The NVRAM, when it is enabled, masks 8K of RAM or ROM and replaces it with its own contents. Four NVRAM pages of 8K are available.
The four lower bits of the byte sent to port &FE82 selects the page: 8 = page0, 9 = page1, A = page2, B = page3, the other values disable the mapping.
In the fourth page, the last 8 bytes are not normal RAM, but can be used to read and write the time from the realtime clock.