This 'Conversion Card' adds 2K or additional RAM and a 16K BBC BASIC ROM to your Atom.
It supports the full set of BBC-type BASIC commands, BASIC syntax is identical so all programs that don't rely on the BBC hardware can be run on the Atom without any modification.
The module is fitted in parallel with Atom BASIC and may be selected by a switch or from the keyboard if certain modifications are made.
The BBC-type BASIC conversion board uses the same 16K BASIC ROM as used in the BBC Microcomputer. The board includes a 4K MOS ROM to provide the correct machine environment on the Atom. The board also includes logic to alter the memory map so that RAM is available from 0000 upwards, and so that the 16K BASIC ROM can reside at 8000 to C000.
The BBC-BASIC TIME function is implemented by means of interrupts, generated by the Atom's 6522 timer (which must therefore be fitted). The BBC-type BASIC board is simply fitted by removing four integrated circuits from the Atom and inserting these in sockets on the BBC-type BASIC board. The board is then plugged into the empty sockets on the Atom.
The board can either be wired permanently in BBC-type BASIC mode, or, with the addition of two wires to the Atom keyboard, you can select between Atom or BBC-type BASIC by pressing CTRL-BREAK or SHIFT-BREAK respectively.
Schematic:
