3.8 Notes

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3.2-3.8 Learning Notes

CSAPP

int compare with unsigned int

If there is a mix of unsigned and signed in single expression,
signed values implicitly cast to unsigned

2’s Complement–>补码

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Expanding

  • Unsigned: add zeros
  • Signed: sign extension:convert w-bit signed int x to w+k-bit int with same value

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Overflow

Add

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Multiplication

Use shift to replace multiply because it’s much faster

u<<3==u*8
(u<<5)-(u<<3)==u*24

How to use the unsigned

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i will go around and become larger than cnt.
It works even if cnt is signed and <0

Integer Puzzles

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  • overflow in + and *
  • TMin (TMin取反后还是自己，因为只有它不对称)
  • transformation between signed and unsigned

Shift

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  • logical shift: Fill with 0 on left
  • arithmetic shift: Replicate most significant bit on left

Division for signed number

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红色是符号位和arithmetic shift
黄色是数字位

+2^k-1可能会让数字位+1,这样商也会+1