The function _DSPvChamada, in this code, is implemented in another file, but the function __Istack hasn't implementation. I don't understand exaclty the target of the funcion __Istack. I hope i have been more clear now.

global _ASMvAjustaStackJmp
global _DSPvChamada;
global __Istack ;

_ASMvAjustaStackJmp:

mov sp,#__Istack ;
mov dptr,#_DSPvChamada;
push dpl ;
push dph ;
reti ;