Perl Bitwise Operators Example
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In digital computer programminga bitwise operation operates on one or more bit patterns or binary numerals at the level of their individual bits. It is a fast, simple action anding 2 binary values perl supported by the processorand is used to manipulate values for comparisons and calculations. On simple low-cost processors, typically, bitwise operations are substantially faster than division, several times faster than multiplication, and sometimes significantly faster than addition.
While modern processors usually perform addition and multiplication just as fast as bitwise operations due to their longer instruction pipelines and other architectural design choices, bitwise operations do commonly use less power because of the reduced use of resources. In the explanations below, any indication of a bit's position is counted from the right least significant side, advancing left.
For example, the binary value decimal 1 has zeroes at every position but the first one. The bitwise NOTor complementis a unary operation that performs logical negation on each bit, forming the ones' complement of the given binary value. Bits that are 0 become 1, and those that are 1 become 0. The bitwise complement is equal to the two's complement of the value minus one. For unsigned integersthe bitwise complement of a number is the "mirror reflection" of anding 2 binary values perl number across the half-way point of the unsigned integer's range.
A simple but illustrative example use is to invert a grayscale image where each pixel is stored as an unsigned integer. A bitwise AND takes two equal-length binary representations and performs the logical AND operation on each pair of the corresponding bits, by multiplying them.
The operation may be used to determine whether a particular bit is set 1 or clear 0. For example, given a bit pattern decimal 3to determine whether the second bit is set we use a bitwise AND with a bit pattern containing 1 only in the second bit:. Because the result is non-zero, we know the second bit in the original pattern was set.
This is often called bit masking. By analogy, the use of masking tape covers, or masksportions that should not be altered or portions that are not of interest. In this case, the 0 values mask anding 2 binary values perl bits that are not of interest.
The bitwise AND may be used to clear selected bits or flags of a register in which each bit represents an individual Boolean state. This technique is an efficient way to store a number of Boolean values using as little memory as possible. For example, decimal 6 can be considered a set of four flags, where the first and fourth flags are clear 0and the second and third flags are set 1. Anding 2 binary values perl second bit may be cleared by using a bitwise AND with the anding 2 binary values perl that has a zero only in the second bit:.
Because of this property, it becomes easy to check the parity of a binary anding 2 binary values perl by checking the value of the lowest valued bit. Using the example above:. A bitwise OR takes two bit patterns of equal length and performs the logical inclusive OR operation on each pair of corresponding bits. The result in each position is 0 if both bits are 0, while otherwise the result is 1.
The bitwise OR may be used to set to 1 the selected bits of the register described above. For example, the fourth bit of decimal 2 may be set by performing a bitwise OR with the pattern with only the fourth bit set:. A bitwise XOR takes two bit patterns of equal length and performs the logical exclusive OR operation on each pair of corresponding bits. The result in anding 2 binary values perl position is 1 if only the first bit is 1 or only anding 2 binary values perl second bit is 1, but will be 0 if both are 0 or both are 1.
In this we perform the comparison of two bits, being 1 if the two bits are different, and 0 if they are the same.
The bitwise XOR may be used to invert selected bits anding 2 binary values perl a register also called toggle or flip. Any bit may be toggled by XORing it with 1. For example, given the bit pattern decimal 2 the second and fourth bits may be toggled by a bitwise XOR with a bit pattern containing 1 in the second and fourth positions:. Assembly language programmers and optimizing compilers sometimes use XOR as a short-cut to setting the value of a register to zero.
Performing XOR on a value against itself always yields zero, and on many architectures this operation requires fewer clock cycles and memory anding 2 binary values perl loading a zero value and saving it to the register. The bit shifts are sometimes considered bitwise operations, because they treat a value as a series of bits rather than as a numerical quantity.
In these operations the digits are moved, or anding 2 binary values perlto the left or right. Registers in a computer processor have a fixed width, so some bits will be "shifted out" of the register at one end, while the same number of bits are "shifted in" from the other end; the differences anding 2 binary values perl bit shift operators lie in how they determine the values of the shifted-in bits.
In an arithmetic shiftthe bits that are shifted out of either end are discarded. In a left arithmetic shift, zeros are shifted in on the right; in a right arithmetic shift, the sign bit the MSB in two's complement is shifted in on the left, thus preserving the sign of the operand.
In the first case, the leftmost digit was shifted past the end of the register, and a new 0 was shifted into the rightmost position. In the second case, the rightmost 1 was shifted out perhaps into the carry flagand a new 1 was copied into the leftmost position, preserving the sign of the number.
Multiple shifts are sometimes shortened to a single shift by some number of digits. A left arithmetic shift by n is equivalent to multiplying by 2 n provided the value does not overflowwhile a right arithmetic shift by n of a two's complement value is equivalent to dividing by anding 2 binary values perl n and rounding toward negative infinity. If the binary number is treated as ones' complementthen the same right-shift operation results in division by 2 n and rounding toward zero.
In a logical shiftzeros are shifted in to replace the discarded bits. Therefore, the logical and arithmetic left-shifts are exactly the same. However, as the logical right-shift inserts value 0 bits into the most significant bit, instead of anding 2 binary values perl the sign bit, it is ideal for unsigned binary numbers, while the arithmetic right-shift is ideal for signed two's complement binary numbers. Another form of shift is the circular shift or bit rotation.
In this operation, the bits are "rotated" as if the left and right ends of the register were joined. The value that is shifted in on the right during a left-shift is whatever value was shifted out on the left, and vice versa. This operation is useful if it is necessary to retain all the existing bits, and is frequently used in digital cryptography.
Rotate through carry is similar to the rotate no carry operation, but the two ends of the register are separated by the carry flag. The bit that is shifted in on either end is the old value of the carry flag, and the bit that is shifted out on the other end becomes the new value of the carry flag. A single rotate through carry can simulate a logical or arithmetic shift of one position by setting up the carry flag beforehand. For this reason, some microcontrollers such as low end PICs just have rotate and rotate through carryand don't bother with arithmetic or logical shift instructions.
Rotate through carry is especially useful when performing shifts on numbers larger than the processor's native word sizebecause if a large number is stored in two registers, the bit that is shifted off the end of the first register must come in at the other end of the second.
With rotate-through-carry, that bit is "saved" in the carry flag during the first shift, ready to shift in during the second shift without any extra preparation. The number of places to shift is given as the second argument to the shift operators.
Shifts can result in implementation-defined behavior or undefined behaviorso care must be taken when using them.
If the first operand is of type uint or ulong, the right-shift is a logical shift. The C-family of languages lack a rotate operator, but one can be synthesized from the shift operators. Care must be taken to ensure the statement is well formed to avoid undefined behavior and timing attacks in software with security requirements. A second try might result in:. However, the branch adds an additional code path and presents an opportunity for timing analysis and attack, which is often not acceptable in high integrity software.
To avoid the undefined behavior and branches under GCC and Clang, the following should anding 2 binary values perl used.
The number of places to shift is given as the second argument. For example, the following assigns x the result of shifting y to the left by two bits:. Bitwise operations are necessary particularly in lower-level programming such as device drivers, low-level graphics, communications protocol packet assembly, and decoding.
Although machines often have efficient built-in instructions for performing arithmetic and logical operations, all these operations can be performed by combining the bitwise operators and zero-testing in anding 2 binary values perl ways. Another example is a pseudocode implementation of addition, showing how to calculate anding 2 binary values perl sum of two integers a and b using bitwise operators and zero-testing:.
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