汇编语言循环机构

[x2061x]

DATA SEGMENT
A DB'Please input a character:','$'
B DB'Input error!','$'
DATA ENDS
CODE SEGMENT
ASSUME CS:CODE,DS:DATA
START:MOV AX,DATA
MOV DS,AX
MOV DX,OFFSET A
; LEA DX,A
MOV AH,9
INT 21H
MOV AH,1
INT 21H
CMP AL,'a'
JB DISPLAY
CMP AL,'z'
JA DISPLAY
SUB AL,20H
MOV DL,AL
MOV AH,2
INT 21H
JMP START
MOV AH,4CH
INT 21H
DISPLAY:LEA DX,B
MOV AH,9
INT 21H
MOV AH,4CH
INT 21H
CODE ENDS
END START

该程序具体如何运行？？？请教！