cmp %al, %cl
js x
I'm confused on what the js (jump on sign) is doing. Is it saying that if al is positive and cl is negative vice versa then jump?
Also, what happens if %cl is 0?
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Peter Cordes
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JS will jump if the sign flag is set (by an earlier instruction). CMP will always modify the flags by performing a subtraction, in this case %cl - %al.
(This is AT&T syntax, so cmp %cl, %al is the same as Intel syntax cmp al, cl)
Because of the operand-size of the instructions, the sign will be bit 7 of the expression %cl-%al (which is thrown away; EFLAGS is updated just like sub, but not al.)
If al == 0, then the temporary value will be cl exactly and the sign will be the sign of the register cl. Thus a jump is taken if cl is negative.
Here's a reference for all the conditional jumps.
Peter Cordes
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Aki Suihkonen
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So al will minus cl; if al was positive and bigger than cl you would get positive, thus no jump. But if al was positive and small than cl, then you would get negative thus jump. Correct? – user3128376 Feb 19 '14 at 06:45
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Yes, that is correct. (except for "al will minus cl", which I interpret as if al was modified, while it isn't -- sorry, I'm not a native speaker of English) – Aki Suihkonen Feb 19 '14 at 06:46
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1The code samples as posted look like AT&T syntax though? – Patrick Pijnappel Apr 24 '19 at 10:51
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This is most certainly AT&T syntax. – S.S. Anne Aug 30 '19 at 21:03