masm - Subtract in assembly Language using a constant variable -

include irvine32.inc cr      equ 13 lf      equ 10    .data currentyear     dword   2017   year            dword   ?   yearmsg     byte    "enter year: ", 0   summsg      byte    "years have passed = ", cr, lf, 0   dig1        byte    ?   digsum      byte    ?   .code main        proc                    ; start            call    clrscr          lea edx, year           ; store         call    writestring          call    readdec             ; input dig1         mov byte ptr year, al          call    crlf          lea edx, yearmsg            ; print "enter year: "         call    writestring         call    readdec             ; input dig1         mov dig1, al          call    crlf          mov al, byte ptr year           ; digsum = year - dig1         sub al, dig1         mov digsum, al          lea edx, summsg         ; print "sum = "; digsum         call    writestring         movzx   eax, digsum         call    writedec          call    crlf                  exit                    ; stop main        endp          end     main 

my intention subtract year user inputs current year.