When J2 is connected and all switches are off then no roms are visible. This now allows you to program the ROMs safely.
Selecting the ROM is done by writing the number to i/o port dfxx. Symbiface decodes the upper 8-bit of the address completely. It looks for the following pattern:
11011111xxxxxxxx -> dfxx
Other expansions do a partial decode of the i/o address only testing bit 13 of the address.
Symbiface 2 also does a partial decode of the current rom number written to #df00. It only processes the lower 5 bits. This means the roms will repeat 4 times through the entire 256 ROM range. This also means they will override Plus cartridge pages.
Symbiface 2 also has the potential to override rom 7 (AMSDOS). On some CPCs this is 100% reliable, on others it is not possible and on some it is unreliable.
== Mechanisms ==