各位大侠帮帮忙

用UVISION 编个程序,老是出现UNDEFINED SYMBOL 为什么,帮我看看
ORG 0000H
JMP START
ORG 0040H
START: MOV PO,#00H; 
  MOV P2,#00H;
step0:SETB P2.4;
  JB P0.3, step1;
  JNB P0.3, step0;
step1: SETB P2.5;
  JB P0.5,step2;
  JNB P0.5,step1;
step2: CLR P2.5
  SETB P2.0;
  MOV R0,#50;
LOOP1: ACALL D100MS;
  JB P0.0,step3
  DJNZ R0,LOOP1
  SETB P2.1;
  MOV R1,#50
LOOP2: ACALL D100MS;
  JB P0.0,step3
  DJNZ R1,LOOP2
  SETB P2.2;
  JMP step2
step3: CLR P2.0;
  SETB P2.3;
  JB P0.2,step4; 
  JNB P0.2,step2;
step4: CLR P2.3;
  JB P0.1,BAI
  JNB P0.1,HEI
BAI: SETB P2.4
  JMP step5
HEI: SETB P2.5
step5: JB P0.6,START;


D100MS: MOV R5,#100;
D1ms: MOV R4,#249
DL: NOP
  NOP
  DJNZ R4,DL
  DJNZ R5,D1ms
  RET 
  END

   

你把P0写成PO了

START: MOV PO,#00H;是零