shl

Shl, short for shift left, is a binary operation that moves each bit in a value toward higher significance by a specified number of positions. Bits shifted out on the left are discarded and zeros are shifted in from the right. The operation is fundamental to low-level bit manipulation and is widely used in arithmetic, graphics, and systems programming.

In programming languages, shift-left is typically implemented as an operator. In C, C++, Java, JavaScript, and C#, the left-shift operator is usually written as <<. For unsigned values it effectively multiplies by 2^n; for signed values the result is subject to the language’s rules about overflow and undefined behavior in some cases. The exact behavior can vary by language and platform, so it is important to consult the relevant specification.

In many assemblers, shl is a mnemonic for the shift-left instruction. In Intel syntax, the instruction can be written as shl reg, imm or shl reg, cl, where the immediate or the value in the count register determines how many positions the bits are moved. The operation may affect processor flags, depending on the architecture and mode.

Examples: In C, int x = 5; int y = x << 1; // y is 10 (assuming no overflow). In x86 assembly, shl eax, 1 doubles the value in the EAX register (subject to width and flags).

See also: Bitwise operations, bit shift, logical shift, rotate left.