Instructor: Erol Sahin

1. Machine Programming – Procedures and IA32 StackCENG331: Introduction to Computer Systems6th Lecture Instructor: ErolSahin • Acknowledgement: Most of the slides are adapted from the ones prepared • by R.E. Bryant, D.R. O’Hallaron of Carnegie-Mellon Univ.

2. IA32 Stack Stack “Bottom” • Region of memory managed with stack discipline • Grows toward lower addresses • Register %espcontains lowest stack address= address of “top” element Increasing Addresses Stack Grows Down Stack Pointer: %esp Stack “Top”

3. IA32 Stack: Push Stack “Bottom” • pushlSrc • Fetch operand at Src • Decrement %espby 4 • Write operand at address given by %esp Increasing Addresses Stack Grows Down -4 Stack Pointer: %esp Stack “Top”

4. IA32 Stack: Pop Stack “Bottom” • poplDest • Read operand at address %esp • Increment %espby 4 • Write operand to Dest Increasing Addresses Stack Grows Down +4 Stack Pointer: %esp Stack “Top”

5. Procedure Control Flow • Use stack to support procedure call and return • Procedure call: call label • Push return address on stack • Jump to label • Return address: • Address of instruction beyond call • Example from disassembly 804854e: e8 3d 06 00 00 call 8048b90 <main> 8048553: 50 pushl %eax • Return address = 0x8048553 • Procedure return: ret • Pop address from stack • Jump to address

6. Procedure Call Example • 804854e: e8 3d 06 00 00 call 8048b90 <main> • 8048553: 50 pushl %eax call 8048b90 0x110 0x110 0x10c 0x10c 0x108 123 0x108 123 0x104 0x8048553 %esp 0x108 %esp 0x108 0x104 %eip 0x804854e %eip 0x804854e 0x8048b90 %eip: program counter

7. Procedure Return Example • 8048591: c3 ret ret 0x110 0x110 0x10c 0x10c 0x108 123 0x108 123 0x104 0x8048553 0x8048553 %esp 0x104 %esp 0x104 0x108 %eip 0x8048591 %eip 0x8048591 0x8048553 %eip: program counter

8. Stack-Based Languages • Languages that support recursion • e.g., C, Pascal, Java • Code must be “Reentrant” • Multiple simultaneous instantiations of single procedure • Need some place to store state of each instantiation • Arguments • Local variables • Return pointer • Stack discipline • State for given procedure needed for limited time • From when called to when return • Callee returns before caller does • Stack allocated in Frames • state for single procedure instantiation

9. Call Chain Example Example Call Chain • yoo(…) • { • • • • • who(); • • • • • } yoo • who(…) • { • • • • • amI(); • • • • • amI(); • • • • • } who amI amI • amI(…) • { • • • • • amI(); • • • • • } amI amI • Procedure amIis recursive

10. Stack Frames Previous Frame • Contents • Local variables • Return information • Temporary space • Management • Space allocated when enter procedure • “Set-up” code • Deallocated when return • “Finish” code Frame Pointer: %ebp Frame for proc Stack Pointer: %esp Stack “Top”

11. Stack Example • yoo(…) • { • • • • • who(); • • • • • } yoo %ebp yoo who %esp amI amI amI amI

12. Stack Example • who(…) • { • • • • • amI(); • • • • • amI(); • • • • • } yoo yoo who %ebp who amI amI %esp amI amI

13. Stack Example • amI(…) • { • • • • • amI(); • • • • • } yoo yoo who who amI amI %ebp amI amI %esp amI

14. Stack Example • amI(…) • { • • • • • amI(); • • • • • } yoo yoo who who amI amI amI amI amI %ebp amI %esp

15. Stack Example • amI(…) • { • • • • • amI(); • • • • • } yoo yoo who who amI amI amI amI amI amI %ebp amI %esp

16. Stack Example • amI(…) • { • • • • • amI(); • • • • • } yoo yoo who who amI amI amI amI amI %ebp amI %esp

17. Stack Example • amI(…) • { • • • • • amI(); • • • • • } yoo yoo who who amI amI %ebp amI amI %esp amI

18. Stack Example • who(…) • { • • • • • amI(); • • • • • amI(); • • • • • } yoo yoo who %ebp who amI amI %esp amI amI

19. Stack Example • amI(…) • { • • • • • • • • • • • } yoo yoo who who amI amI %ebp amI amI %esp amI

20. Stack Example • who(…) • { • • • • • amI(); • • • • • amI(); • • • • • } yoo yoo who %ebp who amI amI %esp amI amI

21. Stack Example • yoo(…) • { • • • • • who(); • • • • • } yoo %ebp yoo who %esp amI amI amI amI

22. IA32/Linux Stack Frame • Current Stack Frame (“Top” to Bottom) • “Argument build:”Parameters for function about to call • Local variablesIf can’t keep in registers • Saved register context • Old frame pointer • Caller Stack Frame • Return address • Pushed by call instruction • Arguments for this call Caller Frame Arguments Frame pointer%ebp Return Addr Old %ebp Saved Registers + Local Variables Argument Build Stack pointer %esp

23. Revisiting swap Calling swap from call_swap int zip1 = 15213; int zip2 = 91125; void call_swap() { swap(&zip1, &zip2); } call_swap: • • • pushl $zip2 # Global Var pushl $zip1 # Global Var call swap • • • • • • Resulting Stack void swap(int *xp, int *yp) { int t0 = *xp; int t1 = *yp; *xp = t1; *yp = t0; } &zip2 &zip1 Rtnadr %esp

24. Revisiting swap swap: pushl %ebp movl %esp,%ebp pushl %ebx movl 12(%ebp),%ecx movl 8(%ebp),%edx movl (%ecx),%eax movl (%edx),%ebx movl %eax,(%edx) movl %ebx,(%ecx) movl -4(%ebp),%ebx movl %ebp,%esp popl %ebp ret Set Up void swap(int *xp, int *yp) { int t0 = *xp; int t1 = *yp; *xp = t1; *yp = t0; } Body Finish

25. swap Setup #1 Entering Stack Resulting Stack %ebp %ebp • • • • • • &zip2 yp &zip1 xp Rtnadr %esp Rtnadr Old %ebp %esp swap: pushl %ebp movl %esp,%ebp pushl %ebx

26. swap Setup #1 Entering Stack %ebp %ebp • • • • • • &zip2 yp &zip1 xp Rtnadr %esp Rtnadr Old %ebp %esp swap: pushl %ebp movl %esp,%ebp pushl %ebx

27. swap Setup #1 Entering Stack Resulting Stack %ebp • • • • • • &zip2 yp &zip1 xp Rtnadr %esp Rtnadr Old %ebp %ebp %esp swap: pushl %ebp movl %esp,%ebp pushl %ebx

28. swap Setup #1 Entering Stack %ebp • • • • • • &zip2 yp &zip1 xp Rtnadr %esp Rtnadr Old %ebp %ebp %esp swap: pushl %ebp movl %esp,%ebp pushl %ebx

29. swap Setup #1 Entering Stack Resulting Stack %ebp • • • • • • Offset relative to %ebp &zip2 12 yp &zip1 8 xp Rtnadr 4 %esp Rtnadr Old %ebp %ebp Old %ebx %esp movl 12(%ebp),%ecx # get yp movl 8(%ebp),%edx # get xp . . .

30. swapFinish #1 swap’s Stack Resulting Stack • • • • • • yp yp xp xp Rtnadr Rtnadr Old %ebp Old %ebp %ebp %ebp Old %ebx Old %ebx %esp %esp movl -4(%ebp),%ebx movl %ebp,%esp popl %ebp ret Observation: Saved and restored register %ebx

31. swapFinish #2 swap’s Stack • • • • • • yp yp xp xp Rtnadr Rtnadr Old %ebp Old %ebp %ebp %ebp Old %ebx Old %ebx %esp %esp movl -4(%ebp),%ebx movl %ebp,%esp popl %ebp ret

32. swapFinish #2 swap’s Stack Resulting Stack • • • • • • yp yp xp xp Rtnadr Rtnadr Old %ebp Old %ebp %ebp %ebp %esp Old %ebx %esp movl -4(%ebp),%ebx movl %ebp,%esp popl %ebp ret

33. swapFinish #2 swap’s Stack • • • • • • yp yp xp xp Rtnadr Rtnadr Old %ebp Old %ebp %ebp %ebp %esp Old %ebx %esp movl -4(%ebp),%ebx movl %ebp,%esp popl %ebp ret

34. swapFinish #3 swap’s Stack Resulting Stack %ebp • • • • • • yp yp xp xp Rtnadr Rtnadr %esp Old %ebp %ebp Old %ebx %esp movl -4(%ebp),%ebx movl %ebp,%esp popl %ebp ret

35. swapFinish #4 swap’s Stack %ebp • • • • • • yp yp xp xp Rtnadr Rtnadr %esp Old %ebp %ebp Old %ebx %esp movl -4(%ebp),%ebx movl %ebp,%esp popl %ebp ret

36. swapFinish #4 swap’s Stack Resulting Stack %ebp • • • • • • yp yp xp xp %esp Rtnadr Old %ebp %ebp Old %ebx %esp movl -4(%ebp),%ebx movl %ebp,%esp popl %ebp ret • Observation • Saved & restored register %ebx • Didn’t do so for %eax, %ecx, or %edx

37. Disassembled swap 080483a4 <swap>: 80483a4: 55 push %ebp 80483a5: 89 e5 mov %esp,%ebp 80483a7: 53 push %ebx 80483a8: 8b 55 08 mov 0x8(%ebp),%edx 80483ab: 8b 4d 0c mov 0xc(%ebp),%ecx 80483ae: 8b 1a mov (%edx),%ebx 80483b0: 8b 01 mov (%ecx),%eax 80483b2: 89 02 mov %eax,(%edx) 80483b4: 89 19 mov %ebx,(%ecx) 80483b6: 5b pop %ebx 80483b7: c9 leave 80483b8: c3 ret Calling Code 8048409: e8 96 ff ff ff call 80483a4 <swap> 804840e: 8b 45 f8 mov 0xfffffff8(%ebp),%eax

38. Register Saving Conventions • When procedure yoo calls who: • yoois the caller • whois the callee • Can Register be used for temporary storage? • Contents of register %edxoverwritten by who • yoo: • • • • • movl $15213, %edx • call who • addl %edx, %eax • • • • • ret • who: • • • • • movl 8(%ebp), %edx • addl $91125, %edx • • • • • ret

39. Register Saving Conventions • When procedure yoo calls who: • yoo is the caller • who is the callee • Can register be used for temporary storage? • Conventions • “Caller Save” • Caller saves temporary in its frame before calling • “Callee Save” • Callee saves temporary in its frame before using

40. IA32/Linux Register Usage • %eax, %edx, %ecx • Caller saves prior to call if values are used later • %eax • also used to return integer value • %ebx, %esi, %edi • Callee saves if wants to use them • %esp, %ebp • special %eax Caller-Save Temporaries %edx %ecx %ebx Callee-Save Temporaries %esi %edi %esp Special %ebp

41. IA 32 Procedure Summary • The Stack Makes Recursion Work • Private storage for each instance of procedure call • Instantiations don’t clobber each other • Addressing of locals + arguments can be relative to stack positions • Managed by stack discipline • Procedures return in inverse order of calls • IA32 Procedures Combination of Instructions+ Conventions • Call / Ret instructions • Register usage conventions • Caller / Callee save • %ebpand %esp • Stack frame organization conventions Caller Frame Arguments Return Addr %ebp Old %ebp Saved Registers + Local Variables Argument Build %esp

42. Today • Arrays • One-dimensional • Multi-dimensional (nested) • Multi-level • Structures

43. Basic Data Types • Integral • Stored & operated on in general (integer) registers • Signed vs. unsigned depends on instructions used Intel GAS Bytes C byte b 1 [unsigned] char word w 2 [unsigned] short double word l 4 [unsigned]int quad word q 8 [unsigned] long int(x86-64) • Floating Point • Stored & operated on in floating point registers Intel GAS Bytes C Single s 4 float Double l 8 double Extended t 10/12/16 long double

44. Array Allocation • Basic Principle TA[L]; • Array of data type T and length L • Contiguously allocated region of L * sizeof(T) bytes char string[12]; x x + 12 int val[5]; x x + 4 x + 8 x + 12 x + 16 x + 20 double a[3]; x x + 8 x + 16 x + 24 char *p[3]; IA32 x x + 4 x + 8 x + 12 x86-64 x x + 8 x + 16 x + 24

45. 1 5 2 1 3 Array Access • Basic Principle TA[L]; • Array of data type T and length L • Identifier A can be used as a pointer to array element 0: Type T* • Reference Type Value val[4] int3 valint * x val+1int * x + 4 &val[2]int * x + 8 val[5]int?? *(val+1)int5 val + iint * x + 4i int val[5]; x x + 4 x + 8 x + 12 x + 16 x + 20 Will disappear Blackboard?

46. 1 5 2 1 3 Array Access • Basic Principle TA[L]; • Array of data type T and length L • Identifier A can be used as a pointer to array element 0: Type T* • Reference Type Value val[4] int3 valint * x val+1int * x + 4 &val[2]int * x + 8 val[5]int?? *(val+1)int5 val + iint * x + 4i int val[5]; x x + 4 x + 8 x + 12 x + 16 x + 20

47. 1 9 0 5 4 2 7 2 1 1 3 2 0 3 9 Array Example typedef int zip_dig[5]; zip_dig cmu = { 1, 5, 2, 1, 3 }; zip_dig mit = { 0, 2, 1, 3, 9 }; zip_dig ucb = { 9, 4, 7, 2, 0 }; • Declaration “zip_digcmu” equivalent to “intcmu[5]” • Example arrays were allocated in successive 20 byte blocks • Not guaranteed to happen in general zip_digcmu; zip_digmit; 56 36 16 40 60 20 64 24 44 68 28 48 52 32 72 56 36 76 zip_digucb;

48. 1 5 2 1 3 Array Accessing Example • Register %edx contains starting address of array • Register %eax contains array index • Desired digit at 4*%eax + %edx • Use memory reference (%edx,%eax,4) zip_digcmu; int get_digit (zip_dig z, int dig) { return z[dig]; } 16 20 24 28 32 36 IA32 # %edx = z # %eax = dig movl (%edx,%eax,4),%eax # z[dig]

49. 1 9 0 5 4 2 2 7 1 3 2 1 9 0 3 Referencing Examples zip_digcmu; • Reference Address Value Guaranteed? mit[3] 36 + 4* 3 = 48 3 mit[5] 36 + 4* 5 = 56 9 mit[-1] 36 + 4*-1 = 32 3 cmu[15] 16 + 4*15 = 76 ?? zip_digmit; zip_digucb; 56 36 16 40 20 60 24 64 44 68 48 28 72 32 52 56 36 76 Will disappear Blackboard?

50. 9 1 0 2 4 5 7 2 1 3 1 2 0 9 3 Referencing Examples zip_digcmu; • Reference Address Value Guaranteed? mit[3] 36 + 4* 3 = 48 3 mit[5] 36 + 4* 5 = 56 9 mit[-1] 36 + 4*-1 = 32 3 cmu[15] 16 + 4*15 = 76 ?? • No bound checking • Out of range behavior implementation-dependent • No guaranteed relative allocation of different arrays zip_digmit; zip_digmit; 36 56 16 40 60 20 64 24 44 68 28 48 32 52 72 56 36 76 Yes No No No