1. Assembly תרגול 7 תכנות באסמבלי, המשך

2. Condition Codes Single bit registers  CF – carry flag  ZF – zero flag  SF – sign flag  OF – overflow flag Relevant only for the most recent operation leal does not alter any condition code In logical operations, CF and OF are set to 0 For shift operations, OF is set to 0, CF is the last shifted out bit

3. Setting Condition Codes

4. 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)

5. 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

6. Accessing the Condition Codes

7. Why does it work? Let’s take setl b,a for example  If there is no overflow and a≥b  SF=0, OF=1  If there is no overflow and a<b  SF=1, OF=0  If there is a negative overflow (a>b)  SF=1, OF=1  If there is a positive overflow (a<b)  SF=0, OF=1

8. a < b in assembly Translate the line: return (a<b); Suppose a is in %edx, b is in %eax: cmpl %eax,%edx # compare a to b setl %al # set %al to 0 or 1 movzbl %al,%eax # set %eax to 0 or 1

9. Jump Instructions

10. Unconditional Jump Direct jump jmp L1 Indirect jump  jmp *%eax  jmp *(%eax)

11. Conditional Jump Can’t use indirect jump Use it to implement  if conditions  loops  switch statements

12. Goto in C

13. If Condition in Assembly

14. Do-While Loops

15. Do-While Loops in Assembly

16. While Loops

17. Exercise InitiallyVariableRegister %eax %ebx %ecx %edx

18. Exercise’s Solution InitiallyVariableRegister aa%eax bb%ebx 0i%ecx aresult%edx

19. Exercise’s Solution Note the optimization done by the compiler!

20. For Loops

21. Exercise

22. Exercise’s Solution Note the optimization done by the compiler!

23. Switch Statements in C

24. Switch Statements in Assembly Building the jump table:

25. Switch Statements in Assembly

27. Exercise

28. Q & A Q: What were the values of the case labels in the switch statement body? What cases had multiple labels in the C code? A: The case labels had values -2,0,1,2,3,4 The case with the labels 2 and 3