使用SAR与SHR在汇编中将二进制数右移 [英] shifting a binary number to the right in assembly with SAR vs. SHR

问题描述

我们知道，当我们向右移动二进制数时，它会被2除. 对于数字:1001 0001.假设它存储在AX寄存器中:

We know that when we shift a binary number to the right, it is divided by 2. For the number: 1001 0001. Let's assume that is stored in AX register:

• 如果我们将其视为无符号数字(等于十进制的145): SHR AX, 1将使AX等于0100 1000，其等于十进制的72，这是正确的.

• If we consider it as unsigned number (which is equal to 145 in decimal): SHR AX, 1 will make AX equal to 0100 1000 which is equal to 72 in decimal, and this is right.

但是，如果我们将其视为带符号的数字(十进制等于-111):SAR AX, 1将使AX等于1100 0100等于十进制-56，并且这是错误的，因为它应等于十进制的-55.

But if we consider it as signed number (which is equal to -111 in decimal): SAR AX, 1 will make AX equal to 1100 0100 which is equal to -56 in decimal, and this is wrong because it should be equal to -55 in decimal.

为什么?

推荐答案

SHR指令执行无符号移位权，移位为零:

The SHR instruction performs an unsigned shift rights, shifting in zeros:

Input:  1001 0001
        |      |\
        +------+ +--> lost
            v
         +------+
         |      |
Output: 0100 1000
        ^
        +-- added by SHR

另一方面，SAR指令执行带符号右移，将最高有效位解释为符号.它会移动符号位的副本:

On the other hand, the SAR instruction performs a signed shift right, interpreting the most significant bit as a sign. It shifts in copies of the sign bit:

Input:  1001 0001
        |      |\
        +------+ +--> lost
        |   v
        |+------+
        v|      |
Output: 1100 1000
        ^
        +-- preserved (copied) by SAR

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