Ассемблерная заметка №1

Об инструкции TEST

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

}