Class 25: More Stack Frames

Held Friday, November 1, 2002

Summary

Today we continue our investigation of stack frames by considering some particular examples.

Notes

Overview

An Example

• Let us continue our discussion of stack frames with a simple example.
• Everyone's favorite recursive procedure is factorial, so we'll try that first.
• I've written slightly more explicit code than is normal to help you think about what is happening.

  function factorial(n: integer): integer;
  var
    tmp1: integer;
  begin
    if (n = 0) then
      factorial := 1
    else
      begin
        tmp := factorial(n-1);
        factorial := n * tmp;
      end
  end;
  

• What will the stack frame for this procedure look like? Here's one simple possibility (along with an indication as to where we hope to have the frame pointer and stack pointer, assuming stacks grow downward):

  +--------------+  <- FP
  |   old FP     |
  +--------------+
  | return value |
  +--------------+
  | ret. address |
  +--------------+
  |      n       |
  +--------------+  <- SP
  

• We'll assume that the local procedure immediately pushes room for two temporaries, giving us the following stack.

  +--------------+  <- FP
  |   old FP     |
  +--------------+
  | return value |
  +--------------+
  | ret. address |
  +--------------+
  |      n       |
  +--------------+
  |     tmp      |
  +--------------+
  |    extra     |
  +--------------+  <- SP  
  

• After a recursive call finishes, we expect to have

  +--------------+  <- FP
  |   old FP     |
  +--------------+
  | return value |
  +--------------+
  | ret. address |
  +--------------+
  |      n       |
  +--------------+
  |     tmp      |
  +--------------+
  |    extra     |
  +--------------+  <- SP  
  |   our FP     |
  +--------------+
  | rec. result  |
  +--------------+
  

• What does the assembly code for factorial look like? In a generic language with four bytes per word

  BEGIN_FACTORIAL:
  # Do anything appropriate for the beginning of a procedure call
  ...
  # if (n = 0)
  START_FACTORIAL_CODE:
    PUSH 8			  # Space for temps
    COMPARE MEM[FP+12], CONSTANT[0]
    JUMP IF EQUAL BASE_CASE
    JUMP RECURSIVE_CASE
  # factorial := 1
  BASE_CASE:
    STORE INTO MEM[FP+4] FROM CONSTANT[1]
    JUMP END_FACTORIAL
  # tmp := factorial(n-1)
  RECURSIVE_CASE:
    # Compute n-1
    SUBTRACT CONSTANT[1] FROM MEM[FP+12], STORE IN MEM[SP-8]
    # Call factorial
    ...
    # Copy the value back
    STORE INTO MEM[SP-8] FROM MEM[SP+4] 
    # factorial := n * tmp
    MULTIPLY MEM[SP-12] BY MEM[SP-8], STORE IN MEM[FP+4]
    JUMP END_FACTORIAL
  # Clean up
  END_FACTORIAL:
    ...
  

• Okay, what do we have to do at the beginning of a procedure? If a procedure knows how much local memory it needs, the procedure should increment the stack pointer

  # Do anything appropriate for the beginning of a procedure call
    ADD CONSTANT[8] to SP
  

• At the end, the procedure should undo what it just did.

  END_FACTORIAL:
    SUBTRACT CONSTANT[8] from SP
  

• It needs to restore the frame pointer

    SET FP to MEM[FP]
  

• It also needs to return to the place that it started.

    JMP MEM[FP+8]
  

• Now to the hard part. What do we do for a recursive procedure call?
• Before jumping to the code for the procedure, we need to store the parameter.

    STORE INTO MEM[SP+12] FROM MEM[SP-8]
  

• We may update the frame pointer, so we store it in the next frame.

    STORE INTO MEM[SP] FROM FP
  

• We also need to update the frame pointer. (We assume that procedures update their own stack pointers.) Conveniently, the end of one frame (given by the stack pointer) is the same as the beginning of the next frame (given by the frame pointer).

    SET FP to SP
  

• We have to advance the stack pointer beyond the pushed stuff

    ADD CONSTANT[16] to SP
  

• We need to tell the procedure where to return to.

    STORE INTO MEM[SP+8], RETURN_HERE
  

• We need to call the procedure. Generically,

    JUMP BEGIN_FACTORIAL
  RETURN_HERE:
  

• We need to recover from the procedure. That means that we need to update the stack pointer to where it was before.

    SUBTRACT CONSTANT[16] from SP
  

• We'll go through an example (and work out the kinks) in class.