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

Well, the code seems to be setting the Stack Pointer (SP) using __Istack, then pushing _DSPvChamada onto the stack.
Then it does a RETI.

Is this some sort of nasty multitasking?

The name of the label is suspicious: "_ASMvAjustaStackJmp" - sounds like Adjust-a-Stack Jump; ie, mess with the Stack, then jump off into oblivion...

Looks like a case of, "If you need to ask the question, you shouldn't be messing with this!"

Where did you get this code?
Can't you contact the author/supplier?