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Bit Masking (Setting, Clearing and Inverting) Using Boolean Logical Functions
(Page 2 of 3)
Clearing Groups of Bits Using the AND Function
To clear a certain pattern of bits,
you do a similar masking operation, but using the AND function
instead. If you AND a bit with zero, it will clear it to zero
regardless of what the bit was before, while ANDing with one
will leave the bit unchanged. So, to take the same example above and
clear the middle six bits, we AND with the reverse bit mask,
111000000111. This is shown in Table 15
and illustrated in Figure 10.
Figure 10: Clearing Bits Using an AND Bit Mask This diagram shows how a bit mask can be used to clear certain bits in a binary number while preserving others. The mask shown here can be likened to a painters mask; each 1 represents a transparent area that keeps the corresponding input bit value, while each 0 is a bit where the original value is to be cleared. After performing an AND on each bit pair, the first three and last three bits are preserved while the middle six, since they were each ANDed with 0, are forced to 0 in the output.
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Table 15: Clearing Bits Using an AND Bit Mask
Input
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1
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0
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1
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0
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0
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1
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0
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1
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1
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0
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1
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0
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Mask
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1
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1
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1
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0
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0
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0
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0
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0
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0
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1
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1
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1
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Result of AND Operation
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1
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0
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1
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0
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0
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0
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0
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0
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0
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0
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1
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0
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We can also look at this
clearing function a different way. We are clearing the bits
where the mask is a zero, and in so doing selecting the
bits where the mask is a one. Thus, ANDing with a bit mask means
that you keep the bits where the mask is a one and remove
the bits where it is a zero.
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Version 3.0 - Version Date: September 20, 2005
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