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Asked: 2026-06-05T14:48:45+00:00

I just started with assembly and it’s going great, but there’s 1 thing that

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I just started with assembly and it’s going great, but there’s 1 thing that I just don’t understand.
How do conditional jumps work?

I have a sample of assembly code here. 

TEST EAX, EAX
FCLEX
JGE SHORT 004022B1

I see that when a certain condition is greater or equal, a jump will be made to 004022B1.

But what is that condition and where is it checked?

I assume the condition must be TEST EAX,EAX. But I’m not sure what it does.

Can anyone explain to me how conditional jumps work and where the condition is checked/stored?

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1 Answer

  1. Editorial Team

    Conditional jumps (and some other instructions) use flags. These flags are bits in the (E/R)FLAGS register. test a, b sets the flags according to the result of and a, b, without updating a with the result. fclex does not change any normal flags (it changes FPU flags of course).

    jge tests* whether the value of the sign flag is equal to the overflow flag. test a, b sets the overflow flag to zero and the sign flag to the signbit of the result of and a, b. So the jump will be taken if eax is positive.

    *: note that it does not test for “greater or equal”. That interpretation is valid when the flags are checked after a cmp (and some other instructions). It really just looks at the flags.

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