computer science Computer science is the study of computation, automation, and information. Computer science spans theoretical disciplines (such as algorithms, theory of computation, information theory, and automation) to Applied science, practical discipli ...

, a logical shift is a bitwise operation that shifts all the bits of its operand. The two base variants are the logical left shift and the logical right shift. This is further modulated by the number of bit positions a given value shall be shifted, such as ''shift left by 1'' or ''shift right by n''. Unlike an

arithmetic shift In computer programming, an arithmetic shift is a shift operator, sometimes termed a signed shift (though it is not restricted to signed operands). The two basic types are the arithmetic left shift and the arithmetic right shift. For binary ...

, a logical shift does not preserve a number's sign bit or distinguish a number's exponent from its significand (mantissa); every bit in the operand is simply moved a given number of bit positions, and the vacant bit-positions are filled, usually with zeros, and possibly ones (contrast with a

circular shift In combinatorial mathematics, a circular shift is the operation of rearranging the entries in a tuple, either by moving the final entry to the first position, while shifting all other entries to the next position, or by performing the inverse oper ...

). A logical shift is often used when its operand is being treated as a

sequence In mathematics, a sequence is an enumerated collection of objects in which repetitions are allowed and order matters. Like a set, it contains members (also called ''elements'', or ''terms''). The number of elements (possibly infinite) is calle ...

of bits instead of as a number. Logical shifts can be useful as efficient ways to perform multiplication or division of unsigned

integer An integer is the number zero (), a positive natural number (, , , etc.) or a negative integer with a minus sign (−1, −2, −3, etc.). The negative numbers are the additive inverses of the corresponding positive numbers. In the language ...

s by powers of two. Shifting left by ''n'' bits on a signed or unsigned binary number has the effect of multiplying it by 2^''n''. Shifting right by ''n'' bits on an ''unsigned'' binary number has the effect of dividing it by 2^''n'' (rounding towards 0). Logical right shift differs from arithmetic right shift. Thus, many languages have different

operators Operator may refer to: Mathematics * A symbol indicating a mathematical operation * Logical operator or logical connective in mathematical logic * Operator (mathematics), mapping that acts on elements of a space to produce elements of another sp ...

for them. For example, in

Java Java (; id, Jawa, ; jv, ꦗꦮ; su, ) is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea to the north. With a population of 151.6 million people, Java is the world's List ...

and

JavaScript JavaScript (), often abbreviated as JS, is a programming language that is one of the core technologies of the World Wide Web, alongside HTML and CSS. As of 2022, 98% of Website, websites use JavaScript on the Client (computing), client side ...

, the logical right shift operator is , but the arithmetic right shift operator is . (Java has only one left shift operator (), because left shift via logic and arithmetic have the same effect.) The

programming language A programming language is a system of notation for writing computer programs. Most programming languages are text-based formal languages, but they may also be graphical. They are a kind of computer language. The description of a programming ...

s C,

C++ C++ (pronounced "C plus plus") is a high-level general-purpose programming language created by Danish computer scientist Bjarne Stroustrup as an extension of the C programming language, or "C with Classes". The language has expanded significan ...

, and Go, however, have only one right shift operator, . Most C and C++ implementations, and Go, choose which right shift to perform depending on the type of integer being shifted: signed integers are shifted using the arithmetic shift, and unsigned integers are shifted using the logical shift. All currently relevant C standards (ISO/IEC 9899:1999 to 2011) leave a definition gap for cases where the number of shifts is equal to or bigger than the number of bits in the operands in a way that the result is undefined. This helps allow C compilers to emit efficient code for various platforms by allowing direct use of the native shift instructions which have differing behavior. For example, shift-left-word in

PowerPC PowerPC (with the backronym Performance Optimization With Enhanced RISC – Performance Computing, sometimes abbreviated as PPC) is a reduced instruction set computer (RISC) instruction set architecture (ISA) created by the 1991 Apple Inc., App ...

chooses the more-intuitive behavior where shifting by the bit width or above gives zero, whereas SHL in

x86 x86 (also known as 80x86 or the 8086 family) is a family of complex instruction set computer (CISC) instruction set architectures initially developed by Intel based on the Intel 8086 microprocessor and its 8088 variant. The 8086 was intr ...

chooses to mask the shift amount to the lower bits ''to reduce the maximum execution time of the instructions'', and as such a shift by the bit width doesn't change the value. Some languages, such as the

.NET Framework The .NET Framework (pronounced as "''dot net"'') is a proprietary software framework developed by Microsoft that runs primarily on Microsoft Windows. It was the predominant implementation of the Common Language Infrastructure (CLI) until bein ...

and

LLVM LLVM is a set of compiler and toolchain technologies that can be used to develop a front end for any programming language and a back end for any instruction set architecture. LLVM is designed around a language-independent intermediate repre ...

, also leave shifting by the bit width and above ''unspecified'' (.NET) or ''undefined'' (LLVM). Others choose to specify the behavior of their most common target platforms, such as C# which specifies the x86 behavior.

Example

If the bit sequence 0001 0111 (decimal 23) is logically shifted by one bit position, then: Note: MSB = Most Significant Bit, LSB = Least Significant Bit

References

{{DEFAULTSORT:Logical shift Binary arithmetic Operators (programming) zh:位操作#逻辑移位