User I/O
--------

 There are no empty spaces in the memory map for
implementing input ports except the switchable RAM bank
area (not an option on the Super Smart Card since it's
RAM bank is always enabled).

 An output only port may be implemented anywhere between
A000-FDFF. If implemented in a RAM area care should be
taken to use an area of RAM not used for anything else.
(FE00 and above can't be used because the CPU doesn't
generate an external /WR for these locations.)

 If you have a cart with an MBC1, a ROM 4Mbit or smaller,
and a RAM 8Kbyte or smaller (or no RAM) then you can use
pins 6 & 7 of the MBC1 for 2 digital output pins for
whatever purpose you wish. To use them you must first
put the MBC1 into 4MbitROM/32KbyteRAM mode by writing
01 to 6000. The two least significant bits you write
to 4000 will then be output to these pins.