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        ORG     1600h

        USE16

        cli                             ; disable the interrupts, just in
                                        ; case they are not disabled yet

        lgdt    [cs:GDTR]               ; load GDT register

        mov     eax,cr0                 ; switch to protected mode
        or      al,1
        mov     cr0,eax

        jmp     CODE_SELECTOR:pm_start


NULL_SELECTOR = 0
DATA_SELECTOR = 1 shl 3                 ; flat data selector (ring 0)
CODE_SELECTOR = 2 shl 3                 ; 32-bit code selector (ring 0)
LONG_SELECTOR = 3 shl 3                 ; 64-bit code selector (ring 0)

GDTR:                                   ; Global Descriptors Table Register
  dw 4*8-1                              ; limit of GDT (size minus one)
  dq GDT                                ; linear address of GDT

GDT rw 4                                ; null desciptor
    dw 0FFFFh,0,9200h,08Fh              ; flat data desciptor
    dw 0FFFFh,0,9A00h,0CFh              ; 32-bit code desciptor
    dw 0FFFFh,0,9A00h,0AFh              ; 64-bit code desciptor

        USE32

pm_start:

        mov     eax,DATA_SELECTOR       ; load 4 GB data descriptor
        mov     ds,ax                   ; to all data segment registers
        mov     es,ax
        mov     fs,ax
        mov     gs,ax
        mov     ss,ax

        mov     eax,cr4
        or      eax,1 shl 5
        mov     cr4,eax                 ; enable physical-address extensions

        mov     edi,70000h
        mov     ecx,4000h shr 2
        xor     eax,eax
        rep     stosd                   ; clear the page tables

        mov     dword [70000h],71000h + 111b ; first PDP table
        mov     dword [71000h],72000h + 111b ; first page directory
        mov     dword [72000h],73000h + 111b ; first page table

        mov     edi,73000h              ; address of first page table
        mov     eax,0 + 111b
        mov     ecx,256                 ; number of pages to map (1 MB)
  make_page_entries:
        stosd
        add     edi,4
        add     eax,1000h
        loop    make_page_entries

        mov     eax,70000h
        mov     cr3,eax                 ; load page-map level-4 base

        mov     ecx,0C0000080h          ; EFER MSR
        rdmsr
        or      eax,1 shl 8             ; enable long mode
        wrmsr

        mov     eax,cr0
        or      eax,1 shl 31
        mov     cr0,eax                 ; enable paging

        jmp     LONG_SELECTOR:long_start

        USE64

long_start:

        mov     rax,'L O N G '
        mov     [0B8000h],rax

        jmp     long_start