The registers at $FDD0-$FDDF contain the ROM configuration for the machine, which normally has the BASIC and kernal ROMs enabled. The ROM configuration is adjusted by writing to the registers (the value is irrelevant). $FDD0 enables or disables BASIC, $FDD1 the low function ROM, $FDD2 the low cartridge ROM, $FDD3 is unused, $FDD4 the kernal, $FDD5 the high function ROM, and $FDD6 the high cartridge ROM. The upper portion of the kernal ROM at $FC00-$FCFF is always enabled no matter what the memory configuration, as are the I/O registers.