CustomBootloader/boot.s

.code16 # tell the assembler that we're using 16 bit mode
.include "gdt.s"

.section .text

.global init # makes our label "init" available to the outside
.globl mainentry

init: # this is the beginning of our binary later.

  #jmp init # jump to "init"
  #ljmpw $0xFFFF, $0 # jumps to the "reset vector", doing a reboot
  call printhelloworld

  jmp switchto32bit

switchto32bit:
  # Load Global Descriptor Table
  lgdt gdt32ptr

     # Load Control Register CR0
     # Enable Protected Mode (Bit0 in CR0)
     # Store Control Register CR0
     movl %cr0, %eax
     orl $(1<<0), %eax
     movl %eax, %cr0

     # Setup all Segment Registers
     # and reload Code Segment, Instruction Pointer
     movw $0x0010, %ax
     movw %ax, %ds
     movw %ax, %es
     movw %ax, %fs
     movw %ax, %gs
     movw %ax, %ss
     ljmp $0x0008, $JumpToPM

.code32
JumpToPM:
   call mainentry

   ljmpw $0xFFFF, $0 # jumps to the "reset vector", doing a reboot

printhelloworld:
  mov $0x0e, %ah # function mode -> display char

  mov $'S', %al
  int $0x10 # video bios interrupt
  mov $'T', %al
  int $0x10 # video bios interrupt
  mov $'A', %al
  int $0x10 # video bios interrupt
  mov $'R', %al
  int $0x10 # video bios interrupt
  mov $'T', %al
  int $0x10 # video bios interrupt

.fill 510-(.-init), 1, 0 # add zeroes to make it 510 bytes long
# . is current position - start position
.word 0xaa55 # magic bytes that tell BIOS that this is bootable
# x86 is little endian - so bytes are swapped in memory!