[Assembly] Enter 1 character test of whether or not there is Assembly

Problem checking input characters with the number or not.

Check character is zero

.model small
.stack 100
.data
    input db 'Nhap vao mot ky tu: $'              
    no db ' Khong phai so$'  
    yes db ' La so$'      
    CLRF db 13,10,'$'
    
.code
    main proc
        mov ax, @data
        mov ds, ax
        
        nhap:
            mov ah, 9
            lea dx, input
            int 21h  
            mov ah, 1   ; nhap ky tu
        tiep:
            int 21h                 
            cmp al, 30h ; so sanh ky tu vua nhap vao voi 30h
            je thoat       ; neu la so 0 thi thoat
            cmp al, 30h 
            jb sai      ; neu nho hon thi nhay den nhan sai
            cmp al, 39h
            jg sai      ; neu lon hon thi nhay den sai
            jmp dung    ; nhay vo dieu kien den tiep
        sai:            
            mov ah, 9
            lea dx, no 
            int 21h
            mov AH,9
            lea DX,CLRF
            int 21H  
            jmp nhap   
        dung:
            mov ah, 9
            lea dx, yes
            int 21h
            mov AH,9
            lea DX,CLRF
            int 21H
            jmp nhap
        thoat:      
            mov ah, 9
            lea dx, yes
            int 21h
            mov ah, 4ch
            int 21h
    main endp
end main

code