Barrel shifter

A barrel shifter is a digital circuit that can shift a data word by a specified number of bits without the use of any sequential logic, only pure combinational logic, i.e. it inherently provides a binary operation. It can however in theory also be used to implement unary operations, such as logical shift left, in cases where limited by a fixed amount (e.g. for address generation unit). One way to implement a barrel shifter is as a sequence of multiplexers where the output of one multiplexer is connected to the input of the next multiplexer in a way that depends on the shift distance. A barrel shifter is often used to shift and rotate n-bits in modern microprocessors,^[1] typically within a single clock cycle.

For example, take a four-bit barrel shifter, with inputs A, B, C and D. The shifter can cycle the order of the bits ABCD as DABC, CDAB, or BCDA; in this case, no bits are lost. That is, it can shift all of the outputs up to three positions to the right (and thus make any cyclic combination of A, B, C and D). The barrel shifter has a variety of applications, including being a useful component in microprocessors (alongside the ALU).

^ Abhilasha; Sudharshan; Anusha, S.L. (2015). "Design and FPGA Implementation of High Performance 8-BIT Vedic Multiplier Using Reversible Logic Based Barrel Shifter". Proceedings of Second International Conference on Signal Processing, Image Processing and VLSI. Singapore: Research Publishing Services. doi:10.3850/978-981-09-6200-5_vlsi-74.

[1] Abhilasha; Sudharshan; Anusha, S.L. (2015). "Design and FPGA Implementation of High Performance 8-BIT Vedic Multiplier Using Reversible Logic Based Barrel Shifter". Proceedings of Second International Conference on Signal Processing, Image Processing and VLSI. Singapore: Research Publishing Services. doi:10.3850/978-981-09-6200-5_vlsi-74.

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