What does the `test` instruction do? [duplicate]

This question already has answers here: 开发者_JAVA技巧 `testl` eax against eax? (8 answers) Closed 5 years ago.

I'm looking at some small assembler codes and I'm having trouble understanding the TEST instruction and its use. I'm looking at the following code at the end of a loop:

8048531:    84 c0                   test   al,al
8048533:    75 dc                   jne    8048511 <function+0x2d>

The way i understand TEST is that it works a bit like the AND operator and it sets some flags. I guess I don't really understand how the flags work. test al,al to me looks like it checks the same lower bits and will always get the same results.

Can someone explain?

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It tests the register against itself, just to set the flags. The result will be different for a zero and a non-zero value.