Dear all, When I type, C:/Keil/C51/BIN/C51 foo.c DB OE NOCOND CD SB WL(1) NOAM INCDIR(a long list of include dir;) Error Message: C51 COMPILER V7.20 - SN: C1S7P-60655E COPYRIGHT KEIL ELEKTRONIK GmbH 1987 - 2004 *** WARNING C500 IN LINE 1 OF foo.c: MISSING DEVICE (PORT IS BUSY) When I disable listing file (i.e. NOPR), it works well. But When I want C51 produce listing files, it fails and error messages is as above. Does any one know how to fix it? Thx for your help.