15 213 The course that gives CMU its Zip Machine Level Programming II Control Flow Sept 11 2003 Topics Condition Codes Setting Testing Control Flow If then else Varieties of Loops Switch Statements class06 ppt Condition Codes Single Bit Registers CF ZF Carry Flag Zero Flag SF OF Sign Flag 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 F 03 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 F 03 Setting Condition Codes cont 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 4 15 213 F 03 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 5 15 213 F 03 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 eax ah al edx dh dl ecx ch cl ebx bh bl esi int gt int x int y return x y Body movl 12 ebp eax cmpl eax 8 ebp setg al movzbl al eax 6 edi esp ebp eax y Compare x y al x y Zero rest of eax Note inverted ordering 15 213 F 03 Jumping jX Instructions Jump to different part of code depending on condition codes 7 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 F 03 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 8 Set Up Finish 15 213 F 03 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 C allows goto as means of transferring control Closer to machine level programming style Generally considered bad coding style movl 8 ebp edx edx x movl 12 ebp eax eax y cmpl eax edx x y jle L9 if x y goto L9 movl edx eax eax x Skipped when x y L9 Done 9 15 213 F 03 Do While Loop Example C Code int fact do int x int result 1 do result x x x 1 while x 1 return result Goto Version 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 10 15 213 F 03 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 11 x result 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 Finish Finish Finish 15 213 F 03 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 12 0 interpreted as true 15 213 F 03 While Loop Example 1 C Code int fact while int x int result 1 while x 1 result x x x 1 return result 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 Is this code equivalent to the do while version Must jump out of loop if test fails 13 15 213 F 03 Actual While Loop Translation 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 14 Second Goto Version 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 15 213 F 03 General While Translation C Code while Test Body Do While Version if Test goto done do Body while Test done 15 Goto Version if Test goto done loop Body if Test goto loop done 15 213 F 03 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 16 Example n 1 times 310 32 38 32 32 2 2 15 213 F 03 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 F 03 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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