1 / 47

Machine-Level Programming 4 Procedures

Topics stack discipline Register saving conventions Creating pointers to local variables. Machine-Level Programming 4 Procedures. Increasing Addresses. Stack Pointer %esp. IA32 Stack. Stack “Bottom”. Region of memory managed with stack discipline Grows toward lower addresses

foy
Download Presentation

Machine-Level Programming 4 Procedures

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. Topics stack discipline Register saving conventions Creating pointers to local variables Machine-Level Programming 4 Procedures

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

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

  4. Increasing Addresses Stack Pointer %esp IA32 Stack Popping Stack “Bottom” • Popping • popl Dest • Read operand at address given by %esp • Increment %esp by 4 • Write to Dest Stack Grows Down +4 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 value • 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 is 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 is program counter

  8. Stack-Based Languages • Languages that Support Recursion • e.g., C, Pascal, Java • Code must be “Reentrant” aka “Thread Safe” • 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 Call Chain • Code Structure • yoo(…) • { • • • • • who(); • • • • • } yoo • who(…) • { • • • • • amI(); • • • • • amI(); • • • • • } who amI amI • amI(…) • { • • • • • amI(); • • • • • } amI amI • Procedure amI is recursive

  10. Frame Pointer %ebp Stack Pointer %esp Stack Frames yoo • Contents • Local variables • Return information • Temporary space • Management • Space allocated when enter procedure • “Set-up” code • Deallocated when return • “Finish” code • Pointers • Stack pointer %esp indicates stack top • Frame pointer %ebp indicates start of current frame who amI proc Stack “Top”

  11. Frame Pointer %ebp Stack Pointer %esp Stack Operation • • • S T A C K Call Chain yoo • yoo(…) • { • • • • • who(); • • • • • } yoo

  12. Frame Pointer %ebp Stack Pointer %esp Stack Operation • • • S T A C K Call Chain yoo • who(…) • { • • • • • amI(); • • • • • amI(); • • • • • } yoo who who

  13. Frame Pointer %ebp Stack Pointer %esp Stack Operation • • • S T A C K Call Chain yoo • amI(…) • { • • • • • amI(); • • • • • } yoo who who amI amI

  14. Frame Pointer %ebp Stack Pointer %esp Stack Operation • • • S T A C K Call Chain yoo • amI(…) • { • • • • • amI(); • • • • • } yoo who who amI amI amI amI

  15. Frame Pointer %ebp Stack Pointer %esp Stack Operation • • • S T A C K Call Chain yoo • amI(…) • { • • • • • amI(); • • • • • } yoo who who amI amI amI amI amI amI

  16. Frame Pointer %ebp Stack Pointer %esp Stack Operation • • • S T A C K Call Chain yoo • amI(…) • { • • • • • amI(); • • • • • } yoo who who amI amI amI amI amI

  17. Frame Pointer %ebp Stack Pointer %esp Stack Operation • • • S T A C K Call Chain yoo • amI(…) • { • • • • • amI(); • • • • • } yoo who who amI amI amI amI

  18. Frame Pointer %ebp Stack Pointer %esp Stack Operation • • • S T A C K Call Chain yoo • who(…) • { • • • • • amI(); • • • • • amI(); • • • • • } yoo who who amI amI amI

  19. Frame Pointer %ebp Stack Pointer %esp Stack Operation • • • S T A C K Call Chain yoo • amI(…) • { • • • • • • • • • } yoo who who amI amI amI amI amI

  20. Frame Pointer %ebp Stack Pointer %esp Stack Operation • • • S T A C K Call Chain yoo • who(…) • { • • • • • amI(); • • • • • amI(); • • • • • } yoo who who amI amI amI amI

  21. Frame Pointer %ebp Stack Pointer %esp Stack Operation • • • S T A C K Call Chain yoo • yoo(…) • { • • • • • who(); • • • • • } yoo who amI amI amI amI

  22. IA32/Linux Stack Frame • Current Stack Frame(“Top” to Bottom) • Parameters for function about to call • “Argument build” • Local variables • If 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 • • • %ebp • • • Resulting Stackbefore jumping to swap void swap(int *xp, int *yp) { int t0 = *xp; int t1 = *yp; *xp = t1; *yp = t0; } &zip2 &zip1 %esp Rtn adr

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

  26. swap Setup #2 Resulting Stack Entering Stack %ebp • • • • • • &zip2 yp &zip1 xp Rtn adr %esp Rtn adr %ebp Old %ebp %esp swap: pushl %ebp movl %esp,%ebp pushl %ebx

  27. swap Setup #3 Resulting Stack Entering Stack %ebp • • • • • • &zip2 yp &zip1 xp Rtn adr %esp Rtn adr %ebp Old %ebp Old %ebx %esp swap: pushl %ebp movl %esp,%ebp pushl %ebx

  28. Effect of swap Setup Entering Stack Resulting Stack %ebp • • • • • • Offset (relative to %ebp) &zip2 12 yp &zip1 8 xp Rtn adr %esp 4 Rtn adr %ebp 0 Old %ebp Old %ebx %esp movl 12(%ebp),%ecx # get yp movl 8(%ebp),%edx # get xp . . . Body

  29. swap Finish #1 • • • swap’s Stack • • • • Observation • Saved & restored register %ebx Offset Offset 12 yp 12 yp 8 xp 8 xp 4 Rtn adr 4 Rtn adr %ebp 0 Old %ebp %ebp 0 Old %ebp -4 Old %ebx %esp -4 Old %ebx %esp movl -4(%ebp),%ebx movl %ebp,%esp popl %ebp ret

  30. swap Finish #2 • • • swap’s Stack • • • swap’s Stack Offset Offset 12 yp 12 yp 8 xp 8 xp 4 Rtn adr 4 Rtn adr %ebp 0 Old %ebp %ebp 0 Old %ebp -4 Old %ebx %esp %esp movl -4(%ebp),%ebx movl %ebp,%esp popl %ebp ret

  31. swap Finish #3 %ebp • • • swap’s Stack • • • swap’s Stack Offset Offset 12 yp 12 yp 8 xp 8 xp 4 Rtn adr 4 Rtn adr %esp 0 Old %ebp %ebp %esp movl -4(%ebp),%ebx movl %ebp,%esp popl %ebp ret

  32. swap Finish #4 %ebp • • • swap’s Stack %ebp • • • • Observation • Saved & restored register %ebx • Didn’t do so for %eax, %ecx, or %edx Exiting Stack Offset 12 yp &zip2 8 xp &zip1 %esp 4 Rtn adr %esp movl -4(%ebp),%ebx movl %ebp,%esp popl %ebp ret

  33. 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

  34. Register Saving Conventions • When procedure yoo calls who: • yoo is the caller, who is the callee • Can Register be Used for Temporary Storage? • Contents of register %edx overwritten by who • yoo: • • • • • movl $15213, %edx • call who • addl %edx, %eax • • • • • ret • who: • • • • • movl 8(%ebp), %edx • addl $91125, %edx • • • • • ret

  35. 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

  36. IA32/Linux Register Usage • Integer Registers • Two have special uses %ebp, %esp • Three managed as callee-save %ebx, %esi, %edi • Old values saved on stack prior to using • Three managed as caller-save %eax, %edx, %ecx • Do what you please, but expect any callee to do so, as well • Register %eax also stores returned value %eax Caller-Save Temporaries %edx %ecx %ebx Callee-Save Temporaries %esi %edi %esp Special %ebp

  37. .globl rfact .type rfact,@function rfact: pushl %ebp movl %esp,%ebp pushl %ebx movl 8(%ebp),%ebx cmpl $1,%ebx jle .L78 leal -1(%ebx),%eax pushl %eax call rfact imull %ebx,%eax jmp .L79 .align 4 .L78: movl $1,%eax .L79: movl -4(%ebp),%ebx movl %ebp,%esp popl %ebp ret Recursive Factorial int rfact(int x) { int rval; if (x <= 1) return 1; rval = rfact(x-1); return rval * x; } • Registers • %eax used without first saving • %ebx used, but save at beginning & restore at end

  38. Caller x Rtn adr %esp %ebp Old %ebp Old %ebx %esp Rfact Stack Setup pre %ebp %ebp pre %ebx Entering Stack rfact: pushl %ebp movl %esp,%ebp pushl %ebx rfact: pushl %ebp movl %esp,%ebp pushl %ebx rfact: pushl %ebp movl %esp,%ebp pushl %ebx pre %ebp Caller pre %ebx 8 x 4 Rtn adr 0 Callee -4

  39. Rfact Body movl 8(%ebp),%ebx # ebx = x cmpl $1,%ebx # Compare x : 1 jle .L78 # If <= goto Term leal -1(%ebx),%eax # eax = x-1 pushl %eax # Push x-1 call rfact # rfact(x-1) imull %ebx,%eax # rval * x jmp .L79 # Goto done .L78: # Term: movl $1,%eax # return val = 1 .L79: # Done: • Registers %ebx Stored value of x %eax • Temporary value of x-1 • Returned value from rfact(x-1) • Returned value from this call Recursion int rfact(int x) { int rval; if (x <= 1) return 1; rval = rfact(x-1) ; return rval * x; }

  40. leal -1(%ebx),%eax pushl %eax call rfact x-1 %esp x-1 %esp Rtn adr Rfact Recursion x Rtn adr x %ebp Old %ebp Rtn adr Old %ebx %esp %ebp Old %ebp x Old %ebx Rtn adr %ebp Old %ebp %eax Old %ebx %ebx x x-1 %eax x-1 %ebx x %eax x-1 %ebx x

  41. imull %ebx,%eax x! Rfact Result Return from Call x x Rtn adr Rtn adr %ebp %ebp Old %ebp Old %ebp Old %ebx Old %ebx x-1 x-1 %esp %esp %eax (x-1)! (x-1)! %eax (x-1)! %ebx x %ebx x Assume that rfact(x-1) returns (x-1)! in register %eax

  42. pre %ebp pre %ebx pre %ebp %ebp 8 x pre %ebx 4 Rtn adr x %ebp 0 Old %ebp Rtn adr %esp %esp %eax x! %ebx Old %ebx %eax x! %ebx Old %ebx Rfact Completion movl -4(%ebp),%ebx movl %ebp,%esp popl %ebp ret movl -4(%ebp),%ebx movl %ebp,%esp popl %ebp ret movl -4(%ebp),%ebx movl %ebp,%esp popl %ebp ret pre %ebp pre %ebx 8 x 4 Rtn adr %ebp 0 Old %ebp -4 Old %ebx -8 x-1 %esp %eax x! %ebx x Old %ebx

  43. Recursive Procedure Top-Level Call void s_helper (int x, int *accum) { if (x <= 1) return; else { int z = *accum * x; *accum = z; s_helper (x-1,accum); } } int sfact(int x) { int val = 1; s_helper(x, &val); return val; } Pointer Code • Pass pointer to update location

  44. %ebp Temp. Space val = 1 Unused %esp Creating & Initializing Pointer Initial part of sfact • Using Stack for Local Variable • Variable val must be stored on stack • Need to create pointer to it • Compute pointer as-4(%ebp) • Push on stack as second argument _sfact: pushl %ebp # Save %ebp movl %esp,%ebp # Set %ebp subl $16,%esp # Add 16 bytes movl 8(%ebp),%edx # edx = x movl $1,-4(%ebp) # val = 1 _sfact: pushl %ebp # Save %ebp movl %esp,%ebp # Set %ebp subl $16,%esp # Add 16 bytes movl 8(%ebp),%edx # edx = x movl $1,-4(%ebp) # val = 1 _sfact: pushl %ebp # Save %ebp movl %esp,%ebp # Set %ebp subl $16,%esp # Add 16 bytes movl 8(%ebp),%edx # edx = x movl $1,-4(%ebp) # val = 1 _sfact: pushl %ebp # Save %ebp movl %esp,%ebp # Set %ebp subl $16,%esp # Add 16 bytes movl 8(%ebp),%edx # edx = x movl $1,-4(%ebp) # val = 1 8 x 4 Rtn adr 0 Old %ebp -4 -8 -12 -16 int sfact(int x) { int val = 1; s_helper(x, &val); return val; }

  45. &val x %esp Passing Pointer Calling s_helper from sfact Stack at time of call 8 x leal -4(%ebp),%eax # Compute &val pushl %eax # Push on stack pushl %edx # Push x call s_helper # call movl -4(%ebp),%eax # Return val • • • # Finish leal -4(%ebp),%eax # Compute &val pushl %eax # Push on stack pushl %edx # Push x call s_helper # call movl -4(%ebp),%eax # Return val • • • # Finish leal -4(%ebp),%eax # Compute &val pushl %eax # Push on stack pushl %edx # Push x call s_helper # call movl -4(%ebp),%eax # Return val • • • # Finish 4 Rtn adr %ebp 0 Old %ebp -4 val = 1 val =x! -8 Unused -12 int sfact(int x) { int val = 1; s_helper(x, &val); return val; } -16

  46. V %edx (accum) %eax x Using Pointer void s_helper (int x, int *accum) { • • • int z = *accum * x; *accum = z; • • • } • Register %ecx holds x • Register %edx holds accum • Assume memory initally has value V • Use access (%edx) to reference memory V*x V*x x %ecx • • • movl %ecx,%eax # z = x imull (%edx),%eax # z *= *accum movl %eax,(%edx) # *accum = z • • •

  47. 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 • Can be 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 • %ebp and %esp • Stack frame organization conventions

More Related