按位运算 - 零填充右移 (>>>) 用法? [英] Bitwise operation - Zero-fill right shift (>>>) usages?

问题描述

一般来说，位移位(>> , <<)允许我们除以/乘以 ^2

Generally speaking, bit shifting (>> , <<) allows us to divide / multiply by ^2

例子:

      9 (base 10): 00000000000000000000000000001001 (base 2)
                   --------------------------------
 9 >> 2 (base 10): 00000000000000000000000000000010 (base 2) = 2 (base 10)

对于负数字:

同样，-9 >>2 产生 -3，因为符号被保留:

Likewise, -9 >> 2 yields -3, because the sign is preserved:

     -9 (base 10): 11111111111111111111111111110111 (base 2)
                   --------------------------------
-9 >> 2 (base 10): 11111111111111111111111111111101 (base 2) = -3 (base 10)

但是看看 >>> 它对于正数的行为相同，但对于负数的行为不同:

But looking at >>> which acts the same for positive numbers, but behaves differently for negative numbers :

mdn

零位从左边移入

我找不到从左侧移动 0 的任何原因/用法(这使得整数为正数):

I can't find any reason / usage for shifting 0 from the left ( which makes the whole number positive) from the left :

       -9 (base 10): 11111111111111111111111111110111 (base 2)
                     --------------------------------
 -9 >>> 2 (base 10): 00111111111111111111111111111101 (base 2) = 1073741821 (base 10)

问题:

在什么情况下我应该使用 >>> ?我不明白我为什么要从左边填充零并弄乱我的负数.

In what scenarios should I use >>> ? I don't understand why should I ever want to pad zeros from the left and mess my negative number.

推荐答案

假设你正在编写一些东西来模仿一个硬件，特别是 移位寄存器.

Let's say you were programming something to mimic a piece of hardware, specifically a shift register.

为了方便起见，我将使用 8 位而不是您的问题中的 32 位.

To make things easier I'll use 8 bits instead of 32 bits as in your question.

每次我们想向这个移位寄存器输入一个高位时，我们可以添加 128，因为它会使最左边的位变为 1.

We can add 128 every time we want to feed a high bit into this shift-register, since it will make the leftmost bit 1.

// Assume n is initialised to 0, so n = 00000000 in binary
n += 128;                    // now n = 10000000 in binary

如果我们每次你想模拟一个时钟周期时都使用 >>> 进行移位，那么在 8 个时钟周期"之后，我们将在最右边有那个 1.如果我们读出最右边的位，那么我们将得到 8 个周期前馈入最左边的位的延迟版本.

If we shift using >>> every time you want to simulate a clock cycle, then after 8 "clock cycles" we will have that 1 at the rightmost bit. If we read that rightmost bit out then we will get a delayed version of what was fed into the leftmost bit 8 cycles ago.

这只是位不被解释为数字的一个例子，我相信还有更多.我认为您会在其他地方找到更多用途，尤其是在旨在模仿硬件电路/构建块的软件中.

This is only one example where the bits are not interpreted as a number, and I am sure there are many more. I think you'll find a few more uses elsewhere, especially in software meant to mimic hardware circuits/building blocks.

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