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 8-bit address latch at &FCFF. This can be expanded to add
additional address latches at &FCFE and &FCFD, as originally use on the
Acorn/CUBE BeebEx and then systems such as the Opus Challenger and
RetroClinic DataCentre. The Morley Electronics RAMdisc has its address
latch at &FCCC-CF.

Acorn  Morley
       &FCCF   Control register
&FCFD  &FCCE   RAM Card Address &xx------
&FCFE  &FCCD   RAM Card Address &--yy----
&FCFF  &FCCC   RAM Card Address &----zz--
&FDxx  &FDxx   RAM card memory at RAM card address &xxyyzz00 to &xxyyzzFF.

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

&FC00  RAM Card Address &------zz
&FC01  RAM Card Address &----yy--
&FC02  RAM Card Address &--xx----
&FC03  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 has three latches, then 16M is addressable.


16K-Wide RAM Card
=================
A Mega-RAM card, such as the Advanced Quarter Meg RAM Card, accesses 16K at
a time via a sideways ROM bank. The 16K bank within the RAM card is selected
via a bank select register. They are usually constructed as a cartridge to
appear in a sideways ROM slot and the page &FC I/O area.

&FCFE       Write enable RAM
&FCFD       Write protect RAM
&FCFC       RAM Card Address %00xxxxxx:xx000000:000000
&8000+yyyy  RAM Card Byte at RAM Card Address %xxxxxx:xxyyyyyy:yyyyyyyy

* With one bank select latch up to 4M is addressable.

Note that as this type of RAM card is usually on a plug-in cartridge, to
access the control registers on a Master requires the cartridge to be paged
into the I/O space by changing the relevant bits in ACCCON.