求助：判断闰年的问题

DATAS SEGMENT
    MESS DB 13,10,'Please input a year: $'
    Y DB 13,10,'YES! $'
    N DB 13,10,'NO! $'
    CONTINUE DB 13,10,'Continue? (y-yes) $'
    w DW 0
    buff DB 5,?,5 DUP(?),10,13,'$'
DATAS ENDS

CODES SEGMENT
    ASSUME CS:CODES,DS:DATAS
START:     MOV AX,DATAS
           MOV DS,AX
Tocon:
           LEA DX,MESS
           CALL DISMSG

           LEA DX,buff
           MOV AH,0AH
           INT 21h
           
           MOV CL, [buff+1]
           LEA DI,buff+2
           PUSH CX         
           DEC CX
           LEA SI,buff+2
     tt1:  INC SI
           LOOP tt1
           
           POP CX
           MOV DH,30h
           MOV BL,10
           MOV AX,1
           CALL ASCDEC
           
           PUSH BX
           PUSH CX
           PUSH DX
              
           MOV AX,w
           MOV CX,AX
              
           MOV DX,0
           MOV BX,4
           DIV BX
           CMP DX,0
           JNZ lab1
              
           MOV AX,CX
           MOV BX,100
           DIV BX
           CMP DX,0
           JNZ lab2
              
           MOV AX,CX
           MOV BX,400
           DIV BX
           CMP DX,0
           JZ lab2
              
     lab1: CLC
           JMP lab3
              
     lab2: STC
              
     lab3: POP DX
           POP CX
           POP BX
           JC a1
           LEA DX,N
           CALL DISMSG

     com:  LEA dx,CONTINUE
           CALL DISMSG
           
           MOV AH,01H
           INT 21H
           CMP Al,'y'
           JE  Tocon            
           JMP exit
    a1:    LEA DX,Y
           CALL DISMSG
           JMP com
    exit:  MOV AH,4ch
           INT 21h

ASCDEC PROC
LOOP1:
           PUSH AX
           SUB BYTE PTR [SI],DH
           MUL BYTE PTR [SI]
           ADD w,AX
           POP AX
           MUL BL
           DEC SI
           LOOP LOOP1
           ret
ASCDEC ENDP

DISMSG PROC
    MOV AH,9H
    INT 21H
    RET
DISMSG ENDP
CODES ENDS
    END START