在硬件级别上如何实现转变？ [英] How are shifts implemented on the hardware level?

问题描述

当要转移的数目未知时，如何在硬件级别实现移位？

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

我无法想象每个转移电路都有单独的电路可以移位的数字（在64位计算机上需要64个移位电路），也不能想象这是一个移位循环（在64位计算机上最多需要64个移位周期）。两者之间是某种妥协还是巧妙的技巧？

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?

推荐答案

该电路称为 桶形移位器-基本上是多路复用器的负载。每个地址位需要移位一层，因此一个8位的桶形移位器需要三位来表示要移位多少，因此需要三层复用器。

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.

这是来自 http://www.globalspec.com/reference/55806/203279/chapter-9-additional-circuit-designs ：

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