All Apple //c and IIc Plus computers except ROM $00 ship with two 16K firmware banks, each visible from $C100-$FFFF. The visible bank is switched by a write to $C028. This is not well-documented by the Apple documentation.

The ROM $00 machines may be upgraded to use a 32K ROM via a pair of solder/trace jumpers on the board.

The aux bank mostly contains firmware supporting the features not present or less mature in the original Apple //c: 3.5" drive/SmartPort, serial and keyboard buffering, memory expansion card, diagnostics, and (for the Plus) accelerator.