What does this Mean ? Here is what the 64K memory map looks like in various configurations (i.e., as seen by the processor):

It's done this way for a reason. The CPU MAPPER restricts the programmer to one offset for each 32K-byte half of a 64K-byte segment. For one chuck of ROM to MAP in another chuck with a different offset, it must do so into the other half of memory from which it is executing. The OS does this by never mapping the chunk of ROM at $C000-$DFFF, which allows this chunk to contain the Interface/MAP code and I/O (having I/O in context is usually desireable, and you can't map I/O anyhow). The Interface/MAP ROM can be turned on and off via VIC register $30, bit 5 (ROM@$C000), and therefore does not need to be mapped itself. Generally, OS functions (such as the Kernel, Editor, and DOS) live in the upper 32K half of memory, and applications (such as BASIC or the Monitor) live in the lower 32K half. For example, when Monitor mode is entered, the OS maps out BASIC and maps in the Monitor. Each has ready access to the OS, but no built-in access to each other. When a DOS call is made, the OS overlays itself with the DOS (except for the magical $C000 code) in the upper 32K half of memory, and overlays the application area with DOS RAM in the lower 32K half of memory.