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Lecture%2017%20-%20%20Bit%20Operations - Lecture 17 Bit...

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Copyright @ 2009 Ananda Gunawardena Lecture 17 Bit Operations In this lecture Background Left Shifting Negative Numbers, one’s complement and two’s complement Right Shifting Bit Operators Masking the Bits Getting the Bits Setting the Bits Binary Files Bit fields More Exercises C is a powerful language and allows programmer many operations for bit manipulation. Data can be accessed at the bit level to make operations and storage more efficient. As you will see, bit operations can be used to do many things including setting flags, encrypting and decrypting images as we will implement in one of the lab assignments. Bit operations can also be used to efficiently pack data into a more compressed form. For example, an entire array of 16 boolean values can be represented by just 2 bytes of data or an IP address such as can be packed into 32 bits of storage. This can come very handy in cases where data needs to be transmitted through a limited bandwidth network or in cases where we simply need to store data more efficiently for better memory management in the application. For example, when programming mobile devices with limited memory, you may want to work at the bit level to make things more efficient and save memory. Also understanding bit operations can be useful in writing device drivers and many other low level applications. First we will discuss the bit shift operations. Left Shifting Think of the following possibility. Suppose we want to multiply an unsigned integer by 2. We can simply shift all bits to the left by one position (assuming no overflow). For example, if a 32-bit unsigned integer 73 is given as
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Copyright @ 2009 Ananda Gunawardena 00000000 00000000 00000000 01001001 Then shifting the bits by 1 position to the left gives us the bit pattern for unsigned 146 00000000 00000000 00000000 10010010 So we can write unsigned int x = 73; x = x<<1; or simply x <<= 1; The operator << is used to shift the bit patterns of a variable. For example, x << n shifts the bit patterns of x by n positions resulting in the number x*2 n assuming there is no overflow. It should be noted that as we left shift, the missing bits on the right are filled by 0’s. Negative Numbers, One’s Complement and Two’s Complement Signed data is generally represented in the computer in their two’s complement. Two’s complement of a number is obtained by adding 1 to its one’s complement. So how do we find one’s complement of a number? Here is the definition One’s complement of x is given by ~x . Obtain the one’s complement of a number by negating each of its binary bits. For example one’s complement of 30 is (represented as a 16- bit short int) 30 = 16 + 8 + 4 + 2 = 00000001 11100000 barb4left binary 30 ~30 = 11111110 00011111 barb4left its one’s complement The two’s complement of the number is obtained by adding 1 to its one’s complement. That is, the two’s complement of 30 is obtained as follows.
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Lecture%2017%20-%20%20Bit%20Operations - Lecture 17 Bit...

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