问题似乎是如果输入小于4，那么CL结束时不是1就是0，这在继续LBL1之前没有检查。而且0和1通常不被认为是素数。下面是一个最小的修复：

.MODEL  SMALL

.DATA
       VAl1     DB      ?
       NL1      DB      0AH,0DH,'ENTER NO:','$'
       NL2      DB      0AH,0DH,'IT IS NOT PRIME','$'
       NL3      DB      0AH,0DH,'IT IS PRIME','$'

       .CODE
   ORG 0100H
MAIN:

        MOV AX,@DATA
        MOV DS,AX

        MOV DX,OFFSET NL1
        MOV AH,09H
        INT 21H

        MOV AH,01H
        INT 21H
        SUB AL,30H
        MOV VAL1,AL
        CMP AL, 2
        JB LBL2 ; 0,1 are not prime
        JE LBL3 ; 2 is prime
        
        MOV AH,00

        MOV CL,2
        DIV CL
        MOV CL,AL
        JMP LBL4 ; check CL

LBL1:
        MOV AH,00
        MOV AL,VAL1
        DIV CL
        CMP AH,00
        JZ LBL2
        DEC CL
LBL4:
        CMP CL,1
        JNE LBL1
        JMP LBL3

LBL2:

        MOV AH,09H
        MOV DX,OFFSET NL2
        INT 21H
        JMP EXIT    

LBL3:
        MOV AH,09H
        MOV DX, OFFSETNL3
        INT 21H

EXIT:
        MOV AH,4CH
        INT 21H

END     MAIN

如果你可以利用你只需要处理0-9的事实，代码可以进一步简化为可能：

.MODEL  SMALL

.DATA
       NL1      DB      0AH,0DH,'ENTER NO:','$'
       NL2      DB      0AH,0DH,'IT IS NOT PRIME','$'
       NL3      DB      0AH,0DH,'IT IS PRIME','$'

       .CODE
   ORG 0100H
MAIN:

        MOV AX,@DATA
        MOV DS,AX

        MOV DX,OFFSET NL1
        MOV AH,09H
        INT 21H

        MOV AH,01H
        INT 21H
        CMP AL, 32H
        JB LBL2 ; 0,1 are not prime
        JE LBL3 ; 2 is prime
        TEST AL, 1
        JZ LBL2 ; even numbers 4,6,8 not prime
        CMP AL, 39H
        JNE LBL3 ; odd except 9 is prime 
LBL2:

        MOV AH,09H
        MOV DX,OFFSET NL2
        INT 21H
        JMP EXIT    

LBL3:
        MOV AH,09H
        MOV DX, OFFSETNL3
        INT 21H

EXIT:
        MOV AH,4CH
        INT 21H

END     MAIN