Hello, I am using C166 with xc161cj. I defined a register variable in file1.a66 like, RegDataSec SECTION DATA at F700h ax: dsw 1 RegDataSec ENDS In file2.a66, I am trying to specify this variable like, extern ax : WORD But it won't work, which means compiler doesn't treat it as a register variable. I cannot write a instruction like, mov ax, #1234 So, how can I specify a register variable? Thanks. David