请问这个程序有什么问题，我是照着书本写的，但编译时出错，其错误提示为：
error L2029:'PRICE':unresolved external
error L2029:'QTY':unresolved external
error L2029:'TOTAL':unresolved external


EXTRN    PRICE:WORD,QTY:WORD,TOTAL:DWORD
PUBLIC    SUBM
CODE    SEGMENT
SUBM    PROC    FAR
        ASSUME    CS:CODE
        PUSH    DX
        XOR        AX,AX
        MOV        AX,PRICE
        MUL        QTY
        MOV        WORD    PTR    TOTAL,AX
        MOV        WORD    PTR    TOTAL+2,DX
        POP        DX
        POP        AX
        RET    
SUBM    ENDP
CODE    ENDS
        END