Acorn BBC/Master RAM Cards
                      ==========================

There are several types of RAM Card that can be plugged into the BBC.
They usually plug into the 1MHz bus.


Page-Wide RAM Card
==================
A page-wide RAM card accesses 256 bytes at a time at &FD00-&FDFF. The page
of RAM that is accessed is specified by writing to an address latch in
&FCxx. The original Acorn specification defined only 64K of memory, with a
single 8bit address latch at &FCFF. This can be expanded to add additional
address latches at &FCFE and &FCFD, as used on the BeebEx.

&FCFD  RAM Card Address &xx------
&FCFE  RAM Card Address &--yy----
&FCFF  RAM Card Address &----zz--
&FDxx  RAM card memory at RAM card address &xxyyzz00 to &xxyyzzFF.

* If the RAM card only has one address latch at &FCFF, then 64K is
  addressable.
* If the RAM card only has two latches at &FCFE/F, then 16M is addressable.
* If the RAM card only has three latches at &FCFD/E/F, then 4G is
  addressable.


Byte-Wide RAM Card
==================
A byte-wide RAM card accesses a single byte at a time. The address of the
byte is specified with one or more 8bit address latches, and the byte is
read or written through an 8bit port.

base+0  RAM Card Address &------zz
base+1  RAM Card Address &----yy--
base+2  RAM Card Address &--xx----
base+3  RAM Card Byte at RAM Card Address &--xxyyzz.

* If the RAM card only has one address latch then 256 bytes are addressable.
* If the RAM card only has two latches, then 64K is addressable.
* If the RAM card only has three latches, then 16M is addressable.