G809 FDC
This floppy disk controller card is capable of either single or double density operation. At the time it was produced the WD1797 control IC was a very new device. It will give about 160k of storage per drive in single density mode or 350k in double density mode (based on 35-track double-sided disk drives). Three software systems were available for this board:

D-DOS, a “floppy tape” OS treating the floppy as a tape. (See Henelec/G805 system for more information).



g809 roms

POLYDOS left the NASCOM memory map alone. The screen memory and monitor workspace didn’t change and much existing software continued to run as before.


CP/M 2.2 for modified NASCOM systems

CP/M 2.2 for Gemini “Multiboard” systems

The reason for having two different versions of CP/M is because the NASCOM and Gemini systems differ in video, keyboard and I/O support systems.

CP/M required that the memory map be re-arranged as it requires free memory from 0000H – where the monitor is normally situated. This prevented existing software from running, but allowed access to CP/M software. A new PROM was used to remap the system on a Nascom 2.

As CP/M expected a screen size of 24 lines of 80 characters the user had to install an IVC (or similar) video board as part of the system.