I now use Keil C51 v6.11. I have written a program and compile in 16bits large mode. It works properly in the hardware. However, when I change it into 24bits Contiguous Mode (512K or 16MB), I find that it become malfunction after calling ISR. Do I need to change anything if I choose to use 24 bits Contiguous mode besides changing ACON and setting in Option/Target/Code ROM sizes?