As Intel’s manual explains, JG interprets the flags as though the comparison was signed, and JA interprets the flags as though the comparison was unsigned (of course if the operation that set the flags was not a comparison or subtraction, that may not make sense). So yes, they’re different. To be precise,
ja
jumps ifCF = 0
andZF = 0
(unsigned Above: no carry and not equal)jg
jumps ifSF = OF
andZF = 0
(signed Greater, excluding equal)
For example,
cmp eax, edx ja somewhere ; will go "somewhere" if eax >u edx ; where >u is "unsigned greater than" cmp eax, edx jg somewhere ; will go "somewhere" if eax >s edx ; where >s is "signed greater than"
>u
and >s
agree for values with the top bit zero, but values with the top bit set are treated as negative by >s
and as big by >u
(of course if both operands have the top bit set, >u
and >s
agree again).