The Activation Stack

1
The Activation Stack
2
Outline

• Prerequisites
• Basic operation of functions
• Physical and logical components of a computer
  system
• Objectives
• Stack as an Abstract Data Type
• Nature of the Activation Stack
• Program execution using the Activation Stack

3
Stacks

• Is a pile or collection or set of items.
• Items can only be added to the top
• Items can only be taken off the top
• Last-in-first-out (LIFO)

Push
Pop
Thing 3
Thing 2
Thing 1
4
What is a Stack?

• A data structure for storing items which are
  to be accessed in last-in first-out order (LIFO).

5
What is a Stack?

• A data structure for storing items which are
  to be accessed in last-in first-out order (LIFO).

6
What is a Stack?

• A data structure for storing items which are
  to be accessed in last-in first-out order (LIFO).

7
What is a Stack?

• A data structure for storing items which are
  to be accessed in last-in first-out order (LIFO).

8
What is a Stack?

• A data structure for storing items which are
  to be accessed in last-in first-out order (LIFO).

9
What is a Stack?

• A data structure for storing items which are
  to be accessed in last-in first-out order (LIFO).

10
What is a Stack?

• A data structure for storing items which are
  to be accessed in last-in first-out order (LIFO).

11
What is a Stack?

• A data structure for storing items which are
  to be accessed in last-in first-out order (LIFO).

12
What is a Stack?

• A data structure for storing items which are
  to be accessed in last-in first-out order (LIFO).

13
Once upon a time

• Many years ago programmers realized that
  programming a complex task would be simpler if
  they could break the big job down into pieces

14
Once upon a time

• This was the birth of modular programming where
  big programs get broken down into smaller pieces
  called modules, methods, subroutines, procedures
  or in the case of Scheme functions.

15
Once upon a time

• Each of these functions has some executable code
  and usually some local data like the parameters
  which are passed in.

16
Once upon a time

• Initially each function had its own little area
  for data called its activation record. Thus that
  was where the data was stored when the function
  was activated or running.

Activation Record for f x
f(x)
Activation Record for g y
g(y)
Activation Record for h z
h(z)
17
Can you see a problem with this approach?
18
Solution

• Instead of giving each function its own unique
  activation record
• Every time a function is called it gets a fresh
  activation record on a stack called the
  activation stack.
• Now we call each of these records on the stack a
  stack frame

19
The Activation Stack

• The white box represents this programs share of
  computer memory.
• The function being executed gets the first frame
  at the bottom of the stack.
• Variables which are used within a function reside
  in that functions stack frame.

foo
20
Adding Modules to the Stack

• When that function calls another function, the
  new function gets its own frame pushed on the
  stack.
• The new functions variables live in that new
  frame.
• ONLY the top frame is active. All frames
  underneath it are stopped.

foo
var1
var2
var3
21
Removing modules from the stack

• When the active function completes its
  instructions, its frame is popped off the stack
  and all of its data dies.
• The only thing that survives is the return value
  result which is passed back to the calling
  function underneath.

foo
var1
var2
var3
22
How Parameters Work

• Formal parameters get a copy of the matching
  actual parameter.
• In the calling function
• (bar this_var)
• In the called function
• (define (bar this_one )
• )

23
How Parameters Work

• Formal parameters receive a copy of the matching
  actual parameter.

4
4
7
9
this_var
that_var
other_var
24
Stack Trace Example
(define (quadratic a b c)(list (pos-root a b
c)   (neg-root a b c))) (define (pos-root a b
c)(/ (pos-numerator a b c)   (denominator
a))) (define (pos-numerator a b c)( (-
b)   (sqrt-of-discriminant a b c))) (define
(denominator a)( a 2.0))
(define (neg-root a b c)(/ (neg-numerator a b
c)    (denominator a)) (define (neg-numerator a
b c)(- (- b)   (sqrt-of-discriminant a b
c))) (define (sqrt-of-discriminant a b c)(sqrt
(- ( b b)   ( 4.0 a c))))
25
Abstraction Example
quadratic
(root1, root2)
26
Further Abstraction
num
pos-numerator
27
Stack Trace Example
(define (quadratic a b c)(list (pos-root a b
c)   (neg-root a b c))) (define (pos-root a b
c)(/ (pos-numerator a b c)   (denominator
a))) (define (pos-numerator a b c)( (-
b)   (sqrt-of-discriminant a b c))) (define
(denominator a)( a 2.0))
(define (neg-root a b c)(/ (neg-numerator a b
c)    (denominator a)) (define (neg-numerator a
b c)(- (- b)   (sqrt-of-discriminant a b
c))) (define (sqrt-of-discriminant a b c)(sqrt
(- ( b b)   ( 4.0 a c))))
28
Positive Root Part I
29
Positive Root Part II
30
Negative Root Part I
31
Negative Root Part II
32
Summary

• You should now know
• Stack as an Abstract Data Type
• Nature of the Activation Stack
• Tracing program execution using the Activation
  Stack

33
Questions?
34
(No Transcript)