汇编语言编程时运行结果都是乱码而且死循环是怎么回事

STACK SEGMENT STACK
DB 100 DUP(?)
STACK ENDS
DATA SEGMENT
MESG DB 'Welcome To our SCHOOL$'
COUNT DB $-MESG
DATA ENDS
CODE SEGMENT
ASSUME SS:STACK,CS:CODE,DS:DATA
START: MOV AX,DATA
MOV DS,AX
MOV BL,0
MOV SI,0
MOV DX,OFFSET MESG
MOV AH,9
INT 21H
MAIN: MOV AL,MESG[SI]
CMP AL,41H
JAE LOOP1
MOV DL,AL
MOV AH,2
INT 21H
INC SI
DEC COUNT
CMP COUNT,0
JAE MAIN
JMP CLH
LOOP1: CMP AL,61H
JBE LOOP2
MOV DL,AL
MOV AH,2
INT 21H
INC SI
DEC COUNT
CMP COUNT,0
JAE MAIN
JMP CLH
LOOP2: ADD AL,32
MOV DL,AL
MOV AH,2
INT 21H
INC BL
INC SI
DEC COUNT
CMP COUNT,0
JAE MAIN
JMP CLH
CLH: MOV DL,BL
MOV AH,2
INT 21H
CODE ENDS
END START