22. ADDR and OFFSET

• The OFFSET operator returns the address of a variable. It is used to specify the location rather than the content of the variable:

          .DATA
              MyVar  DB   77h       ;  byte-sized variable called MyVar initialized to 77h
          .CODE
              .
              mov eax, MyVar        ;  copy 77h into eax
              mov ebx, offset MyVar ;  copy memory address where value 77h stored into ebx
  

• OFFSET can also pass the address of a variable to a procedure in an INVOKE statement.

• However, OFFSET will only work for global variables declared in the .DATA or .DATA? segments.

• OFFSET will fail with local variables, which are declared upon entry into procedure using the LOCAL statement.

• Local variables inside a procedure do not have offset, because they are created on the stack at runtime.

• The ADDR operator solves this problem. It is used exclusively with INVOKE to pass the address of a variable to a procedure.

• For global variables ADDR operator translates to a simple PUSH instruction, just as if OFFSET had been used:

          push GlobalVar
  

• However, for local variables ADDR translates to: lea eax, LocalVar ; load effective address of LocalVar into eax push eax

• Effective address is the physical address of the data in memory.

• It is important to remember that when using ADDR with local variables, the EAX register is modified rather than leaving it available for other usages within the calling procedure.