Instructions can not be executed when using 16 general registers.

I try to use 16 general registers for the soc system, and modify the define "E203_CFG_REGNUM_IS_32" to "E203_CFG_REGNUM_IS_16" in config.v file.

But CPU just executed four instructions at the beginning of the firmware, and then hanged. It seems that exception happened.

Did some guys encounter this issue? Could you give me some suggestions? 
Thanks!