CS270 Colorado State University

====================
Run-time Stack
====================
Ch. 14 and 16

Issues discovered
    - If each function only has one place to store registers, then it is not possible to use that function recursively.
    
    - There are not enough registers to always use registers to pass all of the parameters.


Solution
    Use run-time stack to implement function calls.



Each function instance will have an activation record on the stack

    values for formal parameters
    
    LC3: location for return value
        
        
    stored register values

    locations/slots for local variables


stack grows to lower memory
    stack pointer
    
    
    
    frame pointer
    

    

-------------------
Terminology

function prototypes
    
    
        
    
    declaration versus definition
        
    

formal parameters and arguments
    
        
     
    
     

-----------------------------------
Goal in implementing function calls

caller-independence
    


calling convention
    
    
    
    
    implementation requirements for function calls
        1) pass parameters and return values
        2) call and return


----------------------        
LC3 calling convention
    
    
        
    General
        - stack pointer (R6) always points at top location in stack
            
            
    Caller
        before call
            - push arguments onto stack right-to-left
            
                
        after call 
            - pop return value off the stack
                
                
            - pop arguments off the stack
        
        
    Callee
        - save and restore R7 and R5
            
        - place return value at top of stack on top of arguments
            
        - do NOT modify R4
            
        - keep R6 pointing at top of stack


Full details of callee, 
one possible implementation of the above calling convention

    beginning of function (prologue)
        - allocate memory for return value
            
                    
        - save and restore R7 and R5, push R7 and R5 onto stack
            
            
            
        - set the frame pointer so that it will point at first local
            
        
        - allocate space for locals
            
            
            
    end of function (epilogue)
        - write return value in return value slot
            
            
        - pop local values off the stack
            
            
        - restore frame pointer (R5) by popping it off stack
            
             

-------
Example             

    int main() {
        
        int y = foo(1,2,3);
        
        int z = foo(4,5,6);

        printf( "%d\n", baz() );
    }

    int foo(int p1, int p2, int p3) {
        bar(p1);
        bar(p2);
        bar(p3);
        
        return 77;
    }
    
    void bar(int x) {
        printf("%d\n", x + baz() );
    }
    
    int baz() {
        return 7;
    }


Invocation graph
    
    
    
    
    
    






The run-time stack for the call path: main, foo(1,2,3), bar(p1), baz()
    


















    
    
    

-------------------
Register management
    R0-R3 -- 4 temporary registers, no convention specified
        

    R4 -- points to first address in global data section
    R5 -- frame pointer, callee-saved
    R6 -- stack pointer, should always point at top entry in stack
    R7 -- return address, callee-saved


dynamic link
    
    
    
-----------------------------
Implementing the stack in LC3
        
    
    
    














------------------------
mstrout@cs.colostate.edu, 11/2/08