Bitwise Manipulation - Shifts
Computer Science: OCR A Level, topic 4.1.18
Bitwise Manipulation - Shifts
Bitwise manipulation is the act of modifying data by changing the bits within individual bytes.
Logical shift right
- A logical shift right moves each of the bits in a byte one place to the right.
- The ‘least significant bit’ (LSB) on the right is lost and the empty space for the ‘most significant bit’ (MSB) is filled with a ‘0’.
- For example, 1 0 1 0 1 0 0 0 0 1 0 1 0 1 0 0
- A logical shift right is identified with a triple chevron ‘>>>’.
Logical shift left
- A logical shift left moves each of the bits in a byte one place to the left.
- The MSB on the left is lost and the empty space for the LSB is filled with a ‘0’.
- For example: 1 0 1 0 1 0 0 0 0 1 0 1 0 0 0 0
- A logical shift left is identified with a ‘<<<’.
Arithmetic shift right
- Arithmetic shifts are almost the same as logical shifts, except the MSB is a sign bit and always remains the same.
- An arithmetic shift right has the following result:
- 1 0 1 0 1 0 0 0 1 0 0 1 0 1 0 0
- This has the effect of dividing the original number by 2.
- An arithmetic shift right is identified with a '>>'.
Arithmetic shift left
- An arithmetic shift left has the following result:
- 1 0 1 0 1 0 0 0 1 1 0 1 0 0 0 0
- This has the effect of multiplying the original number by 2.
- An arithmetic shift left is identified with a '<<'.
Circular shift
- With a right circular shift, the same procedure applies.
- However, instead of losing the LSB in a right shift, it ‘wraps around’ and becomes the MSB.
- 1 0 1 0 1 0 0 0 0 1 0 1 0 1 0 0
- Similarly, with a left circular shift, the MSB becomes the new LSB.
- 1 0 1 0 1 0 0 0 0 1 0 1 0 0 0 1