I managed to write a program that reads the information received from the reader and it works just fine..then i added the comparisson loop like this

Compare:JNB 01h,NO_MATCH    ;this bit is set when i've received the tag ID
        MOV R0,#40h
        MOV R6,#10h
        MOV DPTR,#Valid_ID
         

CMP_NXT:        CLR A
                MOVC A,@A+DPTR
                INC DPTR
                MOV 3Fh,@R0             ;i use 3Fh as an auxiliary register
                CJNE A,3Fh,NO_MATCH    ;if the content of A<>content of 3Fh then no match
               
                INC R0
                DJNZ R6,CMP_NXT
                CLR P2.0                            ;if the tag ID =Valid_ID then a LED will glow
                   
NO_MATCH: RET
 

but this doesn't work as it should. please tell me what am i doing wrong in this routine