Assembly - JG JNLE JL JNGE after CMP

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 gt dl  or al-dl gt 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

User · Answer

The command JG simply means  Jump if Greater  The result of the preceding instructions is stored in certain processor flags  in this it would test if ZF 0 and SF OF  and jump instruction act according to their state

User · Answer

Wikibooks has a fairly good summary of jump instructions  Basically  there s actually two stages   cmp instruction op1  op2   Which sets various flags based on the result  and  jmp conditional instruction address   which will execute the jump based on the results of those flags   Compare  cmp  will basically compute the subtraction op1-op2  however  this is not stored  instead only flag results are set  So if you did cmp eax  ebx that s the same as saying eax-ebx - then deciding based on whether that is positive  negative or zero which flags to set   More detailed reference here

User · Answer

When you do a cmp a b  the flags are set as if you had calculated a - b   Then the jmp-type 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  gt  dl but the two choices you have there are mathematically equivalent   al  gt  dl al - dl  gt  dl - dl  subtract dl from both sides  al - dl  gt  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  First the ones where signed-ness is not appropriate    -------- ------------------------------ ------------- --------------------   Instr     Description                    signed-ness   Flags                 -------- ------------------------------ ------------- --------------------    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       Jump if equal                                ZF   1                 JZ       Jump if zero                                                       -------- ------------------------------ ------------- --------------------    JNE      Jump if not equal                            ZF   0                 JNZ      Jump if not zero                                                   -------- ------------------------------ ------------- --------------------    JP       Jump if parity                               PF   1                 JPE      Jump if parity even                                                -------- ------------------------------ ------------- --------------------    JNP      Jump if no parity                            PF   0                 JPO      Jump if parity odd                                                 -------- ------------------------------ ------------- --------------------    JCXZ     Jump if CX is zero                           CX   0                 JECXZ    Jump if ECX is zero                          ECX   0               -------- ------------------------------ ------------- --------------------    Then the unsigned ones    -------- ------------------------------ ------------- --------------------   Instr     Description                    signed-ness   Flags                 -------- ------------------------------ ------------- --------------------    JB       Jump if below                  unsigned      CF   1                 JNAE     Jump if not above or equal                                          JC       Jump if carry                                                      -------- ------------------------------ ------------- --------------------    JNB      Jump if not below              unsigned      CF   0                 JAE      Jump if above or equal                                              JNC      Jump if not carry                                                  -------- ------------------------------ ------------- --------------------    JBE      Jump if below or equal         unsigned      CF   1 or ZF   1       JNA      Jump if not above                                                  -------- ------------------------------ ------------- --------------------    JA       Jump if above                  unsigned      CF   0 and ZF   0      JNBE     Jump if not below or equal                                         -------- ------------------------------ ------------- --------------------    And  finally  the signed ones    -------- ------------------------------ ------------- --------------------   Instr     Description                    signed-ness   Flags                 -------- ------------------------------ ------------- --------------------    JL       Jump if less                   signed        SF  lt  gt  OF               JNGE     Jump if not greater or equal                                       -------- ------------------------------ ------------- --------------------    JGE      Jump if greater or equal       signed        SF   OF                JNL      Jump if not less                                                   -------- ------------------------------ ------------- --------------------    JLE      Jump if less or equal          signed        ZF   1 or SF  lt  gt  OF     JNG      Jump if not greater                                                -------- ------------------------------ ------------- --------------------    JG       Jump if greater                signed        ZF   0 and SF   OF     JNLE     Jump if not less or equal                                          -------- ------------------------------ ------------- --------------------

User · Answer

Addition and subtraction in two s complement is the same for signed and unsigned numbers  The key observation is that CMP is basically subtraction  and      In two s complement  integer representation used by x86   signed and unsigned addition are exactly the same operation   This allows for example hardware developers to implement it more efficiently with just one circuit   So when you give input bytes to the x86 ADD instruction for example  it does not care if they are signed or not   However  ADD does set a few flags depending on what happened during the operation    carry  unsigned addition or subtraction result does not fit in bit size  e g   0xFF   0x01 or 0x00 - 0x01  For addition  we would need to carry 1 to the next level  sign  result has top bit set  I e   is negative if interpreted as signed  overflow  input top bits are both 0 and 0 or 1 and 1 and output inverted is the opposite   I e  signed operation changed sigedness in an impossible way  e g  positive   positive or negative   We can then interpret those flags in a way that makes comparison match our expectations for signed or unsigned numbers   This interpretation is exactly what JA vs JG and JB vs JL do for us   Code example  Here is GNU GAS a code snippet to make this more concrete      0x0            0 in 2 s complement signed      0 in 2 s complement unsigned     mov  0   al     0xFF             -1 in 2 s complement signed      255 in 2 s complement unsigned     mov  0xFF   bl     Do the operation  Is al  lt  bl      cmp  bl   al   Note that AT amp T syntax is  backwards   mov src  dst   So you have to mentally reverse the operands for the condition codes to make sense with cmp   In Intel syntax  this would be cmp al  bl  After this point  the following jumps would be taken    JB  because 0  lt  255 JNA  because   0   255  JNL  because   0  lt  -1  JG  because 0   -1   Note how in this particular example the signedness mattered  e g  JB is taken but not JL   Runnable example with assertions   Equals   Negated versions like JLE   JNG are just aliases  By looking at the Intel 64 and IA-32 Architectures Software Developer s Manuals Volume 2 section  Jcc - Jump if Condition Is Met  we see that the encodings are identical  for example   Opcode  Instruction  Description 7E cb   JLE rel8     Jump short if less or equal  ZF 1 or SF   OF   7E cb   JNG rel8     Jump short if not greater  ZF 1 or SF   OF

[assembly] Assembly - JG/JNLE/JL/JNGE after CMP

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