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This is stage2, here is tripple fault, but in the same code in bootloader everything goes OK,enters pmode OK.
;---------------------Main Code--------------------;
mov ax,cs ; setup ds segment
mov ds,ax
mov es,ax
mov fs,ax


a20:
;---------- Enable A20 Line ---------
   cli
   xor cx, cx
clear_buf:
   in al, 64h
   test al, 02h
   loopnz clear_buf
   mov al, 0D1h
   out 64h, al

clear_buf2:
   in al, 64h
   test al, 02h
   loopnz clear_buf2
   mov al, 0dfh
   out 60h, al
   mov cx, 14h
wait_kbc:
   out 0edh, ax
   loop wait_kbc
;---------- Load GDT ---------

;---------- Enter Protected mode ---------
   mov eax, cr0
   inc eax
   mov cr0, eax
   lgdt[gdt_ptr]

;---------- Load GDT ---------
   jmp codesel:new_datasel
[bits 32]
new_datasel:
   mov ax,datasel ; setup segments to new selector
   mov ds,ax
   mov es, ax
   mov fs, ax
   mov gs, ax


jmp $
;------------GDT Table---------------;
gdt_ptr:
dw gdt_end - gdt - 1
dd gdt

gdt:

; NULL descriptor
dd 0
dd 0
dd 0
dd 0
codesel equ $-gdt
dw 0FFFFh ; segment limit
dw 0 ; base address
db 0
db 0x9A ; priv level 0, code
db 0x0CF ; granurlarity
db 0 ; base address

datasel equ $-gdt
dw 0FFFFh ; segment limit
dw 0 ; base address
db 0
db 92h ; priv level 0, data
db 0x0CF ; gran
db 0 ; base address
gdt_end:

And here is bootloader code:
[bits 16]
org 0x7c00

;---------------------Main Code--------------------;

mov ax,cs ; setup ds segment
mov ds,ax
mov es,ax
mov fs,ax
mov ss,ax ; align stack also
mov sp,0x9000 ; 512 byte stack
;----------- Prints --------------
mov bl,15
call cls
;---------- Load kernel ----------
read:
xor ax,ax ; Floppy Reset BIOS Function
mov dl,0 ; Select floppy that was booted from
int 0x13
jc read

mov ax,0xffff
mov es,ax ; Segment for data
mov bx,0x10 ; Offset
mov ah,2 ; Function to read disk
mov al,20 ; Total sectors to read
mov ch,0 ; Track 0
mov cl,2 ; Sector 2
mov dh,0 ; Head 0 | Drive is already loaded
int 13h ; Call BIOS read disk function
jc read ; motor error, try again

mov dx,0x3f2 ; stop the motor
mov al,0x0C ; from spinning
out dx,al

;---------- Enabling A20 Line ---------
cli ; no more interupts!
xor cx, cx
clear_buf:

in al, 64h ; get input from keyboard status port
test al, 02h ; test the buffer full flag
loopnz clear_buf ; loop until buffer is empty
mov al, 0D1h ; keyboard: write to output port
out 64h, al ; output command to keyboard

clear_buf2:
in al, 64h ; wait 'till buffer is empty again
test al, 02h
loopnz clear_buf2
mov al, 0dfh ; keyboard: set A20
out 60h, al ; send it to the keyboard controller
mov cx, 14h
wait_kbc: ; this is approx. a 25uS delay to wait

out 0edh, ax ; for the kb controler to execute our
loop wait_kbc ; command.

; a20 line is now on, set up GDT and IDT tables
lgdt[gdt_ptr]

mov eax, cr0 ; switch to pmode by
inc ax ; toggling last bit
mov cr0, eax

jmp codesel:flush

[bits 32]
flush:
mov ax,datasel ; setup segments to new selector
mov ds,ax
mov es,ax
mov ss,ax
mov fs,ax
mov gs,ax

jmp codesel:0x100000

;------------GDT Table---------------;
gdt_ptr:
dw gdt_end - gdt - 1
dd gdt

gdt:

; NULL descriptor
dw 0 ; limit 15:0
dw 0 ; base 15:0
db 0 ; base 23:16
db 0 ; type
db 0 ; limit 19:16, flags
db 0 ; base 31:24

; unused descriptor
dw 0
dw 0
db 0
db 0
db 0
db 0

codesel equ $-gdt
dw 0FFFFh ; segment limit
dw 0 ; base address
db 0
db 9Ah ; priv level 0, code
db 0CFh ; granurlarity
db 0 ; base address

datasel equ $-gdt
dw 0FFFFh ; segment limit
dw 0 ; base address
db 0
db 92h ; priv level 0, data
db 0CFh ; gran
db 0 ; base address

gdt_end:
TIMES 510-($-$$) DB 0

SIGNATURE DW 0xAA55

It is because you have to load gdt prior entering protected mode.

so you have to say in your stage 2 thingy:

lgdt [gdt_ptr] -->before activating PM.
mov eax,cr0
inc eax
mov cr0,eax

jmp pm
pm: ...

but pmode entering is after lgdt... :-\

Thunder wrote: This is stage2, here is tripple fault, but in the same code in bootloader everything goes OK,enters pmode OK.
;---------------------Main Code--------------------;
mov ax,cs ; setup ds segment
mov ds,ax
mov es,ax
mov fs,ax


a20:
;---------- Enable A20 Line ---------
???cli
???xor cx, cx
clear_buf:
???in al, 64h
???test al, 02h
???loopnz clear_buf
???mov al, 0D1h
???out 64h, al

clear_buf2:
???in al, 64h
???test al, 02h
???loopnz clear_buf2
???mov al, 0dfh
???out 60h, al
???mov cx, 14h
wait_kbc:
???out 0edh, ax
???loop wait_kbc
;---------- Load GDT ---------

;---------- Enter Protected mode ---------
???mov eax, cr0
???inc eax
???mov cr0, eax
???lgdt[gdt_ptr] -> this has to be done prior to the previous three commands, else the code craps out. BI

;---------- Load GDT ---------
???jmp codesel:new_datasel
[bits 32]
new_datasel:
???mov ax,datasel ; setup segments to new selector
???mov ds,ax
???mov es, ax
???mov fs, ax
???mov gs, ax


jmp $

Note this is your posted code as well as the question.
In the code I mark the error.