15 213 The course that gives CMU its Zip Machine Level Programming II Control Flow Jan 29 2004 Topics class06 ppt Condition Codes Setting Testing Control Flow If then else Varieties of Loops Switch Statements Condition Codes Single Bit Registers CF Carry Flag SF Sign Flag ZF Zero Flag OF Overflow Flag Implicitly Set 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 2 15 213 S 04 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 3 15 213 S 04 Setting Condition Codes cont Explicit Setting by Test instruction testl Src2 Src1 4 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 15 213 S 04 Reading Condition Codes SetX Instructions 5 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 15 213 S 04 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 movzbl to finish job Body int gt int x int y return x y movl 12 ebp eax cmpl eax 8 ebp setg al movzbl al eax 6 eax y Compare x y al x y Zero rest of eax eax ah al edx dh dl ecx ch cl ebx bh bl esi edi esp ebp Note inverted ordering 15 213 S 04 jX Instructions 7 Jumping 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 15 213 S 04 Conditional Branch Example max pushl ebp movl esp ebp int max int x int y if x y return x else return y 8 Set Up movl 8 ebp edx movl 12 ebp eax cmpl eax edx jle L9 movl edx eax Body movl ebp esp popl ebp ret Finish L9 15 213 S 04 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 9 C allows goto as means of transferring control Closer to machine level programming style Generally considered bad coding style edx x eax y x y if x y goto done eax x Skipped when x y done 15 213 S 04 Do While Loop Example C Code Goto Version int fact do int x int result 1 do result x x x 1 while x 1 return result 10 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 15 213 S 04 Do While Loop Compilation Registers Goto Version Assembly edx x eax result int fact goto int x int result 1 fact goto pushl ebp movl esp ebp movl 1 eax movl 8 ebp edx Setup Setup eax 1 edx x loop result x x x 1 if x 1 goto loop L11 imull edx eax decl edx cmpl 1 edx jg L11 result x x Compare x 1 if goto loop return result 11 movl ebp esp popl ebp ret Finish Finish Finish 15 213 S 04 General Do While Translation C Code Goto Version do loop Body if Test goto loop Body while Test Body can be any C statement Typically compound statement 12 Statement1 Statement2 Statementn Test is expression returning integer 0 interpreted as false 0 interpreted as true 15 213 S 04 While Loop Example 1 C Code First Goto Version int fact while int x int result 1 while x 1 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 result x x x 1 return result 13 Is this code equivalent to the do while version Must jump out of loop if test fails 15 213 S 04 Actual While Loop Translation C Code Second Goto Version 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 14 Uses same inner loop as do while version Guards loop entry with extra test 15 213 S 04 General While Translation C Code while Test Body 15 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 15 213 S 04 For Loop Example Compute x raised to nonnegative power p int ipwr for int x unsigned p int result for result 1 p 0 p p 1 if p 0x1 result x x x x 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 zi 1 when pi 0 zi x when pi 1 Complexity O log p n 1 times Example 310 32 38 32 32 2 2 16 15 213 S 04 ipwr Computation Compute x raised to nonnegative power p int ipwr for int x unsigned p int result for result 1 p 0 p p 1 if p 0x1 result x x x x return result result 1 1 9 9 531441 17 x 3 9 81 6561 43046721 p 10 5 2 1 0 15 213 S 04 For Loop Example General Form int result for result 1 p 0 p p 1 if p 0x1 result x x x x for Init Test Update Body Init …
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