name echo         

 .model small

ZAS      SEGMENT   STACK
         DW        640 DUP(?)
ZAS      ENDS

CODE     SEGMENT
         ASSUME    CS:CODE, SS:ZAS
start:   mov ah,1
         int 21h
         cmp al,13
         jz koniec
         mov dl, al
         mov ah, 2
         int 21h
         jmp start
koniec:  mov ah, 4ch
         int 21h
CODE     ENDS
         end start