15213 Machine Level Programming II Control Flow Topics Feb 3 2000 Condition Codes Setting Testing Control Flow If then else Varieties of Loops Switch Statements class06 ppt Condition Single Bit RegistersCodes CF ZF SF OF Carry Flag Zero Flag Sign Flag Overflow Flag Implicit Setting By Arithmetic Operations addl Src Dest C analog t a b CF set if carry out from most significant bit Used to detect unsigned overflow ZF set if t 0 SF set if t 0 OF set if two s complement overflow a 0 b 0 t 0 a 0 b 0 t 0 Not Set by leal instruction class06 ppt 2 CS 213 S 00 Setting Condition Codes Explicit Setting by cont Compare Instruction cmpl Src2 Src1 cmpl b a like computing a b without setting destination CF set if carry out from most significant bit Used for unsigned comparisons ZF set if a b SF set if a b 0 OF set if two s complement overflow a 0 b 0 a b 0 a 0 b 0 a b 0 Explicit Setting by Test instruction testl Src2 Src1 Sets condition codes based on value of Src1 Src2 Useful to have one of the operands be a mask testl b a like computing a b without setting destination ZF set when a b 0 SF set when a b 0 class06 ppt 3 CS 213 S 00 Reading Condition SetX Instructions Codes Set single byte based on combinations of condition codes SetX Condition Description sete ZF Equal Zero setne ZF Not Equal Not Zero sets SF Negative setns SF Nonnegative setg SF OF ZF Greater Signed setge SF OF Greater or Equal Signed setl SF OF Less Signed setle SF OF ZF Less or Equal Signed seta CF ZF Above unsigned setb CF Below unsigned class06 ppt 4 CS 213 S 00 Reading Condition Codes SetX Instructions Cont Set single byte based on combinations of condition codes One of 8 addressable byte registers Embedded within first 4 integer registers Does not alter remaining 3 bytes Typically use andl 0xFF eax to finish job int gt int x int y return x y movl cmpl setg andl 12 ebp eax eax 8 ebp al 255 eax class06 ppt eax ah al edx dh dl ecx ch cl ebx bh bl esi edi esp Body eax y Compare x eax al x y Zero rest of eax 5 ebp Note inverted ordering CS 213 S 00 jX Instructions Jumpi ng Jump to different part of code depending on condition jX codes Condition Description jmp 1 Unconditional je ZF Equal Zero jne ZF Not Equal Not Zero js SF Negative jns SF Nonnegative jg SF OF ZF Greater Signed jge SF OF Greater or Equal Signed jl SF OF Less Signed jle SF OF ZF Less or Equal Signed ja CF ZF Above unsigned jb CF Below unsigned class06 ppt 6 CS 213 S 00 Conditional Branch Example max pushl ebp movl esp ebp int max int x int y if x y return x else return y movl 8 ebp edx movl 12 ebp eax cmpl eax edx jle L9 movl edx eax Body L9 movl ebp esp popl ebp ret class06 ppt Set Up 7 Finish CS 213 S 00 Conditional Branch Example Cont int goto max int x int y C allows goto as means of transferring control Closer to machine level programming style Generally considered bad coding style int rval y int ok x y if ok goto done rval x done return rval movl 8 ebp edx movl 12 ebp eax cmpl eax edx jle L9 movl edx eax L9 class06 ppt edx eax x y if eax Done 8 x y goto L9 x Skipped when x y CS 213 S 00 Do While Loop Example Goto Version fact goto int C Code int fact do int x int result 1 do result x x x 1 while x 1 return result int x int result 1 loop result x x x 1 if x 1 goto loop return result Use backward branch to continue looping Only take branch when while condition holds class06 ppt 9 CS 213 S 00 Do While Loop Compilation Goto Version fact goto int int x int result 1 loop result x x x 1 if x 1 goto loop return result Registers edx eax x result class06 ppt Assembly fact goto pushl ebp movl esp ebp movl 1 eax movl 8 ebp edx Setup Setup eax 1 edx x L11 imull edx eax decl edx cmpl 1 edx jg L11 result x x Compare x 1 if goto loop movl ebp esp popl ebp ret 10 Finish Finish Finish CS 213 S 00 C Code General Do While Translation Goto loop Version Body if Test goto loop do Body while Test Body can be any C statement Typically compound statement Statement1 Statement2 Statementn Test is expression returning integer 0 interpreted as false 0 interpreted as true class06 ppt 11 CS 213 S 00 C Code While Loop Example 1 First Goto Version int fact while goto int x int result 1 loop if x 1 goto done result x x x 1 goto loop done return result int fact while int x int result 1 while x 1 result x x x 1 return result Is this code equivalent to the do while version Must jump out of loop if test fails class06 ppt 12 CS 213 S 00 Actual While Loop Translation Second Goto Version fact while goto2 C Code int fact while int x int result 1 while x 1 result x x x 1 return result Uses same inner loop as do while version Guards loop entry with extra test class06 ppt 13 int int x int result 1 if x 1 goto done loop result x x x 1 if x 1 goto loop done return result CS 213 S 00 General While Translation C Code while Test Body Goto Version Do While Version if Test if Test goto done loop Body if Test goto loop done goto done do Body while Test done class06 ppt 14 CS 213 S 00 While Loop Example 2power x raised to nonnegative Compute int ipwr while int x unsigned p int result 1 while p if p 0x1 result x x x x p p 1 return result p Algorithm Exploit property that p p0 2p1 4p2 2n 1pn 1 Gives xp z0 z1 2 z2 2 2 zn 12 2 2 zi 1 when pI 0 zi x when pI 1 Complexity O log p Example 310 3 2 38 n times 32 32 2 2 class06 ppt 15 CS 213 S 00 ipwr Computation x unsigned p int ipwr int int result 1 while p if p 0x1 result x x x x p p 1 return result result x 1 1 9 9 531441 class06 ppt 16 3 9 81 6561 43046721 p 10 5 2 1 0 CS 213 S 00 While Do While Goto result 1 int while p if p 0x1 result x x x x p p 1 int result 1 if p goto done loop …
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