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#include <stdint.h>
 
void kmain(void)
{
   extern uint32_t magic;
 
   if ( magic != 0x2BADB002 )
   {
      /* Something went not according to specs. Print an error */
      /* message and halt, but do *not* rely on the multiboot */
      /* data structure. */
   }

   volatile unsigned char *videoram = (unsigned char *)0xB8000;
   videoram[0] = 65; /* character 'A' */
   videoram[1] = 0x07; /* light grey (7) on black (0). */

   while (1);
}

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.global loader                          # making entry point visible to linker

# setting up the Multiboot header - see GRUB docs for details
.set ALIGN,    1<<0                     # align loaded modules on page boundaries
.set MEMINFO,  1<<1                     # provide memory map
.set FLAGS,    ALIGN | MEMINFO          # this is the Multiboot 'flag' field
.set MAGIC,    0x1BADB002               # 'magic number' lets bootloader find the header
.set CHECKSUM, -(MAGIC + FLAGS)         # checksum required

.align 4
.long MAGIC
.long FLAGS
.long CHECKSUM

# reserve initial kernel stack space
stack_bottom:
.skip 16384                             # reserve 16 KiB stack
stack_top:
.comm  mbd, 4                           # we will use this in kmain
.comm  magic, 4                         # we will use this in kmain

loader:
    movl  $stack_top, %esp               # set up the stack, stacks grow downwards
    movl  %eax, magic                   # Multiboot magic number
    movl  %ebx, mbd                     # Multiboot data structure

    call  kmain                         # call kernel proper

    cli
hang:
    hlt                                 # halt machine should kernel return
    jmp   hang

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OUTPUT_FORMAT(elf64-x86-64)
ENTRY(kmain)
SECTIONS
{
    . = 10000;

    .text : AT(ADDR(.text) - 10000)
    {
        _code = .;
        *(.text)
        *(.rodata*)
        . = ALIGN(4096);
    }

   .data : AT(ADDR(.data) - 10000)
   {
        _data = .;
        *(.data)
        . = ALIGN(4096);
   }

   .eh_frame : AT(ADDR(.eh_frame) - 10000)
   {
       _ehframe = .;
       *(.eh_frame)
        . = ALIGN(4096);
   }

   .bss : AT(ADDR(.bss) - 10000)
   {
       _bss = .;
       *(.bss)

       /*
        * You usually need to include generated COMMON symbols
        * under kernel BSS section or use gcc's -fno-common
        */

        *(COMMON)
       . = ALIGN(4096);
   }

   _end = .;

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

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$ as -o loader.o loader.s
$ gcc  -m64  -ffreestanding  -nostdlib  -mcmodel=large  -mno-red-zone  -mno-mmx  -mno-sse  -mno-sse2  -mno-sse3  -mno-3dnow -nostartfiles -nodefaultlibs -c kernel.c
$ ld -nostdlib -nodefaultlibs -T link.ld -o kernel kernel.o loader.o

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$ file kernel
kernel: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), statically linked, not stripped
$ qemu-system-x86_64 -kernel kernel
Cannot load x86-64 image, give a 32bit one.

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$ file vmlinuz
vmlinuz: Linux kernel x86 boot executable bzImage, version 3.5.0-23-generic (buildd@komainu) #35-Ubuntu SMP Thu Jan 24 13:, RO-rootFS, swap_dev 0x4, Normal VGA