Hi Olaf,

SJA1000 has 128 RAM locations.

The problem is not adresses loose, but a RAM chip is already decoded at low adresses, and you will get data conflict.

I cannot understand, if you do that :

mov A,#data ; load byte
mov R0,#reg_no ; load register number
clr Pxx ; enable chip
movx @R0,A ; write byte into chip
setb Pxx ; disable chip

I think you do not need any decoding
as you select the chip with a port.
But this is not the question.

Thanks.