Boot loade

[Kurt Wagner]

HI, I'm trying to make a boot loader in assembly NASM for a little OS following the bootloader of linux 0.1 by linus Torwalds.
I have a problem with LGDT instruction in PROTECTED MODE, because if i place the GDT structure or the GDT descriptor after the page directory and the page tables ( as Linus Torwalds do) at ORG 0x7000 (in poor words), the value loaded in the GDT register is not correct and i get an exception at run time trying to load DS. The code of the boot loader is loaded at 0x00000. But if i place GDT and GDT descriptor before Page directory (near 0x00000) it works well.
But in this way the paging setup will overwirte this low area because the page directory starts at 0x000000 and its entry will overwrite.

[Keith Kanios]

Check for stack corruption and if you are correctly loading the GDTR. Check out the example below.

;...

ldgt[GDTR]

;...

GDT:

;Null Descriptor
   DW 0x00
   DW 0x00
   DB 0x00
   DB 0x00
   DB 0x00
   DB 0x00

;System (Ring 0) Code Descriptor
   DW 0xFFFF   ;Limit (Lower 16 bits)
   DW 0x0000   ;Base (Lower 16 bits)
   DB 0x00      ;Base (Middle 8 bits)
   DB 0x9A      ;Present, Ring 0, Code, Non-Conforming, Readable
   DB 0xCF      ;Granularity, Big, 0, AVL, Limit (Upper 4 bits)
   DB 0x00      ;Base (Upper 8 bits)

;System (Ring 0) Data Descriptor
   DW 0xFFFF   ;Limit (Lower 16 bits)
   DW 0x0000   ;Base (Lower 16 bits)
   DB 0x00      ;Base (Middle 8 bits)
   DB 0x92      ;Present, Ring 0, Data, Expand-Up, Writable
   DB 0xCF      ;Granularity, Big, 0, AVL, Limit (Upper 4 bits)
   DB 0x00      ;Base (Upper 8 bits)
.end:

GDTR: DW GDT.end - GDT - 1  ;Size of GDT (minus 1 byte)
      DD GDT                ;Location of GDT

[Keith Kanios]

There always has to be one little typo...

ldgt[GDTR]

... SHOULD BE ...

lgdt[GDTR]

HtH.

[Kurt Wagner]

I've found a problem in my code, in PROTECTED MODE, with a GDT already correctly set:

[BITS 32]         ; Tell NASM to assemble 32-bit code
startup_32:

mov eax,0x10      ; Load eax with 0x10(16=2*8) pointer in the GDT
   mov es,ax
   mov ds,ax
   mov fs,ax
   mov gs,ax
   mov ss,ax

mov esp,pg_dir-4      ; Set the stack pointer
   call setup_gdt
         .....
         .....

times 0x7000-($-$$) db 0           ; AFTER PAGE DIRECTORY AND PAGE TABLES

align 16
_gdt:
    dd  0x00000000
    dd  0x00000000   ; NULL descriptor

dw   0x0FFF      ; second descriptor for kernel code segment 0xFFF (0x1000 - 1)  16MB
   dw   0x0000      ; base address=0
   dw   0x9A00      ; code, read/exec
   dw   0x00C0      ; granularity=4096, 386

dw   0x0FFF      ; third descriptor for kernel data segment 0xFFF (0x1000 - 1)  16MB
   dw   0x0000      ; base address=0
   dw   0x9200      ; data read/write
   dw   0x00C0      ; granularity=4096, 386

dd  0x00000000
    dd  0x00000000   ; TEMPORARY

times  506     dd     0   

gdt_descr:
   dw    0x7FF             ; 256 entries
   dd   _gdt             ; GDT address

setup_gdt:
; Procedure setup_gdt()
 ; Descritpion: Load the GDT descriptor.
   lgdt [gdt_descr]
   ret


The call to this procedure is not correct, because it jumps to the location 0x90012 and then i get an exception.
What's the problem?