see Bank Switching
This is an extended version of the CBS RAM Plus bankswitching scheme, supported by the Harmony cartridge.
There are six (or seven) 4K banks, accessible by read/write to $1FF5 -$1FFA (or $1FFB), and 256 bytes of RAM.
The 256 bytes of RAM can be loaded/saved to Harmony cart flash by accessing $1FF4.
RAM read port is $1100 - $11FF, write port is $1000 - $10FF.
For 29K versions of the scheme, the first 1K is ARM code.
Also supported is a 32K variant. In any event, only data at 1K - 29K of the ROM is used.