assume cs:code,ds:data

data segment 

db "Beginner's All-purpose Symbolic Instruction Code.",0

data ends

code segment
start:        mov ax,data
              mov ds,ax
              mov si,0
              call letterc

              mov ax,4c00h
              int 21h

letterc:      push si
              push cx
              mov ch,0
s:            mov cl,[si]
              jcxz ok
              cmp cl,61h
              jb s1
              cmp cl,86h
              ja s1
              and byte ptr [si],11011111b
s1:           inc si
              loop s
ok:           pop cx
              pop si
              ret
code ends
end start