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ECS50-L3

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📗 -> 9/30/24:

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Lecture Slides

🎤 Vocab

❗ Unit and Larger Context

Bitwise operations are useful:

  • They’re very fast
  • Very useful for boolean representations
    • Read/Write/Execution Permissions:
      • 755 (rwxr-xr-x): 111 / 101 / 101
      • 700 (rwx------): 111 / 000 / 000

Bitwise operations to accomplish a task:

Uses for AND:

Check to see if a bit is a 0 or 1

  • Create a new variable called mask that has 1’s in the positions you want to check and 0’s in all other positions
  • if(A & mask) then at least one of the bits in that was set to 1 in the mask is set to 1 in A
  • If not then all of the positions in A you were trying to check were 0’s
    Change the value of a bit to 0
  • Create a new variable called mask that has 0’s in the positions that you want to change to 0 and 1’s everywhere else
  • A &= mask

Uses for OR:

Setting bits to 1

  • Create a new variable called mask that has 1’s in the positions that you want to change and 0’s everywhere else

  • A |= mask;

  • OR with 1

  • Bits will become 1 if 0, and remain 1 if 1

Uses for XOR:

Flip particular bits and leave the rest unchanged

  • Create a new variable called Mask that has

    • 1’s at the position you want to flip
    • 0’s at the positions you want to leave unchanged

  • Variables ^= Mask

  • XOR with a string of bits, with 1 being the bit to be flipped

    • Only the bits with 1s are flipped, rest are kept the same

Uses for bit shift:

  • There are times when you may have multiple fields packed into a single variable and will need to extract them
  • To extract a field use the following procedure
    • Shift the variable containing the field to the right so that the least significant bit of the field ends up at index 0
    • Then and the field with a mask that has as many 1’s in it as there are bits in the field

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