asm

section .text

org 100h
start:

mov cx, 0h
mov ah, 09h
mov dx, v_give_a_string ;shows "give me a string"
int 21h
giveagain:
call p_giving_a_string ;reads a given letter and counts how many letters a user gave
cmp al, 65
jb j_error_or_ending ;if the user hits enter, program will end
jae j_check_whether_its_a_letter ;if the user types something what is no a letter, program shows an error

ending:
mov ax, 4c00h
int 21h

j_deleting_stack:
pop bx
cmp bx, 0
je start
jne j_deleting_stack

j_error_or_ending:
cmp al, 13
je j_tu
jne j_error

j_check_whether_its_a_letter:
cmp al, 90
jbe j_its_a_letter
cmp al, 122
ja j_error
cmp al, 97
jb j_error

j_its_a_letter:
push ax
jmp giveagain

j_error:
mov ah, 09h
mov dx, v_thats_an_error_dude
int 21h
jmp j_deleting_stack

j_tu:
mov ax, 2
mul cx
mov si, 0

j_change_cases: ;i think that is the segment, where i got a problem
mov ax, [bp + si]
add si, 2
cmp ax, 0
je ending
cmp ax, 95
jb j_uppercase
ja j_lowercase

j_uppercase:
sub ax, 32
mov ah, 02h
mov dx, ax
int 21h
jmp j_change_cases

j_lowercase:
sub ax, 32
mov ah, 02h
mov dx, ax
int 21h
jmp j_change_cases

p_giving_a_string:
mov ah, 01h
int 21h
inc cx
ret

section .data
v_give_a_string db 10, "give me a string: $", 10
v_thats_an_error_dude db 10, "thats not what we wanted. try again sweetie $", 10