x86 - Difference between JA and JG in assembly

Question

Can you please tell me the difference between JUMP IF ABOVE AND JUMP IF GREATER in Assembly language? when do i use each of them? do they give me different results?

Answer 1

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 if CF = 0 and ZF = 0 (unsigned Above: no carry and not equal)
• jg jumps if SF = OF and ZF = 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).