OSDev.org

Due to recent developments, I need to port my os from real mode to unreal (flat-real) mode. I found this code to switch to unreal mode (too lazy to code myself )

   cli                ;Don't allow interrupts during this
   push ds            ;Save real mode selectors
   push es
   xor  eax, eax      ;Patch the GDT psuedo-descriptor
   mov  ax, ds        ;  (assuming ds points the segment containing the GDT)
   shl  eax, 4
   add  [GDT+2], eax
   lgdt [GDT]         ;Load the GDT
   mov  eax, cr0      ;Switch to pmode
   inc  ax
   mov  cr0, eax
   mov  bx, flat_data ;Our only pmode selector
   mov  ds, bx        ;Install 4Gb limits (warning)
   mov  es, bx
   dec  ax            ;Switch back to real mode
   mov  cr0, eax
   pop  es            ;Restore real mode selectors
   pop  ds
   sti                ;Notice, I never needed to change the CS while in pmode.

Notice how you need the GDT functions for switching to unreal mode? Well I've tried defining them like this:

           %include "gdtn.inc"
GDT        start_gdt
flat_data  desc 0, 0xFFFFF, D_DATA+D_WRITE+D_BIG_LIM
           end_gdt

AND like this:

GDT        dw  0xF, GDT, 0, 0, 0xFFFF, 0, 0x9200, 0x8F
flat_data  equ 8

But neither of them work. My OS just hangs when it loads the kernel. It should display "Kernel Loaded..." before it tries to enter unreal mode, but it crashes right when the kernel loads. I'm not sure if this is a glitch with the unreal mode routine or the GDT, but I would really like it if someone could tell me if they notice something wrong.

Thanks!