Help! Problem with CPL downgrate in..

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narada

Joined: 15 Feb 2008
Posts: 77
Location: Ukraine, Dnepropetrovsk

narada 23 May 2010, 19:28

Very siiimple english Embarassed

1) Needed info^

Code:

os_code_l db 0xFF,0xFF,0x00,0x00,0x00,10011000b,01001111b,0x00
os_ss_l db 0x80, 0x00, 0x00, 0x00, 0x00, 10010010b, 01001111b, 0x00
os_data_l db 0xFF, 0xFF, 0x00, 0x00, 0x00, 10010010b, 01001111b, 0x00
app_code_l db 0xFF, 0xFF, 0x00, 0x00, 0x00, 11111000b, 01001111b, 0x00 
app_ss_l db 0x80, 0x00, 0x00, 0x00, 0x00, 11110010b, 01001111b, 0x00
app_data_l db 0xFF, 0xFF, 0x00, 0x00, 0x00, 11110010b, 01001111b, 0x00

I need to start CPL3-code (app_code:u_mode) from CPL0 (this code running in CPL0 of course) with IRETD:

Code:

      mov      dx, os_ss
  mov      ss, dx
     mov     esp, os_stack

   mov      ax, app_data
       mov      ds, ax

 mov     eax, app_ss ; SS
    push    eax

     mov     eax, app_stack ; STACK
      push    eax

     push    dword 0x13202 ; EFLAGS

  mov     eax, app_code ; CS
  push    eax

     mov     eax, u_mode ; EIP
   push    eax

     IRETD

In this case I got GPL. But, if I am using os_code (DPL0) instead of app_code (DPL3) all gone right ok. So why Ring0 -> Ring0 = Ok, but Ring0 -> Ring3 != Ok??? Question

[/code]

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23 May 2010, 19:28

baldr

Joined: 19 Mar 2008
Posts: 1651

baldr 24 May 2010, 11:44

narada,

Make sure that priority levels meet the requirements (app_code and app_ss selectors should have RPL==3, i.e. app_code = app_code_l+3 or something like that).

24 May 2010, 11:44

narada

Joined: 15 Feb 2008
Posts: 77
Location: Ukraine, Dnepropetrovsk

narada 24 May 2010, 12:55

No... with RPL all fine...
May be this is not an answer, but when I initialized TR register (added tss_selector and LTR ... ... ) all start working propertly...

it's very interesting... as I understood, IRETD instruction or another (instead of Jxx) needed a TSS...

(Times ago all worked fine, but I decided to do scheduler withous TSS, as software task swith... and now I am using sowtware t.sw., but TSS structure just must be in system... as think...)

24 May 2010, 12:55

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