CODEWITHSUNDEEP
assemblyx86reverse-engineeringollydbgcmp
43.52.4D.
43.52.4D.

Reputation: 980

JNZ & CMP Assembly Instructions

Correct me if I am wrong.

This is my understanding of JNZ and CMP.

JNZ - The jump WILL take place if the Z Flag is NOT zero (1)

CMP - If the two values are equal, the Z Flag is set (1) otherwise it is not set (0)

Olly DBG

This is a flash tutorial I am watching. It is teaching the solution to a simple CrackMe.

As you can see, the previous instruction compared AL with 47h. They were equal which set the Z flag. (You can see it in the Registers windows on the right side)

The next instruction is a JNZ. My understanding was that the jump will take place if the Z flag is set. The Z flag IS set, but the jump doesn't take place!

Why?

Upvotes: 33

Views: 159781

Answers (5)

johnfound
johnfound

Reputation: 7061

I will make a little bit wider answer here.

There are generally speaking two types of conditional jumps in x86:

  1. Arithmetic jumps - like JZ (jump if zero), JC (jump if carry), JNC (jump if not carry), etc.

  2. Comparison jumps - JE (jump if equal), JB (jump if below), JAE (jump if above or equal), etc.

So, use the first type only after arithmetic or logical instructions:

sub  eax, ebx
jnz  .result_is_not_zero 

and  ecx, edx
jz   .the_bit_is_not_set

Use the second group only after CMP instructions:

cmp  eax, ebx
jne  .eax_is_not_equal_to_ebx

cmp  ecx, edx
ja   .ecx_is_above_than_edx

This way, the program becomes more readable and you need never be confused.

Note, that sometimes these instructions are actually synonyms. JZ == JE; JC == JB; JNC == JAE and so on. The full table is following. As you can see, there are only 16 conditional jump instructions, but 30 mnemonics - they are provided to allow creation of more readable source code:

Mnemonic Condition tested Description
jo OF = 1 overflow
jno OF = 0 not overflow
jc, jb, jnae CF = 1 carry / below / neither above nor equal
jnc, jnb, jae CF = 0 not carry / not below / above or equal
je, jz ZF = 1 equal / zero
jne, jnz ZF = 0 not equal / not zero
jbe, jna (CF or ZF) = 1 below or equal / not above
ja, jnbe (CF or ZF) = 0 above / neither below nor equal
js SF = 1 sign
jns SF = 0 not sign
jp, jpe PF = 1 parity / parity even
jnp, jpo PF = 0 not parity / parity odd
jl, jnge (SF xor OF) = 1 less / neither greater nor equal
jge, jnl (SF xor OF) = 0 greater or equal / not less
jle, jng ((SF xor OF) or ZF) = 1 less or equal / not greater
jg, jnle ((SF xor OF) or ZF) = 0 greater / neither less nor equal

Upvotes: 32

Suhailan Safei
Suhailan Safei

Reputation: 1

You can read JNE/Z as *

Jump if the status is "Not set" on Equal/Zero flag

"Not set" is a status when "equal/zero flag" in the CPU is set to 0 which only happens when the condition is met or equally matched.

Upvotes: 0

Charlotte Russell
Charlotte Russell

Reputation: 1475

JNZ     Jump if Not Zero    ZF=0

Indeed, this is confusing right.

To make it easier to understand, replace Not Zero with Not Set. (Please take note this is for your own understanding)

Hence,

JNZ     Jump if Not Set     ZF=0

Not Set means flag Z = 0. So Jump (Jump if Not Set)

Set means flag Z = 1. So, do NOT Jump

Upvotes: 0

43.52.4D.
43.52.4D.

Reputation: 980

At first it seems as if JNZ means jump if not Zero (0), as in jump if zero flag is 1/set.

But in reality it means Jump (if) not Zero (is set).

If 0 = not set and 1 = set then just remember:
JNZ Jumps if the zero flag is not set (0)

Upvotes: 3

Michael F
Michael F

Reputation: 40829

JNZ is short for "Jump if not zero (ZF = 0)", and NOT "Jump if the ZF is set".

If it's any easier to remember, consider that JNZ and JNE (jump if not equal) are equivalent. Therefore, when you're doing cmp al, 47 and the content of AL is equal to 47, the ZF is set, ergo the jump (if Not Equal - JNE) should not be taken.

Upvotes: 47

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