你怎么只设置在C字节的某些位而不影响其他人呢？ [英] How do you set only certain bits of a byte in C without affecting the rest?

问题描述

说我有这样的1010XXXX，其中X值可以是任何一个字节。我想设置低四位为特定图案，说1100，同时使上四比特不受影响。我将如何做到这一点的最快用C？

Say I have a byte like this 1010XXXX where the X values could be anything. I want to set the lower four bits to a specific pattern, say 1100, while leaving the upper four bits unaffected. How would I do this the fastest in C?

推荐答案

您可以将所有这些位通过按位安定设置为0的4位和所有其他设置为1（设置为0，这是对4的补设置为1比特）。然后，您可以按位或位，你通常会。

You can set all those bits to 0 by bitwise-anding with the 4 bits set to 0 and all other set to 1 (This is the complement of the 4 bits set to 1). You can then bitwise-or in the bits as you would normally.

即

 val &= ~0xf; // Clear lower 4 bits. Note: ~0xf == 0xfffffff0
 val |= lower4Bits & 0xf; // Worth anding with the 4 bits set to 1 to make sure no
                          // other bits are set.

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