mystring_len=($-mystring)怎么放在.data块里啊,'='不是伪指令吗?

.data
mystring byte "this is a long string"
         byte "any number of characters"
mystring_len=($-mystring)

.code
main proc   
    mov eax,10000h
    mov ebx,mystring_len
        ret
main endp
end main

mystring_len=($-mystring)怎么放在.data块里啊,'='不是伪指令吗?