely what you had done previously. As an example of how you could do it the more complex way, I slapped together an example as a proof of concept. The kernel disk reading routine is a hack for demonstration purposes; it uses the fast method of turning on A20 for demo purposes. It has a boot.asm that enters into protected mode, zeroes the BSS section, then transfers to a kernel entry point called kmain. It should display MDP in the upper left with white on magenta.

You can find the sample code here: http://www.capp-sysware.com/misc/osdev/linkedboot/

Edit: Had some time this morning to update the code a bit. The kernel loading is less hackish.