钨极氩弧焊机基本组成:程序中L4中的LEA DX,DS:MESS2语句中为什么要有ds

程序如下：
DATA SEGMENT
OUT1 DB 'Input string1:',13,10,'$'
OUT2 DB 'Input string2:',13,10,'$'
OUT3 DB 'The two strings are the same?',13,10,'$'
OUT4 DB 'Yes',13,10,'$'
OUT5 DB 'No',13,10,'$'
MESS1 DB 50 DUP(?)
DATA ENDS ;8

EXTRA SEGMENT
MESS2 DB 50 DUP(?)
EXTRA ENDS ;12

CODE SEGMENT
ASSUME CS:CODE,DS:DATA,ES:EXTRA
START:MOV AX,DATA
MOV DS,AX

MOV AX,EXTRA
MOV ES,AX

LEA DX,OUT1
MOV AH,9
INT 21H

MOV SI,0
MOV CL,0
L1:MOV AH,01H
INT 21H
CMP AL,0DH
JE L2
MOV MESS1[SI],AL
INC SI
INC CL
JMP L1

L2:MOV MESS1[SI],BYTE PTR '$'
LEA DX,MESS1
MOV AH,9
INT 21H

MOV DL,0AH
MOV AH,2
INT 21H
MOV DL,0DH
MOV AH,2
INT 21H

LEA DX,OUT2
MOV AH,9
INT 21H

MOV DI,0
MOV CH,0
L3:MOV AH,01H
INT 21H
CMP AL,0DH
JE L4
MOV MESS2[DI],AL
INC DI
INC CH
JMP L3

L4:MOV MESS2[DI],BYTE PTR '$'
LEA DX,DS:MESS2
MOV AH,9
INT 21H

MOV DL,0AH
MOV AH,2
INT 21H
MOV DL,0DH
MOV AH,2
INT 21H

LEA DX,OUT3
MOV AH,9
INT 21H

CMP CL,CH
JNZ L5

LEA SI, MESS1
LEA DI, MESS2
SUB CH,CH
CLD
REPZ CMPSB
JNZ L5

LEA DX,OUT4
MOV AH,9
INT 21H
JMP L6

L5:LEA DX,OUT5
MOV AH,9
INT 21H

L6:MOV AH,4CH
INT 21H
CODE ENDS
END START

该程序是用来比较从键盘输入的两个字符串是否相同，请问在程序中l4中的LEA DX,DS:MESS2语句中为什么要有ds呢， 没有的话显示就不对