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

I'm assembling them like so:

nasm -f bin abc.asm

I'd like to write some of these using GAS instead. Two questions:

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

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

Answer 1

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