.IF comparing strings MASM

Question

How do I achieve something like this:

abc db "abc",0
def db "def",0
textnotequal db "strings are not equal",0
textequal db "strings are equal",0

 .if abc != def
    invoke MessageBox, NULL, addr textnotequal, addr textnotequal, MB_OK

 .elseif abc == def
     invoke MessageBox, NULL, addr textequal, addr textequal, MB_OK
 .endif

Do I need to mov abc & def into something first or is this generally not possible ?

Answer 1

You can write your version of cmpstr function in assembly. For example:

abc db "abc",0
def db "def",0
...
mov ecx,3     #the length of the abc and def strings
cld           #set the direction flag so that EDI and ESI will increase using repe
mov esi, offset [abc]  #moves address of abc string into esi
mov edi, offset [def]  #exact syntax may differ depending on assembler you use
                       #I am not exactly sure what MASM accepts but certainly something similar to this
repe cmpsb     #repeat compare [esi] with [edi] until ecx!=0 and current chars in strings match
               #edi and esi increase for each repetition, so pointing to the next char
cmp ecx,0      #test if the above command passed until the end of strings
je strings_are_equal  #if yes then strings are equal
# here print the message that strings are not equal (i.e. invoke MessageBox)
jmp end
strings_not_equal:
# here print the message that strings are equal (i.e. invoke MessageBox)
end: