Loop and decrease counter not working - assembly - loops

Question

I am new to assembly language and was asked to have a program that will allow only three chances of error when typing the password in.
s:
main proc
mov ax, #data
mov ds, ax
mov cx, 3
again:
mov ah, 9
lea dx, a
int 21h
mov ah, 2
mov dl, '*'
int 21h
mov ah, 8
int 21h
mov bl, al
mov ah, 2
cmp bl, 'Y'
je second
cmp bl, 'y'
je second
jne iinvalid
second:
mov ah, 2
mov dl, '*'
int 21h
mov ah, 8
int 21h
mov bh, al
mov ah, 2
cmp bh, 'E'
je third
cmp bh, 'e'
je third
jne iinvalid
third:
mov ah, 2
mov dl, '*'
int 21h
mov ah, 8
int 21h
mov cl, al
mov ah, 2
cmp cl, 'S'
je welcome
cmp cl, 's'
je welcome
jne iinvalid
iinvalid:
jmp invalid
checking:
mov ah, 9
lea dx, i
int 21h
jmp exit
togo:
loop again
welcome:
mov ah,9
lea dx, c
int 21h
lea dx, d
int 21h
mov ah, 1
int 21h
mov bl, al
newp:
mov ah, 9
lea dx, i
int 21h
jmp exit
invalid:
mov ah, 9
lea dx, b
int 21h
mov ah, 1
int 21h
mov cl, al
cmp cl, 'Y'
je togo
cmp cl, 'y'
je togo
jne exit
exit:
mov ah, 4ch
int 21h
main endp
end s

Answer 1

you destroy the content of cx inside the loop.
easiest way to fix it:
mov cx, 3
again:
push cx ; save loop cnt on stack
mov ah, 9
...
togo:
pop cx ; restore cnt from stack
loop again
keep in mind, that if you jump out of the loop, cx must still be removed from stack
welcome:
pop cx ; purge cnt from stack
mov ah,9
just a hint:
you compare the characters entered, to be either lowercase, or uppercase
uppercase and lowercase characters are (in ascii) identically, except of bit 0x20. you could "mask out" this bit by an AND with 0xDF ( 1101 1111 binary), and check the characters with just a single branch (instead of 3):
cmp bl, 'Y'
je second
cmp bl, 'y'
je second
jne iinvalid
--->
and bl, 0xdf ; make lowercase to uppercase
cmp bl, 'Y' ; THEN here both 'y' and 'Y' match
jne iinvalid