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 Flag is NOT zero  1   CMP - If the two values are equal  the Z Flag is set  1  otherwise it is not set  0     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

User · Answer

You can read JNE Z as    Jump if the status is  quot Not set quot  on Equal Zero flag   quot Not set quot  is a status when  quot equal zero flag quot  in the CPU is set to 0 which only happens when the condition is met or equally matched

User · Answer

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

User · Answer

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

User · Answer

I will make a little bit wider answer here    There are generally speaking two types of conditional jumps in x86    Arithmetic jumps - like JZ  jump if zero   JC  jump if carry   JNC  jump if not carry   etc  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

User · Answer

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

[assembly] JNZ & CMP Assembly Instructions

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