1. Run-Time Environments Chapter 7
Lecture # 22
Run-Time Environments
Chapter 7

2. Runtime Environments
Before code generation static source text of a program needs to be related to the actions that must occur at runtime to implement the program.
As execution proceeds same name in the source text can denote different data objects in the target machine.
The allocation and de allocation of data objects is managed by the runtime support package.

3. 7.1 Source Language Issues
Each execution of a function is referred to as an activation of the procedure/function
If the function is recursive several of its activations may be alive at the same time

4. Procedure Activation and Lifetime
A procedure is activated when called
The lifetime of an activation of a procedure is the sequence of steps between the first and last steps in the execution of the procedure body
A procedure is recursive if a new activation can begin before an earlier activation of the same procedure has ended

5. Activation Tree
We use an activation tree to depict the way control enters and leaves activations. In an activation tree:
Each node represents activation of a procedure
The root represents activation of main program
The node for a is the parent of the node for b if the control flows from activation a to b
The node for a is to the left of node for b if the lifetime of a occurs before the lifetime of b

6. Procedure Activations: Example
program sort(input, output) var a : array [0..10] of integer; procedure readarray; var i : integer; begin for i := 1 to 9 do read(a[i]) end; function partition(y, z : integer) : integer var i, j, x, v : integer; begin … end procedure quicksort(m, n : integer); var i : integer; begin if (n > m) then begin i := partition(m, n); quicksort(m, i - 1); quicksort(i + 1, n) end end; begin a[0] := -9999; a[10] := 9999; readarray; quicksort(1, 9) end.
Activations: begin sort enter readarray leave readarray enter quicksort(1,9) enter partition(1,9) leave partition(1,9) enter quicksort(1,3) … leave quicksort(1,3) enter quicksort(5,9)
… leave quicksort(5,9) leave quicksort(1,9) end sort.

7. Activation Trees: Example
q(1,9)
p(1,9)
q(1,3)
q(5,9)
p(1,3)
q(1,0)
q(2,3)
p(5,9)
q(5,5)
q(7,9)
p(2,3)
q(2,1)
q(3,3)
p(7,9)
q(7,7)
q(9,9)
Activation tree for the sort program Note: also referred to as the dynamic call graph

8. Control Stack
The flow of control of a program corresponds to depth first traversal of the activation tree
Control stack is used to keep track of the live procedure activations.
The idea is to push the node when activation begins and pop when it ends

9. Factorial Example discussed
public static int Factorial(int num) { if (num == 1) fact=1; } else fact=fact+num*Factorial(--num); return fact;

10. Addition of Elements of Array recursively
public static int Add(int index) {
if (index == 0)
sum = arr[index]; }
else
sum = sum + Add(arr[index]);
return sum;

11. Control Stack Activation tree: Control stack:
Activations: begin sort enter readarray leave readarray enter quicksort(1,9) enter partition(1,9) leave partition(1,9) enter quicksort(1,3) enter partition(1,3) leave partition(1,3) enter quicksort(1,0)
leave quicksort(1,0) enter quicksort(2,3) … s s
q(1,9)
q(1,3)
q(2,3) r
q(1,9)
p(1,9)
q(1,3)
p(1,3)
q(1,0)
q(2,3)

12. Scope Rules
Environment determines name-to-object bindings: which objects are in scope?
program prg; var y : real; function x(a : real) : real; begin … end; procedure p; var x : integer; begin x := 1; … end; begin
y := x(0.0); … end.
Variable x locally declared in p
A function x

13. Binding of Names
If a name is declared once in a program it can hold different values at runtime
The term environment refers to a function that maps name to a storage location
The term state is referred to a function that maps a storage location to the value held there

14. Mapping Names to Values
environment
state
name
storage
value
var i; … i := 0; … i := i + 1;

15. Mapping Names to Values
At compile time
At run time
environment
state
name
storage
value
var i; i := 0; i := i + 1;
Environment and states are different
An assignment changes the state but not the environment

16. Static and Dynamic Notions of Bindings
Static Notion
Dynamic Notion
Definition of a procedure
Activations of the procedure
Declaration of a name
Bindings of the name
Scope of a declaration
Lifetime of a binding

17. Storage Organization Runtime storage may be subdivided as:
The generated target code
Data objects
Control stack to keep track of procedure activations
The size of target code is fixed at compile time
Similarly size of data objects may be known

18. Typical subdivision of runtime memory
A separate area of runtime memory called Heap holds all other runtime information
Code
Static Data
Stack
Heap

19. Stack Allocation
Activation records (subroutine frames) on the run-time stack hold the state of a subroutine
Calling sequences are code statements to create activations records on the stack and enter data in them
Caller’s calling sequence enters actual arguments, control link, access link, and saved machine state
Callee’s calling sequence initializes local data
Callee’s return sequence enters return value
Caller’s return sequence removes activation record

20. Activation Records (Subroutine Frames)
fp (frame pointer)
Returned value
Actual parameters
Optional control link
Optional access link
Save machine status
Local data
Temporaries
Caller’s responsibility to initialize
Callee’s responsibility to initialize

21. Control Links fp sp Stack growth
The control link is the old value of the fp
Caller’s activation record fp
Callee’s activation record
Control link sp
Stack growth

22. Access Links (Static Links)
s a x
q(1,9) access k v
s a x
q(1,9) access k v
q(1,3) access k v
s a x
q(1,9) access k v
q(1,3) access k v
p(1,3) access i j
s a x
q(1,9) access k v
q(1,3) access k v
p(1,3) access i j
e(1,3) access
The access link points to the activation record of the static parent procedure:
s is parent of r, e, and q q is parent of p

23. Accessing Nonlocal Data
To implement access to nonlocal data a in procedure p, the compiler generates code to traverse np - na access links to reach the activation record where a resides
np is the nesting depth of procedure p
na is the nesting depth of the procedure containing a

24. Parameter Passing Modes
Call-by-value: evaluate actual parameters and enter r-values in activation record
Call-by-reference: enter pointer to the storage of the actual parameter