JNZ & CMP Assembly Instructions

Question

Correct me if I am wrong.

This is my understanding of JNZ and CMP.

JNZ - The jump WILL take place if the Z Fl

Answer 1

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 will 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 / not above nor equal
jnc, jae, jnb   CF = 0            not carry / above or equal / not below
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 and ZF = 0      above / not below or 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 / not 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 / not less nor equal