Runtime Environments

1. Runtime Environments What is in the memory?

2. Outline • Memory organization during program execution • Static runtime environments • Stack-based runtime environments • Without local procedure • With local procedure • Parameter passing Runtime Environment

3. Memory organization during program execution • Main memory • Store large amount of data • Slower access • Registers • Very small amount of data • Faster access Code area Global/static area stack Data area Free space registers Heap Main memory Runtime Environment

4. proc 1 proc 2 proc n Code Area • Addresses in code area are static (i.e. no change during execution) for most programming language. • Addresses are known at compile time. entry point entry point entry point Runtime Environment

5. Data Area • Addresses in data area are static for some data and dynamic for others. • Static data are located in static area. • Dynamic data are located in stack or heap. • Stack (LIFO allocation) for procedure activation record, etc. • Heap for user allocated memory, etc. Runtime Environment

6. Registers • General-purpose registers • Used for calculation • Special purpose registers • Program counter (pc) • Stack pointer (sp) • Frame pointer (fp) • Argument pointer (ap) Runtime Environment

7. Calling Sequence • Sequence of operations that must be done for procedure calls • Call sequence • Sequence of operations performed during procedure calls • Find the arguments and pass them to the callee. • Save the caller environment, i.e. local variables in activation records, return address. • Create the callee environment, i.e. local variables in activation records, callee’s entry point. • Return sequence • Sequence of operations performed when return from procedure calls • Find the arguments and pass them back to the caller. • Free the callee environment. • Restore the caller environment, including PC. Runtime Environment

8. Issues in Call Sequence • Which part of the call sequence is included in the caller code? Which is in the callee code? • Save space in the code segment if the call sequence is included in the callee code. • Normally, the caller finds the arguments and provides them to the callee. • Which operation is supported in hardware? • The more operations supported in hardware, the lower cost (i.e. execution time and space for code) is. Runtime Environment

9. Static runtime environments • Static data • Both local and global variables are allocated once at the beginning and deallocated at program termination • Fixed address • No dynamic allocation • No recursive call • Procedure calls are allowed, but no recursion. • One activation record for each procedure, allocated statically • Example:FORTRAN 77 Runtime Environment

10. Memory Organization for Static Runtime Environment PROGRAM TEST COMMON MAX INTEGER MAX REAL TAB(10), TEMP … QMEAN(TAB, 3, TEMP) … END SUBROUTINE QMEAN(A, SIZE, MEAN) COMMON MAX INTEGER MAX, SIZE REAL A(SIZE), MEAN, TEMP INTEGER K … END MAX TAB(1) TAB(2) … TAB(10) TEMP 3 A SIZE MEAN Return addr TEMP K Global area Activation record for main Activation record for QMEAN Runtime Environment

11. Stack-based runtime environments • Handle recursive calls • Activation records are allocated in stack, called rumtime stack or call stack • One procedure can have more than one activation records in the stack at one time • Call sequence is more complex than the sequence in static environment Runtime Environment

12. Stack-based Environments Without Local Procedures • Maintain pointer to the current activation record • Store frame pointer in an fp register • Record the link from an activation record to the previous record • In each activation record, a link from an activation record to the activation record of the caller, called a dynamic link or control link is stored. • Sometimes, the area for parameters and local variables need to be identified. • A stack pointer is maintained in an sp register. SP local var.s return addr control link parameters FP Runtime Environment

13. sp sp sp sp sp sp sp sp sp arguments arguments control link control link fp fp fp fp local var.s local var.s return addr return addr Call Sequence in Stack-based Environments Without Local Procedures Global area • Calling sequence • Push arguments • Push fp as control link • Copy sp to fp • Store return address • Jump to callee • Reserve space for local variables • Return sequence • Copy fp to sp • Load control link into fp • Jump to return address • Change sp to pop arguments activation record of main Direction of stack growth Calling sequence Return sequence Store fp as control link Compute arguments and push into stack Move fp Reserve area for local variables Load fp into sp (to pop local var.s and return addr) Load control link into fp Jump to return addr Push return address Pop arguments Runtime Environment

14. Global area activation record for main activation record for g(2) activation record for f(1) activation record for g(1) activation record for g(1) Activation Tree main() { …; g(x); return(0); } void g(int m) { … f(y); … g(y); … } void f(int n) { g(n); … } Runtime Environment

15. Address of local variables and parameters are calculated dynamically during execution time Offset of variables and parameters from fp is: static known during compile time Stored in symbol table Address of x =fp+xOffset Address of a =fp+aOffset parameters control link return address local variables Calculating Address in Stack-Based Environments a aOffset fp xOffset x sp Runtime Environment

16. Considerations in Address Calculation • Sizes of variables depend on data types • Addressing elements in array • Number of dimensions • Size of array • Ordering in storage • Row-major order • Column-major order Runtime Environment

17. Local Temporaries X = (a+b)/((c-d)*f(i)) • Values of (a+b) and (c-d) must be stored before f can be called. • Thus, area for temporary variables must be reserved in the stack. • Usually, temporaries are pushed into stacks after local variables. Runtime Environment

18. Nested Declarations • Local variables of blocks can be defined. void f { int x; int i; … { char x; int j; … } … } • A block could be treated as a procedure, but it is inefficient. • Another simpler method is to: • push local variables in stack when the block is entered • pop the local variables when the block is exited rest of stack control link of f activation record of f return addr of f x i x j local var. of block Runtime Environment

19. program main; var a[10]:int; proc cal(a[10]:int); var ans:real; func sum(e[10]:int); var t: int; begin … return(t); end func ave(b[10],n:int); var ans:int; begin ans=sum(b); return(total/n); end begin ans=ave(a,10); return; end begin input(a); cal(a); return; end Local Procedures Runtime Environment

20. Stack-based Environments with Local Procedures • Access link/static link must be included in each activation records • Represents the defining environment • Access link and control link need not be the same • Control link • represents the calling environment parameters access link activation record control link return addr local var. Runtime Environment

21. a[10] access link activation record for cal control link return addr ans b[10] access link activation record for ave control link return addr ans e[10] access link activation record for sum control link return addr t Call Sequence in Stack-based Environments With Local Procedures global area activation record for main program main; var a[10],n :int; proc cal(a[10]:int); var ans:real; func sum(e[10]:int); var t: int; begin … return(t); end func ave(b[10]); var ans:int; begin ans=sum(b); return(ans/n); end begin ans=ave(a,10)); return; end begin n=10; input(a); cal(a); return; end ans Runtime Environment

22. Calling sequence Push arguments Push access link Push fp as control link Copy sp to fp Store return address Jump to callee Reserve space for local variables How to find access link Return sequence Copy fp to sp Load control link into fp Pop access link Jump to return address Change sp to pop arguments Call Sequence Runtime Environment

23. Accessing Variables in Local Procedures • Access Chaining • Follow access links from one activation record until the required variable is found • Inefficient • Display • Access links are stored in an array called display Runtime Environment

24. Nesting level Indicates the depth of the procedure in program definition Level 0: outermost Need to be stored in the symbol table To find the activation record for the nth nesting level from the activation record in the ith nesting level Follow the access link (n-i) times Examples: To access ans in cal Follow links 2-1 times To access n in main Follow links 2-0 times a[10] access link activation record for cal nesting level: 1 control link return addr ans b[10] access link activation record for ave nesting level: 2 control link return addr ans e[10] access link activation record for sum nesting level:2 control link return addr t Accessing Chaining global area activation record for main Runtime Environment

25. Access links are stored in the array display. The activation record of a procedure in the ith nesting level is stoted in display[i]. a[10] access link display[0] control link display[1] return addr display[2] display[2] ans b[10] access link control link return addr ans e[10] access link control link return addr t Display global area activation record for main cal ave sum Runtime Environment

26. Parameter Passing • Pass by value • Pass by reference • Pass by value-result • Pass by name Runtime Environment

27. Value parameters are not changed during the execution Only the value is sent into the procedure, and are used locally When the control is returned from the callee, the value of the parameter is not passed back to the caller. void change(int x) { x++; return; } void main() { int y=0; change(y); printf(“%d\n”,y); return; } Output: 0 Pass by Value Runtime Environment

28. The reference to a variable is passed to the callee. The callee used the reference (address) to refer to the variable. Indirect addressing is needed to refer to parameters The variable in the caller and the referenced memory in the callee share the same memory location. The value of the variable in the caller is also changed when the referenced in the callee is changed. void change (int &x) { x++; return; } void main() { int y=0; change(y); printf(“%d\n”,y); return; } Output: 1 Pass by Reference Runtime Environment

29. The value of the parameter is copied into the callee when the callee is entered. New memory location is provided for the parameter in the callee’s activation record. No indirect address is needed When the control is returned to the caller, the value is copied back. void change(int x) { x++; return; } void main() { int y=0; change(y); printf(“%d\n”,y); return; } Output: 1 Pass by Value-Result Runtime Environment

30. The argument is replaced by the textual representation of the parameter passed from the caller. The evaluation of the actual parameters must be delayed until the execution. A procedure (thunk) is called to find the value of the parameter in the callee. void change(int x) { i=4; x++; return; } void main() { int i=0; int a[5]={0,0,0,0,0}; change(a[i]); return; } After the call: a={0,0,0,0,1} Pass by Name Runtime Environment