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

Answer 2

What directives should I use in GAS? I've found the '.org' directive but GAS doesn't seem to have a '.bits' directive.

The assembler defaults to 64--bit for me, you can use --32 or --64 to chose on the command line. Have a look at the manual for as to see how you can change the architecture inside the code if needed (e.g. .code16 can be used to generate real mode code for a boot loader).

You most likely don't want to use the .org directive to specify where the code is located, but will probably want to use a link script or specify where the text and data segments are loaded on the command line. (org 0x0000000000200000 results in a 2+ MB binary file).

What should I pass to gcc or as to generate a plain binary file? I.e. what the -f bin option does with NASM.

$ cat test.S
.section .text
.globl _start
_start:
        xor %rax, %rax
        mov test, %rax
        ret

test: .quad 0x1234567812345678


$ as --64 -o test.o test.S
$ ld -Ttext 200000 --oformat binary -o test.bin test.o

$ objdump -D -b binary -m i386:x86-64 test.bin
test.bin:     file format binary
Disassembly of section .data:


0000000000000000 <.data>:
   0:   48 31 c0                xor    %rax,%rax
   3:   48 8b 04 25 0c 00 20    mov    0x20000c,%rax
   a:   00 
   b:   c3                      retq

   c:   78 56                   js     0x64
   e:   34 12                   xor    $0x12,%al
  10:   78 56                   js     0x68
  12:   34 12                   xor    $0x12,%al