emu8086为什么不能编译length指令？

     DATA  SEGMENT
       COUNT  EQU       10
         NUM  DB        10 DUP(?)
        IBUF  DB        7,0,6 DUP(?)
        OBUF  DB        6  DUP(?)
      INFOR1  DB        "PLEASE INPUT 10 NUMBERS:",0AH,0DH,'$'
      INFOR2  DB        "THE MAX  FOUND IN THE 10 NUMBERS IS $"
      INFOR3  DB        0AH,0DH,'$'
        DATA  ENDS
       STACK  SEGMENT   STACK
       DATA2  DW        40  DUP(?)
         TOP  EQU       LENGTH DATA2
       STACK  ENDS
        CODE  SEGMENT
              ASSUME    CS:CODE,DS:DATA,SS:STACK
      START:  MOV       AX,DATA
              MOV       DS,AX
              MOV       AX,STACK
              MOV       SS,AX
              MOV       AX,TOP
              MOV       SP,AX