GNU AS value not in assigned section

Question

I am currently trying to build a two-stage bootloader in GNU assembler. I use the as command to compile the assembly files and the ld command to link them to the final binary. Some information beforehand:

$ as --version
GNU assembler (GNU Binutils for Debian) 2.28 [...]
This assembler was configured for a target of `x86_64-linux-gnu'

And ld:

$ ld --version
GNU ld (GNU Binutils for Debian) 2.28 [...]

The affected assembly file:

.code16

.section .test

label: .short 0x1234

.section .text

    .global _setup32

_setup32:
    cli

    xor %ax, %ax

    # Some more code [...]

And the linker script used to build the binary file: Before anyone asks, the following linker script is the stage-2 part of my bootloader, hence the origin of 0x8000.

OUTPUT_FORMAT(binary)
ENTRY(_setup32)

SECTIONS
{
    . = 0x8000;
    _SETUP_START = .;

    .test ALIGN(0x10) : {
        *(.test*)
    }

    .text ALIGN(0x10) : {
        _TEXT_START = .;
        *(.text*)
        _TEXT_END = .;
    }

    .rodata ALIGN(0x10) : {
        _RODATA_START = .;
        *(.rodata*)
        _RODATA_END = .;
    }

    .data ALIGN(0x10) : {
        _DATA_START = .;
        *(.data*)
        _DATA_END = .;
    }

    .bss ALIGN(0x10) : {
        _BSS_START = .;
        *(.bss*)
        _BSS_END = .;
    }

    /DISCARD/ : {
        *(.comment*)
    }

    _SETUP_END = .;
}

However, after the final setup.bin file has been built, the value 0x1234 is not located at the beginning of the hexdump. In fact, it is not even included in the final built. The bootloader is working absolutely fine, I'm just unable to assign variables or in general, bytes to individual sections.

I am using the following, very basic rules in my Makefile:

$(SETUPBIN): $(SETUP).o
    $(LD) -T setup.ld $(SETUP).o -o $@

%.o: %.asm
    $(AS) $< -o $@

Furthermore: I know about GNU AS directives .text and .data, but they only store the instructions and bytes in fixed locations of the binary: Everything in the .text directive is located at the beginning of the binary whereas everything in .data is found at the end of the binary.

How do I put everything in a custom section?

x86 is little-endian. You did check for the byte sequence `0x34 0x12`, right? I'm not sure if you need to do anything special beyond that linker script to make a custom section appear in the output binary. (e.g. for ELF output you need an `"@progbits"` attribute to get a section placed in a segment that will be mapped, but in a flat binary all the sections except .bss are just in the binary.) — Peter Cordes, Jan 04 '19 at 22:06
@PeterCordes correct. But said value is not found anywhere in the flat binary. I too wondered if its just for flat binaries that the output is somewhat cropped or optimized away, but I haven't found anything about that either. — CRoemheld, Jan 04 '19 at 22:26
You need to use `.section .test, "a"` to mark it allocatable. — Jester, Jan 04 '19 at 22:47

GNU AS value not in assigned section

0 Answers0