Is there a particular reason for clearing Accumulator after incrementing DPTR ? You have created a never-ending loop with the CJNE instruction.

From what i understand your code is continuously reading the S0BUF (??) and writing its data to the external memory.
This is absolutely wrong especialy if what you are trying to do is receive a byte through the serial port and then write it to RAM. Look for more information on the use of the serial port.

You should put the code from the label START and forth inside an interrupt servicing routine for the serial interface, although you can do the same thing without interrupts if you check the RI status bit and then proceed with reading the SBUF.

Panos Kenterlis
Computer Systems Engineer