Problem with GDT pointer

[billion_boi]

hey,
Ive managed to compile my bootloader using NASM with no errors , but the code doesnt run beyond the GDTpointer that ive marked below...I can print a string befor the line...but not after...Any ideas as to why?

Its not like ive loaded the segment register that my string function uses with the descriptor offsett yet...

Code: Select all

GDT:

.Gdt_null:

	Dw 0

	Dw 0

	Dw 0

	Dw 0

.Gdt_code:

	Dw 1

	Dw 0

	Db 0

	Db 10011010

	Db 11011111

	Db 0

.Gdt_data:

	Dw 1

	Dw 0

	Db 0

	Db 10010010

	Db 11011111

	Db 0

.Gdt_end:

.Gdt_pointer:

	Dw .Gdt_end - .Gdt_code -1 ;****Error here

	Dd .Gdt_code

[jzgriffin]

You aren't supposed to "run" the GDT--you're supposed to load it with lgdt [gdtpointer]. Perhaps you could post the rest of your code?

[billion_boi]

Code: Select all

[BITS 16]



[ORG 0X7E00] ;









jmp main



main:

	;****segment DS register is already set at 00000 for you by the bootloader



	mov si, BootstrapLoaded ;Load into the index register the offsett of the string

	call PrintMsg

	

	Call GDT
	mov si,GDTLoaded

	Call PrintMsg

	jmp $





;The following GDT notes are written in Notes3 file

GDT:

.Gdt_null:

	Dw 0

	Dw 0

	Dw 0

	Dw 0

.Gdt_code:

	Dw 1

	Dw 0

	Db 0

	Db 10011010

	Db 11011111

	Db 0

.Gdt_data:

	Dw 1

	Dw 0

	Db 0

	Db 10010010

	Db 11011111

	Db 0

.Gdt_end:

.Gdt_pointer:

	Dw .Gdt_end - .Gdt_code -1

	Dd .Gdt_code

.Gdt_load

	CLI

	Lgdt [.Gdt_pointer]

	Ret







PrintMsg:	;The datasegment register for the bootstrap is loaded in the first stage bootsector

	mov ah, 0x0e

	mov bh, 0x00

	mov bl, 0x07

.NextChar:

	mov al, [ds:si] 

	inc si ;

	or al, al  

	jz .Break 

	int 0x10  

	jmp .NextChar   

.Break:

	ret





BootstrapLoaded: db "The bootstrap(extended bootloader) is running", 13, 10, 0

GDTLoaded: db "GDT is loaded",13,10,0

[Combuster]

Code: Select all

call GDT

should be

Code: Select all

call GDT.Gdt_load

for the reason previously explained

[billion_boi]

i see, so i could however enable the pmode bit in the Cr0 register first...and then run and load the GDt right?

[Combuster]

have you tried?

[jzgriffin]

You need to load the GDT prior to enable protected mode. See the Wiki page for Protected mode.

[Combuster]

Actually, you don't need a GDT in order to set the PE bit successfully.

[billion_boi]

Here is my restructured code in which im only loading the GDT not running it.
But i still have the issue that the code doesnt run beyond the pointer.

Code: Select all

[BITS 16]

	

[ORG 0X7E00]




jmp main



main:

	;****segment DS register is already set at 00000 for you by the bootloader



	mov si, BootstrapLoaded

	call PrintMsg



	mov si, MSGPmode

	Call PrintMsg



	Call GDT_load



	jmp $




;The following GDT notes are written in Notes3 file

GDT_run:

.Gdt_null:    ;Offset 0, descriptor code 0

	Dw 0

	Dw 0

	Dw 0

	Dw 0

.Gdt_code: ;Offset 8 bytes down, 0x8h

	Dw 1

	Dw 0

	Db 0

	Db 10011010

	Db 11011111

	Db 0

.Gdt_data: ;Offsett 16 bytes down, 0x10h

	Dw 1

	Dw 0

	Db 0

	Db 10010010

	Db 11011111

	Db 0

.Gdt_end:



GDT_load:



.Gdt_pointer:

	Dw GDT_run.Gdt_end - GDT_run.Gdt_code -1

	Dd GDT_run.Gdt_code
;*******************************Problem here....String doesnt print...*******************

mov si, OK;Load into the index register the offsett of the string

Call PrintMsg


.Gdt_load

	CLI

	Lgdt [.Gdt_pointer]

	Ret









PrintMsg:	

	mov ah, 0x0e

	mov bh, 0x00

	mov bl, 0x07

.NextChar:

	mov al, [ds:si] ;DS (segment selecter not necessar, NASM will assume current data segment)

	inc si 

	or al, al ; if al=0 (the last character=null) then 0 flag is set

	jz .Break 

	int 0x10  

	jmp .NextChar   

.Break:

	ret





BootstrapLoaded: db "The bootstrap(extended bootloader) is running", 13, 10, 0
MSGPmode: db "Loading Protected Mode",13,10,0

OK: db "[OK]",13,10,0

[nekros]

you are "running" the gdt pointer. Call .gdt_load gdt_load doesn't do anything.

[billion_boi]

nekros, Jeremiah, Combuster...I think i figured out what youve been trying to tell me this whole time...So now in what i hope is the last example, im not running the GDT, ive only set up the pointer...But still i cant get the string to print...Any ideas as to why?

Code: Select all

	
Call GDT_run.Gdt_pointer

jmp $

GDT_run:

.Gdt_null:    ;Offset 0, descriptor code 0

	Dw 0

	Dw 0

	Dw 0

	Dw 0

.Gdt_code: ;Offset 8 bytes down, 0x8h

	Dw 1

	Dw 0

	Db 0

	Db 10011010

	Db 11011111

	Db 0

.Gdt_data: ;Offsett 16 bytes down, 0x10h

	Dw 1

	Dw 0

	Db 0

	Db 10010010

	Db 11011111

	Db 0

.Gdt_end:

	ret
.Gdt_pointer:

	Dw GDT_run.Gdt_end - GDT_run.Gdt_code -1

	Dd GDT_run.Gdt_code
	;*******************************Problem here....String doesnt print...*******************

	mov si, OK;Load into the index register the offsett of the string
	Call PrintMsg

	ret

[jzgriffin]

You're still trying to run the GDT. Replace the call with this:

Code: Select all

   lgdt [GDT_run.Gdt_pointer]
   mov ax, 0x10
   mov ds, ax
   mov es, ax
   mov fs, ax
   mov gs, ax
   mov ss, ax
   jmp 0x08:Flush

Flush:
   mov si, OK;Load into the index register the offsett of the string
   Call PrintMsg

[nekros]

Please tell me you understand that :'( If you do not, RTFM. The idea is not to execute the pointer. It is to LOAD the pointer into the gdtr.

[billion_boi]

Since in the code i havent used thr LGDT instruction, or called the function that sets up the descriptors.... rather just set the limit and base adress in the pointer. Why or how is it that im running the GDT?

[jzgriffin]

You're jumping to the address of the GDT pointer, where you've dw/dd'd some bytes. When the processor sees those bytes it tries to execute them as opcodes. You are trying to run data as code.