assemblyx86eflags

Assembly - JG/JNLE/JL/JNGE after CMP

I don't understand the JG/JNLE/JL/JNGE instructions, which come after CMP.

for example, If I have:

CMP al,dl
jg label1

When al=101; dl =200.

On what we ask the jg? Is it on al>dl? or al-dl>0?

Same prolbem on the next code:

test al,dl
jg label1

I don't understand what we compare, and on what we ask the "jg".

In other words, I don't understand when we would jump to label1, and when we wouldn't.

For AT&T syntax, note that the subtraction does the other direction, so read right to left instead of left to right:

Solution

  • When you do a cmp a,b, the flags are set as if you had calculated a - b. Then the conditional jump instructions check those flags to see if the jump should be made.

    In other words, the first block of code you have (with my comments added):

    cmp  al, dl     ; set flags based on the comparison.
jg   label1     ; then jump based on the flags.

    would jump to label1 if and only if al was greater than dl.

    You're probably better off thinking of it as al > dl but the two choices you have there are mathematically equivalent:

    al          >   dl
(al - dl)   >   (dl - dl) [subtract dl from both sides]
(al - dl)   >   0         [cancel the terms on the right hand side]

    You need to be careful when using jg inasmuch as it assumes your values were signed. So, if you compare the bytes 101 (101 in two's complement) with 200 (-56 in two's complement), the former will actually be greater. If that's not what was desired, you should use the equivalent unsigned comparison.

    See here for more detail on jump selection, reproduced below for completeness, in the order of:

    Instruction Description Signed-ness Conditions checked
    JO Jump if overflow OF = 1
    JNO Jump if not overflow OF = 0
    JS Jump if sign SF = 1
    JNS Jump if not sign SF = 0
    JE
    JZ    		 Jump if equal
    Jump if zero    		 ZF = 1
    JNE
    JNZ    		 Jump if not equal
    Jump if not zero    		 ZF = 0
    JP
    JPE    		 Jump if parity
    Jump if parity even    		 PF = 1
    JNP
    JPO    		 Jump if no parity
    Jump if parity odd    		 PF = 0
    JCXZ
    JECXZ
    JRCXZ    		 Jump if CX is zero
    Jump if ECX is zero
    Jump if RCX is zero    		 CX = 0
    ECX = 0
    RCX = 0
    JB
    JNAE
    JC    		 Jump if below
    Jump if not above or equal
    Jump if carry    		 unsigned CF = 1
    JNB
    JAE
    JNC    		 Jump if not below
    Jump if above or equal
    Jump if not carry    		 unsigned CF = 0
    JBE
    JNA    		 Jump if below or equal
    Jump if not above    		 unsigned CF = 1 or ZF = 1
    JA
    JNBE    		 Jump if above
    Jump if not below or equal    		 unsigned CF = 0 and ZF = 0
    JL
    JNGE    		 Jump if less
    Jump if not greater or equal    		 signed SF <> OF
    JGE
    JNL    		 Jump if greater or equal
    Jump if not less    		 signed SF = OF
    JLE
    JNG    		 Jump if less or equal
    Jump if not greater    		 signed ZF = 1 or SF <> OF
    JG
    JNLE    		 Jump if greater
    Jump if not less or equal    		 signed ZF = 0 and SF = OF