MSP430FR2355: SFR_8BIT macro??

I am trying to write some higher level library functions and am basing them off of a base address.  The issue I am running into is that P1 and P2 share the same base address.  If I point to this address and I don't necessarily have a configuration parameter that says Port 1 or Port 2 (as this seems redundant given I am already pointing to the base address) I somehow need to know how to shift <<8 for P2 writes/reads.....Same goes with P3/P4 and P5/P6.

Can someone tell me how I can track down how TI has done it?  I have gotten to the part but don't know where to look past this:

/* External references resolved by a device-specific linker command file */
#define SFR_8BIT(address)   extern volatile unsigned char address