boot/x86/old/protected.s

   1 ; Everything between real mode and the C kernel
   2
   3         bits 16
   4
   5 switch_to_pm
   6         mov bx, 0B800h
   7         mov es, bx ; set extra segment to starting address of video RAM
   8         mov byte [es:0], 'L' ; print an L to screen, to let us know that we actually got here
   9
  10         cli ; disable interrupts
  11         xor ax, ax ; clear accumulator
  12         mov ds, ax ; clear data segment, this makes sure the next instruction reads from the right place
  13         lgdt [gdt_desc] ; load the Global Descriptor Table
  14
  15         mov eax, cr0 ; move Control Register 0 to accumulator
  16         or eax, 1 ; flip the bit which controls memory protection
  17         mov cr0, eax ; move the accumulator back to CR0
  18         jmp CODE_SEG:protected ; not quite there yet, need to do a far jump to clean the pipeline from any 16-bit instructions
  19                                                    ; note that the jump does not have to be physically far away, it just needs to use a segmented address
  20
  21         bits 32 ; we are finally in 32-bit mode
  22 protected
  23         mov ax, DATA_SEG ; put the selector for the data segment in the accumulator
  24         ; in real mode, segmentation works by taking one of these segment registers, shifting it right by 4 bits or 1 hex digit
  25         ; and then adding a 16-bit offset to form a 20-bit address
  26         ; example: 1234h:5678h
  27         ; 1234h is shifted to 12340h, 12340h + 5678h is 179B8h
  28         ; in 32-bit protected mode, these segment registers do not hold the segment address itself, but a selector in the GDT
  29         ; so we have to update the segment registers accordingly
  30         mov ds, ax
  31         mov ss, ax
  32         mov es, ax
  33         mov fs, ax
  34         mov gs, ax
  35
  36         ; reinitialize the stack at a safe location
  37         mov ebp, 090000h
  38         mov esp, ebp
  39
  40         ; transfer control to the kernel
  41         call KERNEL_OFFSET
  42
  43         ; above call should not return in normal circumstances, but if it does hang forever
  44         jmp $
  45
  46 video_memory equ 0B8000h
  47
  48 ; the actual Global Descriptor Table
  49 ; refer to Volume 3, Chapter 2, 2.1.1 of Intel's 64 and IA-32 Software Developer's Manual for more info
  50 gdt_start
  51 null_seg
  52         dq 0
  53 code_seg
  54         dw 0FFFFh
  55         dw 0
  56         db 0
  57         db 10011010b
  58         db 11001111b
  59         db 0
  60 data_seg
  61         dw 0FFFFh
  62         dw 0
  63         db 0
  64         db 10010010b
  65         db 11001111b
  66         db 0
  67 gdt_end
  68 gdt_desc
  69         dw gdt_end - gdt_start - 1
  70         dd gdt_start
  71 CODE_SEG equ code_seg - gdt_start
  72 DATA_SEG equ data_seg - gdt_start
  73