cpl3->cpl0

HardEnough · Post by **HardEnough** » Sun Oct 23, 2005 12:45 pm

hi guys
sorry for a new thread.
but if i want to context switch from cpl 3 -> cpl 0 i should push ss3 and esp3 , right ?
so for the first time i create a task i need to initialize its kernel stack.
so which one should point to the userstack and changes automatically as the stack grows or shrinks with the task, is it (2) or (3) ??
please can any one make it clear to me ?
thank you

Code: Select all

create_task:
   push   ebp
   mov   ebp, esp
   mov   esp, dword [ebp+08]
  ;---------------------;
  ; push dword ss3   ;      -----------> (1) ?
  ; push dword esp3 ;      -----------> (2) ?
  ;---------------------;
   push   dword   0202h  ; eflags
   push   dword   08h   ; cs
   mov   eax, dword [ebp+12]
   push   dword   eax   ; eip
   push   dword   00h   ; eax
   push   dword   00h   ; ecx
   push   dword   00h   ; edx
   push   dword   00h   ; ebx
   mov   eax, dword [ebp+16]
   push   dword   eax   ; esp  -------------> (3)  ?
   push   dword   00h   ; ebp
   push   dword   00h   ; esi
   push   dword   00h   ; edi
   push   dword   10h   ; gs
   push   dword   10h   ; fs
   push   dword   10h   ; es
   push   dword   10h   ; ds
   push   dword   00h   ; error code
   push   dword   20h   ; interrupt number
   mov   eax, esp
   mov   esp, ebp
   pop   ebp
   ret

Cjmovie · Post by **Cjmovie** » Sun Oct 23, 2005 1:47 pm

Ugh, I've no idea what you're doing.

But I can point out that you (It doesn't seem so, at least) don't have a TSS installed.
When the processor switching from Ring 3 to Ring 0 code, it will load all of the values (kernel stack, etc.) from the first TSS enty.

Hope I was of, at least, some help.

HardEnough · Post by **HardEnough** » Sun Oct 23, 2005 2:07 pm

But I can point out that you (It doesn't seem so, at least) don't have a TSS installed.
When the processor switching from Ring 3 to Ring 0 code, it will load all of the values (kernel stack, etc.) from the first TSS enty.

ofcourse i know that, i'll update the TSS (ss0,esp0) during context switching.
i meant what should point to the real esp of user process which changes automatically as the process do pushes and pops , is it (1) or (2) ?

and the more important issues, when creating a new process which one ( (1) or (2) ) should i point to the end of the memory allocated for the process user stack ?

HardEnough · Post by **HardEnough** » Mon Oct 24, 2005 10:16 am

hi again,

it seems that no one understood me. i will try to make it clearer.
for example if a user process called a syscall by an int instruction, and the parameters where pushed on the process stack rather than the registers, which is the same technique used by *BSD. so which should point to the real user stack of the process esp3 number (2) or esp number (3) ?
another thing what should i initially set the eso3 number (2) & esp number (3) when i create a new process ?

please can any one help me.
thanks

distantvoices · Post by **distantvoices** » Mon Oct 24, 2005 2:37 pm

well, the first part I for one do otherwise: I pass pointers to the system call by putting these values into registers. the systemcall pries the parameters from the saved processor state image in tcb->esp0 then.

second: Hm. You allocate a stack area. Then you pass your ring3 selector into ss3 field and your freshly allocated stack image+sizeof stackimage-4 to the esp3 field. Voila.

third: for ring3->ring0 transitions, you don't need to save off esp3/ss3 (althou in the irq stub one also pushes esp). That's done by int [xx]. YOu need the tss to provide the processor with info about where is the esp0 stack of that task and the according ss0 value.

stay safe.

HardEnough · Post by **HardEnough** » Mon Oct 24, 2005 6:56 pm

hi beyond infinity,

Here is my GDT

Code: Select all

gdt:
   null_desc   dd   0h
            dd      0h
   flat_code    dw       0ffffh
         dw     0h
         db   0h
         db   10011110b ; 9eh
         db   11001111b ; cfh
         db   0h
   flat_data     dw     0ffffh
         dw   0h
         db   0h
         db   10010010b ; 92h
         db   11001111b ; cfh
         db   0h
   user_code  dw      0ffffh
         dw   0h
         db   0h
         db   11111110b
         db   11001111b
         db   0h
   user_data   dw      0ffffh
         dw   0h
         db   0h
         db   11110010b
         db   11001111b
         db   0h
   task_state   dd        0h
         dd   0h

and here is my TSS

Code: Select all

tss:
   back_link   dw   0h
         dw   0h
   esp0      dd   0h
   ss0      dw   20h ;------> constant 
         dw   0h
   esp1      dd   0h
   ss1      dw   0h
         dw   0h
   esp2      dd   0h
   ss2      dw   0h
         dw   0h
   ccr3      dd   0h
   eip      dd   0h
   eflags      dd   0h
   eeax      dd   0h
   eecx      dd   0h
   eedx      dd   0h
   eebx      dd   0h
   eesp      dd   0h
   eebp      dd   0h
   eesi      dd   0h
   eedi      dd   0h
   ees      dw   0h
         dw   0h
   ecs      dw   0h
         dw   0h
   eds      dw   0h
         dw   0h
   efs      dw   0h
         dw   0h
   egs      dw   0h
         dw   0h
   eldt      dw   0h
         dw   0h
         dw   0h
   bitmap      dw   0h

Here is my IRQ0 handler

Code: Select all

irq0:
   pusha
   push   gs
   push   fs
   push   es
   push   ds
   mov   ax, 10h
   mov   gs, ax
   mov   fs, ax
   mov   es, ax
   mov   ds, ax
   push   dword 00h
   push   dword 20h
   push   dword esp
   call   interrupt
   mov   esp, [eax+4]
   mov   [tss+4], dword eax
   mov   al, 20h
   out   20h, al
   add   esp, 08h
   pop   ds
   pop   es
   pop   fs
   pop   gs
   popa
   iret

Code: Select all

create_task:
   push   ebp
   mov   ebp, esp
   mov   esp, dword [ebp+08]
   push   dword   00h   ; ss3  i don't know what to put here as initial value
   push   dword   00h   ; esp3 also here
   push   dword   0202h   ; eflags
   push   dword   18h   ; cs
   mov   eax, dword [ebp+12]
   push   dword   eax   ; eip
   push   dword   00h   ; eax
   push   dword   00h   ; ecx
   push   dword   00h   ; edx
   push   dword   00h   ; ebx
   mov   eax, dword [ebp+16]
   push   dword   eax   ; esp   ;------------> i don't what should is                   put here as an initial value
   push   dword   00h   ; ebp
   push   dword   00h   ; esi
   push   dword   00h   ; edi
   push   dword   20h   ; gs
   push   dword   20h   ; fs
   push   dword   20h   ; es
   push   dword   20h   ; ds
   push   dword   00h   ; error code
   push   dword   20h   ; interrupt number
   mov   eax, esp
   mov   esp, ebp
   pop   ebp
   ret

Here the Code

Code: Select all

void koko(void); /* a test process */
void wawa(void); /* another test process */
int create_task(int, int, int);

int koko_kstack[76];  /* there is no mm yet */
int wawa_kstack[76];
int koko_ustack[90];
int wawa_ustack[90];

#define TOP 0 /* Stack Top */
#define END 1 /* Stack Buttom */

int koko_s[2];
int wawa_s[2];

#define NONE -1
#define KOKO 1
#define WAWA 2

int task;

main()
{
  koko_s[TOP]=(int)&koko_kstack[0];
  wawa_s[TOP]=(int)&wawa_kstack[0];

  koko_s[END]=create_task((int)&koko_kstack[75],(int)&koko, (int)&koko_ustack[89]);
  wawa_s[END]=osdk_create_task((int)&wawa_kstack[75],(int)&wawa, (int)&wawa_ustack[89]);

 task=NONE;
 for(;;);
}

int interrupt(int esp, int num, int err)
{
 if(task!=NONE)
    {  
      if (task==KOKO){
   koko_s[END]=esp;
   task=WAWA;
   return (int)&wawa_s[TOP];
       }

    else{
      wawa_s[END]=esp;
      task=KOKO;
      return (int)&koko_s[TOP];
      }
    }else{
      task=KOKO;
      return koko_s[TOP];
    }
}

void koko()
{

  kputchar('K');
  while(1){
    if(seconds==10){
      kputchar('K');
      while(1){
   if(seconds==11){
     kputchar('W');
     break;
   }
      }
    }
  }
}

void wawa()
{
  kputchar('W');
  while(1){
    if(seconds==12){
      kputchar('W');
      while(1){
   if(seconds=15){
     kputchar('W');
     break;
   }
      }
    }
  }
}

Please can any body figure what i've foolishly done :-p
thank alot

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