This is the code of a program that can be used to write upcoming serial data to external RAM:

MRAM EQU 0000H ;RAM first address
BAUD EQU 0E6H ;1200 bauds at 12Mhz

ORG 0000H
LJMP START


ORG 0023H ;Serial Interrupt
LJMP SERI

ORG 0027H
START: MOV AUXR,#03h
MOV PCON,#00h
MOV IE,#90H
MOV IP,#10H
MOV SP,#07H
MOV DPTR,#MRAM
MOV SCON,#54H
MOV TMOD,#20H
MOV TL1,#BAUD
MOV TH1,#BAUD
SETB TR1

ESPERA: CPL P3.3
SJMP ESPERA


ORG 0050H
SERI: MOV A,SBUF
MOVX @DPTR,A
INC DPTR
CLR RI
RETI

END

This code is not functioning, the ports P0 and P2 become at a high level all the time, if I include the line "mov P1,A" I see that the acumulator have the right serial data. But is not sending it to the external RAM.

I need to set some type of software or hardware to use MOVX instructions?