The point of test eax eax

Question

Possible Duplicate    x86 Assembly -    testl    eax against eax        I m very very new to assembly language programming  and I m currently trying to read the assembly language generated from a binary   I ve run across    test    eax  eax   or test  rdi   rdi  etc  etc  I m very confused as to what this does   Isn t the values in  eax   eax the same   What is it testing   I read somewhere that it is doing the AND operation     but since they are the same value  wouldn t it just return  eax   The following is just one instance where I found this usage      400e6e        85 c0                   test    eax  eax    400e70        74 05                   je     400e77  lt phase 1 0x23 gt    I thought je jumps if the two values being compared are equal      well  because   eax is well  itself  in what situation would we NOT jump   I m a beginner to programming in general  so I d appreciate it very much if someone could explain this to me   Thanks

User · Answer

This checks if EAX is zero  The instruction test does bitwise AND between the arguments  and if EAX contains zero  the result sets the ZF  or ZeroFlag

User · Answer

You are right  that test  and s the two operands  But the result is discarded  the only thing that stays  and thats the important part  are the flags  They are set and thats the reason why the test instruction is used  and exist     JE jumps not when equal  it has the meaning when the instruction before was a comparison   what it really does  it jumps when the ZF flag is set  And as it is one of the flags that is set by test  this instruction sequence  test x x  je     has the meaning that it is jumped when x is 0   For questions like this  and for more details  I can just recommend a book about x86 instruction  e g  even when it is really big  the Intel documentation is very good and precise

User · Answer

Some x86 instructions are designed to leave the content of the operands  registers  as they are and just set unset specific internal CPU flags like the zero-flag  ZF   You can think at the ZF as a true false boolean flag that resides inside the CPU    in this particular case  TEST instruction performs a bitwise logical AND  discards the actual result and sets unsets the ZF according to the result of the logical and  if the result is zero it sets ZF   1  otherwise it sets ZF   0    Conditional jump instructions like JE are designed to look at the ZF for jumping notjumping so using TEST and JE together is equivalent to perform a conditional jump based on the value of a specific register    example     TEST EAX EAX JE some address     the CPU will jump to  some address  if and only if ZF   1  in other words if and only if AND EAX EAX    0 which in turn it can occur if and only if EAX    0   the equivalent C code is     if eax    0        goto some address

User · Answer

CMP subtracts the operands and sets the flags  Namely  it sets the zero flag if the difference is zero  operands are equal    TEST sets the zero flag  ZF  when the result of the AND operation is zero  If two operands are equal  their bitwise AND is zero when both are zero  TEST also sets the sign flag  SF  when the most significant bit is set in the result  and the parity flag  PF  when the number of set bits is even   JE  Jump if Equals  tests the zero flag and jumps if the flag is set  JE is an alias of JZ  Jump if Zero  so the disassembler cannot select one based on the opcode  JE is named such because the zero flag is set if the arguments to CMP are equal   So   TEST  eax   eax JE   400e77  lt phase 1 0x23 gt    jumps if the  eax is zero

User · Answer

test is a non-destructive and  it doesn t return the result of the operation but it sets the flags register accordingly  To know what it really tests for you need to check the following instruction s   Often out is used to check a register against 0  possibly coupled with a jz conditional jump

[assembly] The point of test %eax %eax

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