CS 105 Tour of the Black Holes of Computing Machine Level Programming II Control Flow Topics 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 b t CF set if carry out from most significant bit Detects unsigned overflow also used for multiprecision arithmetic 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 CS 105 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 CS 105 Setting Condition Codes cont Explicit setting by Test instruction testl Src1 Src2 Sets condition codes based on value of Src1 Src2 Intel thought it useful to have one of the operands be a mask Compiler usually sets Src1 and Src2 the same 4 testl a b like computing a b without setting destination ZF set when a b 0 SF set when a b 0 CS 105 Reading Condition Codes SetX instructions 5 Set single byte based on combinations of condition codes CS 105 Reading Condition Codes Cont SetX instructions Set single byte based on combinations of condition codes eax ah al One of 8 addressable byte registers edx dh dl ecx ch cl ebx bh bl 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 esi edi esp ebp eax y Compare x y al x y Zero rest of eax Note inverted ordering CS 105 jX instructions 7 Jumping Jump to different part of code depending on condition codes CS 105 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 CS 105 Conditional Branch Ex 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 edx eax x y if eax Done Nearly always very bad coding style x y goto L9 x Skipped when x y CS 105 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 10 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 CS 105 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 x eax result 11 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 CS 105 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 CS 105 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 13 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 CS 105 Actual While Loop Translation C Code int fact while int x int result 1 while x 1 result x x x 1 return result 14 Uses same inner loop as do while version Guards loop entry with extra test 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 CS 105 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 CS 105 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 CS 105 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 17 CS 105 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 result 1 Body Test p 0 Update p p 1 if p 0x1 result x x x x 18 CS 105 For While For Version for Init Test Update Body Do While Version Init if Test goto done do Body Update while Test done 19 While Version Init while Test Body Update Goto Version Init if Test goto done loop Body Update if Test goto loop done CS 105 For Loop Compilation Goto Version result 1 if p 0 goto done loop if p 0x1 result x x x x p p 1 if p 0 goto loop done Init if Test goto done loop Body Update if Test goto loop done Init result 1 Test p 0 Body if p 0x1 result x x x x Update p p 1 20 CS 105 typedef enum ADD MULT MINUS DIV MOD BAD op type char unparse symbol op type op switch …
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