This controls which RAM page is swapped into the 8000-BFFFh area. Reading the port will also return the RAM page currently swapped in.
On the TI-83 Plus, RAM pages are 0 and 1, which for this port are 40h and 41h (bit 6 identifying it as a RAM page). On the TI-83 Plus Silver Edition, it is bit 7, thus the 8 RAM pages are 80h-87h. Make sure your inputs and outputs follow this.