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?

Solution 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).

Solution 2

JA is used for jumping if the last "flag changing" instruction was on unsigned numbers. but on the other hand, JG is used for jumping if the last "flag changing" instruction was on signed numbers.

Comments

[Peter Cordes]

Above (ja) is unsigned, Greater (jg) is signed. See Understanding Carry vs. Overflow conditions/flags for signed vs. unsigned to learn more about how exactly they get set that way by cmp, sub, add, or other instructions. See also jcc in the instruction set reference. Other links in the x86 tag wiki.

[Ciro Santilli OurBigBook.com]

Possible duplicate of Assembly - JG/JNLE/JL/JNGE after CMP

[Peter Cordes]

Assembly Language: difference between ja and jg? has an 8-bit example pointing out that 0x80 is 128 as unsigned, and -128 as signed. Good dup target for questions where the problem is not realizing that numbers with their high bit set are negative for jl / jg and compare less than any number without that bit set.

[user3157687]

Could you give me an example please? does that mean JA ignores the negative sign?

[harold]

@user3157687 there is no sign. There are only condition flags. ja ignores the sign flag (SF) though. Example incoming..

[user3157687]

sorry but what do you mean by u, s ?:)

[harold]

@user3157687 unsigned and signed

[user3157687]

So, if i say for example CMP 5, -6 JA somewhere. it will not jump right? because 6>5 ?

[harold]

@user3157687 no, if you have cmp 5, -6 \ ja somewhere (ignore the syntax error), it will not jump (in that you are right), but the reason is that -6 (aka 0xfffffffa) is much bigger than 5, not that 6 is bigger than 5. cmp 5, -1 wouldn't jump either, -1 is even bigger than -6. "Unsigned bigger", of course.

[harold]

@user3157687 well maybe you could look at it like that, but really you should just look at 1) how it interprets the flags, and 2) what those flags mean in context

[harold]

Yes, 4 is clearly bigger than 2, signedness doesn't even matter

[harold]

@user3157687 Yes, and it would also jump if greater (-1 is both greater than -6 and above -6)

[harold]

@TeeSee Intel, AMD, they don't say vastly different things, mainly the same things in a different format, though there are some actual differences.