15 213 The course that gives CMU its Zip Machine Level Programming II Control Flow Sept 14 2000 Topics Condition Codes Setting Testing Control Flow If then else Varieties of Loops Switch Statements class06 ppt Condition Codes Single Bit Registers 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 F 00 Setting Condition Codes cont Explicit Setting by 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 F 00 Reading Condition Codes SetX Instructions 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 F 00 Reading Condition Codes Cont SetX Instructions 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 eax ah al edx dh dl ecx ch cl ebx bh bl esi int gt int x int y return x y movl cmpl setg andl 12 ebp eax eax 8 ebp al 255 eax class06 ppt edi esp Body eax y Compare x eax al x y Zero rest of eax 5 ebp Note inverted ordering CS 213 F 00 Jumping jX Instructions Jump to different part of code depending on condition codes jX 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 F 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 F 00 Conditional Branch Example Cont int goto max int x int y 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 C allows goto as means of transferring control Closer to machine level programming style Generally considered bad coding style x y goto L9 x Skipped when x y CS 213 F 00 Do While Loop Example Goto Version C Code int fact do int x int result 1 do result x x x 1 while x 1 return result int fact goto 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 F 00 Do While Loop Compilation Goto Version int fact goto 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 F 00 General Do While Translation Goto Version C Code do loop Body if Test goto loop 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 F 00 While Loop Example 1 First Goto Version C Code int fact while int x int result 1 while x 1 result x x x 1 return result 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 Is this code equivalent to the do while version Must jump out of loop if test fails class06 ppt 12 CS 213 F 00 Actual While Loop Translation Second Goto Version C Code int fact while int x int result 1 while x 1 result x x x 1 return result int fact while goto2 int x int result 1 if x 1 goto done loop result x x x 1 if x 1 goto loop done return result Uses same inner loop as do while version Guards loop entry with extra test class06 ppt 13 CS 213 F 00 General While Translation C Code while Test Body Do While Version Goto Version if Test goto done do Body while Test done if Test goto done loop Body if Test goto loop done class06 ppt 14 CS 213 F 00 While Loop Example 2 Compute x raised to nonnegative power p 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 Algorithm Exploit property that p p0 2p1 4p2 2n 1pn 1 Gives xp z0 z1 2 z2 2 2 zn 12 2 2 Example zi 1 when pI 0 310 zi x when pI 1 Complexity O log p n times class06 ppt 15 32 38 32 32 2 2 CS 213 F 00 ipwr Computation int ipwr int x unsigned p 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 F 00 While Do While Goto int result 1 while p if p 0x1 result x x x x p p 1 …
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