How to generate plain binaries like nasm -f bin with the GNU GAS assembler?

Question

I have some NASM files that generally have the structure:

        [BITS 64]
        [ORG 0x0000000000200000]

start:
        ...

        ret

Answer 1

objcopy -O binary

A good option is:

as -o test.o test.S
ld -Ttext 0x7C00 -o test.elf test.o
objcopy -O binary kernel.elf kernel.bin

The advantage over ld --oformat binary is that it is easier to use the symbols to debug via:

qemu-system-i386 -hda main.img -S -s &
gdb main.elf -ex 'target remote localhost:1234'

See also: https://stackoverflow.com/a/32960272/895245

Linker script

-Ttext is fine for quick and dirty testing, but for serious work you should use a script instead to increase robustness.

Otherwise, ld will use a default script (ld --verbose) intended for userland application, which does not look like your application.

Without further information, the minimal script I can give is:

SECTIONS
{
    . = 2M;
    .text :
    {
        *(.*)
    }
}

And then use it with -T:

as --64 -o test.o test.S
ld -T linker.ld --oformat binary -o test.bin test.o

But you will likely want to modify that script based on your exact application.

See also: Is there a way to get gcc to output raw binary?

I have a repository with working examples for some common use cases:

• boot sectors
• multiboot interfacing with C