about task stack initialize ?

hi soin :

I have read the function OSTaskStkInit in os_cpu_c.c, but i can't unstand some

code

1: stk = frame_pointer - 13;

2: stk[12] = (INT32U)task; /* task address (ra) */

3: stk[11] = (INT32U) pdata; /* first register argument (r4) */

4:#if OS_THREAD_SAFE_NEWLIB

stk[10] = (INT32U) local_impure_ptr; /* value of _impure_ptr for this thread */# endif /* OS_THREAD_SAFE_NEWLIB */

5: stk[0] = ((INT32U)&OSStartTsk) + 4;/* exception return address (ea) */

i undstand these codes simulate the complier's action.

in line 1: why 13 , only initialize 4 of 13 at most, stk[9]...stk[1] is for what ?

in line 5:why need initialize ea