Separate kernel stack using GDT

alix · **Joined:** Tue Nov 13, 2012 3:56 pm **Posts:** 12

Hello,

I'm playing a bit with OS programming since some time and currently i'm experimenting with the stack segments. I would like to have the kernel stack starts at physical location 0x200000 (2MB) and grows down up to 0x100000 (1MB). The kernel is loaded at physical address 0x200000 and grows up, no paging yet.

For this i setup the following gdt entries.

Code:

; GDT system code descriptor
gdt_cs:
   dw 0FFFFh      ; maximum limit 0FFFFFh (1 meg or 4 gig)
   dw 0                 ; base address; gets patched above
   db 0
   db 9Ah         ; present,ring 0,code,non-conforming,readable
   db 0CFh         ; page-granular (4 gig limit), 32-bit
   db 0

; GDT system data descriptor
gdt_ds:
   dw 0FFFFh      ; maximum limit 0FFFFFh (1 meg or 4 gig)
   dw 0                 ; base address; gets patched above
   db 0
   db 92h         ; ring 0, data, expand-up, writable
   db 0CFh         ; page-granular (4 gig limit), 32-bit
   db 0
; GDT system stack descriptor
gdt_ss:
   dw 0FFFFh               ; maximum limit 0FFFFFh (1 meg or 4 gig)
   dw 0
   db 0
   db 96h ;                   ; ring 0, data, expand-down, writable
   db 04Fh                   ; page-granular (1 meg limit), 32-bit
   db 0

gdt_end:

gdt:
    dw gdt_end - gdt_begin - 1
    dd gdt_begin

And i'm loading these entries the following way:

Code:

    ....
    ....
    jmp      08h:pmode
[BITS 32]
pmode:
    mov      ax, 0x10      ; set data segments to data selector (0x10)
    mov      ds, ax
    mov      es, ax
    mov      gs, ax
    mov      fs, ax
    mov            ax, 0x18
    mov      ss, ax

Everything is perfectly working with Qemu. I can trace some C function calls and see how my stack is growing down from the 2MB limit. But bochs is sending me isr 13 (general protection fault) as soon as i execute any code after loading the stack segment select with mov ss, ax
With the same setup if i load the stack descriptor with the same values as for the code (separated as above) everything works in bochs but except that my stack is somewhere else not as i want below 2MB.

Any idea? It is very important for me to understand if i'm doing something wrong.

gerryg400 · **Posted:** Tue Nov 13, 2012 5:00 pm

Expand down segments don't work like that. Actually they aren't very useful unless the stack is at the top of the segment and you intend to modify the size of the stack segment at runtime.

Use an expand-up segment.

If you want your stack to be between phys 0x100000 and 0x200000 and to use segment protection then set the base to 0x100000 and the limit to 0x100000 (actually 0xfffff).

When you initialise the ESP reg set it to 0x100000 also.

alix · **Joined:** Tue Nov 13, 2012 3:56 pm **Posts:** 12

Thanks for your reply. It should work as you said and Qemu and Bochs.

Not sure why Qemu behaves the setup described above. But anyway i'll not spend more time on this as there is much much more to learn/do

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Separate kernel stack using GDT

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