如何在x86程序集中设置或清除溢出标志? [英] How can I set or clear overflow flag in x86 assembly?

问题描述

我想编写一个简单的代码(或算法)来设置/清除溢出标志.对于设置OF，我知道我可以使用带符号的值.但是我该如何清除呢?

I want to write a simple code (or algorithm) to set/clear overflow flag. For setting OF, I know that I can use signed values. But how can I clear that?

推荐答案

有许多可能的解决方案.

There are many possible solutions.

例如， test al, al 将清除OF标志，而不会影响寄存器的内容.

For instance, test al, al will clear the OF flag without affecting register contents.

或者，如果您不想影响其他标志，则可以直接修改*FLAGS寄存器.例如，在32位中，看起来像这样:

Or, if you don't want to affect the other flags, you can just directly modify the *FLAGS register. For example, in 32-bit, this would look like:

pushfd                   ; Push EFLAGS onto the stack
and dword [esp], ~0x800  ; Clear bit 11 (OF)
popfd                    ; Pop the modified result back into EFLAGS

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_{根据 Peter Cordes 的建议，将or al, al更改为test al, al. (效果是相同的，但出于性能方面的考虑，后者更好)}

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_{Changed or al, al to test al, al per Peter Cordes' recommendation. (The effects are the same but the latter is better for performance reasons)}

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