How can I limit the display characters only within the alphabet

Question

Well, say I have a program that only accepts letters, regardless of its case (uppercase or lowercase), converts and displays its opposite case (eg. A to a, a to A). The program then displays the next letters through ascending and descending order BUT ONLY WITHIN THE ALPHABET.

For example, if i input 'l' and choose ascending order, it will output its corresponding uppercase equivalent as well as the next letters after it only until Z.

Input: l
Display
[a]ascending
[d]descending
Choice:a
L
M
N
O
P
Q
R
S
T
U
V
W
X
Y
Z

likewise if I input E and choose descending order, it will output its corresponding lowercase equivalent as well as the next letters before it only until a.

Input:E
Display
[a]ascending
[d]descending
Choice:d
e
d
c
b
a

Here is a snippet of code I have accomplished so far, I have managed to convert the input letters to its uppercase and lowercase equivalents and was also able to display them vertically. My only problem is how to limit the output only to the alphabet.

    startOver:
    call cls
    mov ah,9
    lea dx,string8      ;Display the string "Input:"
    int 21h

    mov ah,1        ;input starting letter
    int 21h
    mov bl,al

    call nlcr               ;nlcr is a procedure for new line and carriage return

    mov ah,9
    lea dx,string9      ;Display the string "Display"
    int 21h

    call nlcr

    mov ah,9
    lea dx,string10     ;Display the string "[a]ascending"
    int 21h

    call nlcr

    mov ah,9
    lea dx,string11     ;Display the string "[d]descending"
    int 21h

    call nlcr       ;nlcr is a procedure for newline and carriage return

    mov ah,9        ;Display the string "Choice:"
    lea dx,string12     
    int 21h

    mov ah,1        ;input display choice if it's ascending or descending
    int 21h         
    mov cl,al
    call nlcr

    cmp cl,'a'      ;validate the display choice
    je ascending
    cmp cl,'d'
    je descending

    checkd:
    cmp cl,'d'
    je descending
    cmp cl,'d'

    mov ah,9
    lea dx,string14
    int 21h
    jne startOver

    ascending:      ;display in ascending order
    xor bl,20h      ;converts letter to uppercase or lowercase
    mov ah,2
    mov dl,bl
    int 21h

    mov cx,15          ;display the letters vertically, I put a default of 15 for I am not sure how to limit it to the alphabet only          
    asc:
    mov bl,dl   
    call nlcr
    mov dl,bl
    inc dl
    int 21h
    loop asc
    jmp exit

    descending:     ;display in descending order
    xor bl,20h      ;converts letter to uppercase or lowercase
    mov ah,2
    mov dl,bl
    int 21h

    mov cx,15   
    desc:
    mov bl,dl   
    call nlcr
    mov dl,bl
    dec dl
    int 21h
    loop desc
    jmp exit

    exit:
    ;call cls
    mov ah,4ch
    int 21h
    main endp 

Thanks for the replies!

Answer 1

I'd change it to something like this:

  asc:
  mov bl,dl   
  call nlcr
  mov dl,bl
  inc dl
  int 21h
  and bl,0DFh  ; make sure bl is upper case 
  cmp bl,'Z'
  jb asc       ; loop until the next character would be 'z'+1 or 'Z'+1
  jmp exit

And similarily for descending order; execpt then you'd compare against 'A' and use ja instead of jb.