Programmers zero a register by combining it with itself
To set a processor register to zero, assembly programmers and compilers often skip loading a zero at all. They XOR the register with itself, which always yields nothing, and on many chips it takes fewer cycles and less memory. It is a small example of how much work gets done by flipping individual bits.
Bitwise operations treat a number as a row of ones and zeros and act on each position separately. NOT flips every bit. AND gives 1 only where both inputs have a 1; OR gives 1 where either does; XOR gives 1 only where the two inputs differ. On cheap, simple processors these are much quicker than division, several times quicker than multiplication and sometimes faster than addition. Modern chips have closed most of that speed gap, but bitwise instructions still tend to draw less power.
Each has a practical job. AND acts as a mask: combine a value with a pattern holding a single 1, and a non-zero answer reveals whether that bit was set, the way masking tape shields parts of a surface. Checking only the lowest bit tells you instantly whether a number is odd or even; 6 AND 1 gives zero, so 6 is even. OR switches chosen bits on, and XOR toggles them. Packing many true-or-false flags into the bits of one register saves memory, a trick used heavily in low-level code.
NOT has a neat visual side. For 8-bit unsigned values it amounts to 255 minus the number, so applying it to every pixel of a greyscale image produces a photographic negative. In two's complement arithmetic, the scheme most computers use for signed numbers, NOT x equals minus x minus one.
Shifts slide bits left or right within a fixed-width register. Shifting left by n places multiplies by 2 to the power n, barring overflow, and an arithmetic right shift copies the sign bit in so negative numbers stay negative. Rotations wrap bits from one end around to the other so nothing is lost, which makes them a staple of cryptography. With two inputs there are exactly 16 possible logical operations; with three, 256.
Source: Bitwise operation
