This is the status port, from which hardware information is read. It is a read-only port.

Bit 0 is the battery status. It is set for good batteries, and reset for low.
Bit 1 is the LCD driver status. While this bit is zero, the LCD is not ready to receive a command.
Bit 5 is the model. TI-84+/SE models have this bit set while 83+/SE models do not.
Bit 7 is the hardware type. TI-83 Plus Silver Editions have this bit set, while TI-83 Pluses have it zero.