SER_INT
	clr	ET0 ;comment 3
	clr	RI
	mov	A,	SBUF
	mov	R1,	A ;comment 1
	jnb	RI,	$ ;comment 2
	mov	A,	SBUF
	CLR	RI
	mov	R3,	A ;comment 1
	setb	ET0
	ret

comment1: it is a bad idea to transfer characters between an ISR and the main code in registers. Invariably, some day, something will be added that will make it blow up
comment 2: NEVER "hang" in an interrupt
comment 3: a) WHY b) clr/setb - what if it was disabled

how about
mov a,sbuf
mov @Ri,A
inc Ri

Erik