A damn simple bootloader that prints "Hello, world!".

boot.c

__asm__(
        ".code16gcc\n"
        "jmpl $0, $main\n"
);

static const char * const msg = "Hello, world!";

void char_out(const char ch) {
    __asm__(
            "mov $0x0e, %%ah\n"
            "mov %0, %%al\n"
            "int $0x10\n"
            : : "r"(ch) : "ax"
    );
}

void main() {
    const char *cur = msg;
    for (;;) {
        char ch = *(cur++);
        if (!ch) break;
        char_out(ch);
    }
    
    __asm__(
            "hlt\n"
    );
}

linker.ld

ENTRY(main);
SECTIONS
{
    . = 0x7c00;
    .text : AT(0x7c00)
    {
        *(.text);
    }
    .data :
    {
        *(.data);
        *(.rodata*);
    }
    .magic : AT(0x7dfe)
    {
        SHORT(0xaa55);
    }
}