1. Runtime Environments Source language issues Storage organization
trace the execution of a program
activation trees
the scope of a name
parameter passing
Storage organization
the control stack
symbol tables
dynamic storage allocation
by Neng-Fa Zhou

2. Procedures and Functions
Structural programming
combine shared code
parameterized
Advantages over a flat language
Modularity
blocks for building large systems
Information hiding
Separate compilation
by Neng-Fa Zhou

3. Procedure Definitions and Calls
formal parameters
Function definition
Function call
void sort(int a[], int l, int u){
... }
return type
actual parameters
sort(a,0,100)
by Neng-Fa Zhou

4. Trace the Execution of Programs
void qsort(int l, int u){
int i;
if (l>=u) return;
i = partition(l,u);
qsort(l,i-1);
qsort(i+1,u); }
>qsort(1,9)
>partition(1,9)
<partition(1,9)
>qsort(1,3)
>partition(1,3)
<partition(1,3)
...
<qsort(1,3)
>qsort(5,9)
<qsort(5,9)
<qsort(1,9)
activation
by Neng-Fa Zhou

5. Activation Trees q(1,9) p(1,9) q(1,3) q(5,9) p(1,3) q(1,0) q(2,3)
each node represents an activation
the root represents the main program
node A is a parent of node B if A calls B
the order of siblings is determined by the lifetimes of the activations
by Neng-Fa Zhou

6. The Scope of a Name most closely nested rule main(){ int a = 0;
int b = 0; {
int b = 1;
int a = 2;
printf("%d %d\n",a,b); }
int b = 3;
most closely nested rule
by Neng-Fa Zhou

7. Parameter Passing Call by value Call by reference Call by name
primitive types, pointers, structures in C
all data types in Java
Call by reference
arrays in C
var arguments in Pascal
Call by name
in-line expansion
by Neng-Fa Zhou

8. Call by Value formal and actual are independent actual a
1. evaluate the actual parameter
2. pass it to the activation record a
3. use the formal parameter as a local variable
formal
formal and actual are independent
by Neng-Fa Zhou

9. Call by Reference formal becomes an alias for actual
when formal is updated, actual is also updated a
formal
by Neng-Fa Zhou

10. Call by Name The body is substituted for the call
The local names of the called procedure are renamed if necessary to keep them different from those in the calling procedure
The actual parameter are surrounded by parentheses if necessary
by Neng-Fa Zhou

11. Questions to Ask May procedures be recursive?
What happens to the values of local names when control returns from an activation of a procedure?
May a procedure refer to nonlocal names?
How are parameters passed?
May procedures be passed as parameters?
May procedures be returned as results?
Must storage be deallocated explicitly?
by Neng-Fa Zhou

12. Storage Organization Code Code: program + library Static Data
Static data: global variables
Stack: activation frames
Heap: dynamically allocated memory
Static Data
Stack
Heap
by Neng-Fa Zhou

13. Control Stack
>qsort(1,9)
>partition(1,9)
<partition(1,9)
>qsort(1,3)
>partition(1,3)
qs(1,9)
qs(1,3)
top_sp
pa(1,3)
A frame is pushed onto the stack each time a procedure is called
The frame is popped off after the execution ends
by Neng-Fa Zhou

14. Activation Records (Frames)
returned value
actual parameters
optional control link:
parent frame etc.
access link:
frame of the outer procedure
machine status:
program counter
registers
optional control link
optional access link
saved machine status
local data
temporaries
by Neng-Fa Zhou

15. Calling and Returning Sequences
Calling sequence
evaluate actual parameters
set callee's frame (arguments,return address,parent)
reset the top_sp register
save registers including the program counter
initialize local data
begin execution
caller
callee
by Neng-Fa Zhou

16. Calling and Returning Sequences
place return value
restore the state including P and top_sp
continue execution
callee
caller
by Neng-Fa Zhou

17. Dangling References main(){ int *p; p = dangle(); } int *dangle(){
int i=23;
return &i;
by Neng-Fa Zhou