Selection from instruction sheet
The VIXEN cartrige must always be inserted or removed from the VIC 20 computer with the power off.
The cartridge must be set to the desired configuration prior to insertation into the expansion port at the rear of the computer. If a powered Memory Expansion Board is in use, then this should be turned off propr to insertion or removal.
On switching on the computer, the screen will display the number of bytes free. (...)
The VIXEN can be used wither as a 16K RAM cartridge or an 11K RAM Cartridge. In the 11K Mode, 3K RAM fills the 3K gap in the Memory Map with the remaining 8K locating in block 1 or 3.
Two VIXENs switched in opposition will give full 27K memory expansion.