How are shifts implemented on the hardware level?

cpu processor bit-shift circuit alu
Matt picture Matt · Jun 7, 2012 · Viewed 7.7k times · Source

How are bit shifts implemented at the hardware level when the number to shift by is unknown?

I can't imagine that there would be a separate circuit for each number you can shift by (that would 64 shift circuits on a 64-bit machine), nor can I imagine that it would be a loop of shifts by one (that would take up to 64 shift cycles on a 64-bit machine). Is it some sort of compromise between the two or is there some clever trick?

Answer

Martin Thompson picture Martin Thompson · Jun 7, 2012

The circuit is called a "barrel shifter" - it's a load of multiplexers basically. It has a layer per address-bit-of-shift-required, so an 8-bit barrel shifter needs three bits to say "how much to shift by" and hence 3 layers of muxes.

Here's a picture of an 8-bit one from http://www.globalspec.com/reference/55806/203279/chapter-9-additional-circuit-designs:

enter image description here

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