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The shift operators, << and
>> move the bits in the left-side operand as far as specified by the
right-side operand. If m is a mask that allows passage of bits 0
and 3, you can shift the on bits two places to the left, creating a mask
that allows passage of bits 2 and 5:
m
*-*-*-*-*-*-*-*-*
|0|0|0|0|1|0|0|1| ----*
*-*-*-*-*-*-*-*-* |
| | |
*---* *---* v Action of m << 2
| | |
v v |
*-*-*-*-*-*-*-*-* |
|0|0|1|0|0|1|0|0| <---*
*-*-*-*-*-*-*-*-*
Similarly, you can shift the on bits two places to the right, losing the 0 bit, creating a mask that allows passage of bit 1 only:
m
*-*-*-*-*-*-*-*-*
|0|0|0|0|1|0|0|1| ----*
*-*-*-*-*-*-*-*-* |
| |
| |
*---* v Action of m >> 2
| |
v |
*-*-*-*-*-*-*-*-* |
|0|0|0|0|0|0|1|0| <---*
*-*-*-*-*-*-*-*-*