Question

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.

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

• Regarding cmp / jg, jle, etc in AT&T syntax assembly
• What does JL mean in at&t syntax?

Answer 1

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:

• ones that aren’t signed or unsigned compares when used after CMP.
• unsigned ones.
• signed ones.

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