DATAS SEGMENT
    STRING1 DB 'Please input four hex numbers:',0AH,'$'
    ER        DB 0AH,0DH,'$'
    STRING2 DB 'The hex numbers converted to binary numbers are:',0AH,'$'  
DATAS ENDS

STACKS SEGMENT
    ;此处输入堆栈段代码
STACKS ENDS

CODES SEGMENT
    ASSUME CS:CODES,DS:DATAS,SS:STACKS
START:
    MOV AX,DATAS
    MOV DS,AX
;**********************输入数字提示语*******************************   
    LEA DX,STRING1
    MOV AH,09H
    INT 21H   
   
    MOV CL,4
    MOV DX,CX
    MOV BX,0
INPUT:
    MOV AL,0
    MOV AH,1
    INT 21H

    CMP AL,'9'               
    JBE NEXT2                ;键入值小于等于‘9’（‘0’~‘9’）则减30H
    CMP AL,'a'
    JB NEXT1                ;键入值小于‘a’（‘A’~‘F’）则减37H               
    SUB AL,20H                ;键入值在‘a’~‘f’则先减20H，再减37H
NEXT1:
    SUB AL,7
NEXT2:
    SUB AL,30H
    SHL    BX,CL
    ADD BX,AX
    DEC DX
    JNZ INPUT
;****************************在屏幕上打印换行符*****************************************     
    LEA DX,ER
    MOV AH,09H
    INT 21H
   
    LEA DX,STRING2
    MOV AH,09H
    INT 21H
      
    SUB BX,4352;调整，减去1000100000000 = 4352,不知道为什么会出现这种情况
    MOV CX,16
AGAIN:
    SHL BX,1
    JC NEXT3
    MOV DL,'0'
    MOV AH,6
    INT 21H
    JMP NEXT4   
NEXT3:
    MOV DL,'1'
    MOV AH,6
    INT 21H   
NEXT4:
    LOOP AGAIN      
      
    MOV AH,4CH
    INT 21H
CODES ENDS
    END START

为什么SUB BX,4352;调整，减去1000100000000 = 4352, 这里要这样调整，还有就是NEXT2:
    SUB AL,30H
    SHL    BX,CL
    ADD BX,AX
    DEC DX
    JNZ INPUT

这里怎么知道他会结束？