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Assembly

Assembly. תרגול 7 תכנות באסמבלי, המשך. 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

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Assembly

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  1. Assembly תרגול 7תכנות 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) || (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

  18. Exercise’s Solution

  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

  26. 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

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