Ctorwy31 BBC Master ROM RAM cartridge

Introduction

This card is an easy to use upgrade for the BBC Master. It is effectively the familiar ‘ROM
cartridge’ but with a modern static RAM to provide additional benefits. For example, most
cartridges can only accept two ROMs and, with a large number available for the machine,
this leads to a lot of swapping of cartridges. However, constantly inserting and removing
cartridges imposes wear on both the connector in the machine and also on the cartridge edge
connector itself. On the other hand the use of RAM on the cartridge means that you can load
different ROM images from disk (or solid state media) as and when needed. The RAM
appears as two banks of 16KB in sockets 0/1 or 2/3, depending on which slot the card
occupies.

The RAM chip itself can be thought of as containing four pairs of 16KB. The machine can
only ‘see’ one pair at any time, and two switches are used to decide which of the four pairs
are visible to the computer. You don’t have to the turn the machine off when altering the
switches although a Ctrl-Break is usually needed.

The optional battery backup is provided by a single, inexpensive CR2032 cell and means
that the cartridge data is retained even when the computer is turned off. Contrast this with
the machine’s internal sideways RAM (link selectable in banks 4, 5, 6 and 7) which is
always lost in the absence of power. Furthermore, and unlike the internal sideways RAM,
the RAM on the cartridge can easily be write protected at the flick of a switch. Partial write
protection allows one of the banks to be write protected and the other to be written to as
normal. There is also a read inhibit facility so that you can easily recover from a crashed
machine caused by a corrupted or problematic ROM image. The CR2032 should maintain
the data for at least three years before it needs replacing.

An additional bonus is the presence of space for a single 28-pin socket which can accept a
16KB EPROM. This can be an ordinary leaf contact socket or even a ZIF socket. A single
switch disables one of the banks of RAM and allows the ROM socket to be active instead. A
ZIF socket can be especially handy when copying several ROM images onto a suitable filing
system for use later in sideways RAM.

The standard Master command *SRLOAD can be used to load data into the RAM banks. A
short program, described later, can save the data in any RAM bank to disk.

Download a ISO of the CD that comes with the BBC Master ROM RAM cartridge from HERE.