[PDF] [PDF] Procedures and the Call Stack

%esp%eip0x104%esp%eip0x80485910x80485910x1040x1040x1080x10c0x1100x8048553123ProcedureCall/Return:40x1080x10c0x1101238048591:c3 ret0x8048553%eip: programcounter31ret0x80485530x108

Revisitingswapintzip1 = 02481;intzip2 = 98195;void call_swap() {swap(&zip1, &zip2);}void swap(int*xp, int*yp) {intt0 = *xp;intt1 = *yp;*xp= t1;*yp= t0;}call_swap:• • •pushl$zip2# Global Varpushl$zip1# Global Varcall swap• • •&zip2&zip1Rtnadr%espResultingStack•••Callingswapfromcall_swap34

Revisitingswapvoid swap(int*xp, int*yp) {intt0 = *xp;intt1 = *yp;*xp= t1;*yp= t0;}swap:pushl%ebpmovl%esp,%ebppushl%ebxmovl12(%ebp),%ecxmovl8(%ebp),%edxmovl(%ecx),%eaxmovl(%edx),%ebxmovl%eax,(%edx)movl%ebx,(%ecx)movl-4(%ebp),%ebxmovl%ebp,%esppopl%ebpretBodySetUpFinish35

1284swapBody&zip2&zip1Rtnadr%espEnteringStack•••%ebpypxpRtnadrOld%ebp•••ResultingStackOld%ebxmovl 12(%ebp),%ecx # get ypmovl 8(%ebp),%edx # get xp. . .Offsetrelativetonew%ebp39Bodycall_swapframe%esp%ebp

Disassembledswap080483a4 :80483a4: 55 push %ebp80483a5: 89 e5 mov%esp,%ebp80483a7: 53 push %ebx80483a8: 8b 55 08 mov0x8(%ebp),%edx80483ab: 8b 4d 0c mov0xc(%ebp),%ecx80483ae: 8b 1a mov(%edx),%ebx80483b0: 8b 01 mov(%ecx),%eax80483b2: 89 02 mov%eax,(%edx)80483b4: 89 19 mov%ebx,(%ecx)80483b6: 5b pop %ebx80483b7: c9 leave 80483b8: c3 ret 8048409: e8 96 ff ff ff call 80483a4 804840e: 8b 45 f8 mov0xfffffff8(%ebp),%eaxCallingCode44mov%ebp,%esppop %ebprelativeaddress(littleendian)

RegisterSavingConventionsWhenprocedureyoocallswho:yooisthecallerwhoisthecalleeWillregistercontentsstillbethereafteraprocedurecall?yoo:• • •movl$12345, %edxcall whoaddl%edx, %eax• • •retwho:• • •movl8(%ebp), %edxaddl$98195, %edx• • •ret47?

IA32/LinuxRegisterSavingConventions%eax, %edx, %ecxCallersavespriortocallifneedsvaluesaftercall%eaxalsousedtoreturnintegervalue%ebx, %esi, %ediCalleesavesifwilluse%esp, %ebpspecialformofcalleesaverestoredtooriginalvaluesbeforereturning%eax%edx%ecx%ebx%esi%edi%esp%ebpCaller-SaveTemporariesCallee-SaveTemporariesSpecial49

APuzzle*p = d;return x -c;movsbl12(%ebp),%edxmovl16(%ebp),%eaxmovl%edx,(%eax)movswl8(%ebp),%eaxmovl20(%ebp),%edxsubl%eax,%edxmovl%edx,%eaxWritetheCfunctionheader,types,andorderofparameters.Cfunctionbody:assembly:movsbl=movesign-extendingabytetoalong(4-byte)movswl=movesign-extendingaword(2-byte)toalong(4-byte)

Example:PointerstoLocalVariables51void s_helper(intx, int*accum) {if (x <= 1) {return;} else {intz = *accum* x;*accum= z;s_helper(x-1,accum);}}intsfact(intx) {intval= 1;s_helper(x, &val);return val;}Top-LevelCallRecursiveProcedurePasspointertoupdatelocationsfact(3)val=1s_helper(3,&val)val=3s_helper(2,&val)val=6s_helper(1,&val)val=6.

Temp.Space%espCreating&InitializingPointer52intsfact(intx) {intval= 1;s_helper(x, &val);return val;}_sfact:pushl %ebp# Save %ebpmovl %esp,%ebp# Set %ebpsubl $16,%esp# Add 16 bytes movl 8(%ebp),%edx# edx = xmovl $1,-4(%ebp)# val = 1InitialpartofsfactxRtnadrOld%ebp0 4 8 -4 val = 1Unused-12 -8 -16_sfact:pushl%ebp# Save %ebpmovl%esp,%ebp# Set %ebpsubl$16,%esp# Add 16 bytes movl8(%ebp),%edx# edx= xmovl$1,-4(%ebp)# val= 1_sfact:pushl%ebp# Save %ebpmovl%esp,%ebp# Set %ebpsubl$16,%esp# Add 16 bytes movl8(%ebp),%edx# edx= xmovl$1,-4(%ebp)# val= 1_sfact:pushl%ebp# Save %ebpmovl%esp,%ebp# Set %ebpsubl$16,%esp# Add 16 bytes movl8(%ebp),%edx# edx= xmovl$1,-4(%ebp)# val= 1%esp%esp%ebpMuststorevalinmemory(stack);registersdonothaveaddresses.

PassingPointer53intsfact(intx) {intval= 1;s_helper(x, &val);return val;}leal-4(%ebp),%eax# Compute &valpushl%eax# Push on stackpushl%edx# Push xcall s_helper# callmovl-4(%ebp),%eax# Return val•••# FinishCallings_helperfromsfactxRtnadrOld%ebp%ebp0 4 8 val = 1-4 Unused-12 -8 -16%espx&valStackattimeofcall:leal-4(%ebp),%eax# Compute &valpushl%eax# Push on stackpushl%edx# Push xcall s_helper# callmovl-4(%ebp),%eax# Return val•••# Finishleal-4(%ebp),%eax# Compute &valpushl%eax# Push on stackpushl%edx# Push xcall s_helper# callmovl-4(%ebp),%eax# Return val•••# Finishval=x!Muststorevalinmemory(stack);registersdonothaveaddresses.

IA32/LinuxProcedureSummarycall, ret, push, popStackdisciplinefitsprocedurecall/return.*IfPcallsQ,QreturnsbeforeP,includingrecursion.Conventionssupportarbitraryfunctioncalls.Safelystoreper-callvaluesinlocalstackframeandcallee-savedregistersPushfunctionargumentsattopofstackbeforecallResultreturnedin%eax*Take251tolearnaboutlanguageswhereitdoesn't.ReturnAddrSavedRegisters+LocalVariablesArgumentBuildOld%ebpArgumentsCallerFrame%ebp%esp54