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The very fastest shifters are implemented as full crossbars, in a manner similar to the 4-bit shifter depicted above, only larger. These incur the least delay, with the output always a single gate delay behind the input to be shifted (after allowing the small time needed for the shift count decoder to settle; this penalty, however, is only incurred when the shift count changes).
For this reason, some microcontrollers such as low end PICs just have rotate and rotate through carry, and 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 size, because if a large number is stored in two registers, the ...
This motivates the following general definition: For a string s over an alphabet Σ, let shift(s) denote the set of circular shifts of s, and for a set L of strings, let shift(L) denote the set of all circular shifts of strings in L. If L is a cyclic code, then shift(L) ⊆ L; this is a necessary condition for L being a cyclic language.
In assembly languages these instructions are represented by mnemonics such as ADD/SUB, ADC/SBC (ADD/SUB including carry), SHL/SHR , ROL/ROR (bit rotates), RCR/RCL (rotate through carry), and so on. [2] The use of the carry flag in this manner enables multi-word add, subtract, shift, and rotate operations.
The two basic types are the arithmetic left shift and the arithmetic right shift. For binary numbers it is a bitwise operation that shifts all of the bits of its operand; every bit in the operand is simply moved a given number of bit positions, and the vacant bit-positions are filled in.
However, do note that a shift operand value which is either a negative number or is greater than or equal to the total number of bits in this value results in undefined behavior. For example, when shifting a 32 bit unsigned integer, a shift amount of 32 or higher would be undefined. Example:
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Logical shift: a logic zero is shifted into the operand. This is used to shift unsigned integers. Rotate: the operand is treated as a circular buffer of bits in which its least and most significant bits are effectively adjacent. Rotate through carry: the carry bit and operand are collectively treated as a circular buffer of bits.