boot.asm

ORG 0x7c00
BITS 16

CODE_SEG  equ gdt_code - gdt_start
DATA_SEG  equ gdt_data - gdt_start

_start:
     jmp short start
     nop

 times 33 db 0 

start:
    jmp 0:step2

step2:
  cli ; clear interrupts
  mov ax, 0x00
  mov ds, ax
  mov es, ax
  mov ss, ax 
  mov sp, 0x7c00
  sti ; enables interrupts


.load_protected:
    cli
    lgdt[gdt_descriptor]
    mov eax, cr0
    or eax, 0x1
    mov cr0, eax
    jmp CODE_SEG:load32

;GDT
gdt_start:
gdt_start:
    dd 0x0
    dd 0x0

; offset 0x0
gdt_code:     ; cs should point to this
    dw 0xffff ; segment limit first 0-15 bits
    dw 0      ; base 0 to 15 bits
    db 0      ; base 16 to 23 bits
    db 0x9a   ; access byte
    db 11001111b ; high 4 bit flags and low 4 bit flags
    db 0       ; base 24 to 31 bits  
    
  ; offset 0x10
  gdt_data:   ; DS, SS, ES, FS, GS
    dw 0xffff ; segment limit first 0-15 bits
    dw 0      ; base 0 to 15 bits
    db 0      ; base 16 to 23 bits
    db 0x92   ; access byte
    db 11001111b ; high 4 bit flags and low 4 bit flags
    db 0       ; base 24 to 31 bits
  gdt_end:

  gdt_descriptor:
      dw gdt_end -  gdt_start-1  
      dd gdt_start

 [BITS 32]
 load32:
     mov ax, DATA_SEG
     mov ds, ax
     mov es, ax 
     mov fs, ax
     mov gs, ax
     mov ss, ax
     mov ebp, 0x00200000
     mov esp, ebp 
     ; Enable a20 line
     in al, 0x92
     or al, 2
     out 0x92, al
     jmp $


times 510-($ - $$) db 0
dw 0xAA55