This was a mono video control card with various operating modes and embellishments. It was an enhanced version of the earlier IVC card (G812) and was upwardly compatible with it. It was actually Gemini’s first video card to be specifically designed for their own system. No attempt was made to remain compatible with the NASCOM video system.
Available (software configurable) configurations were:
  • 4 pages of 25 lines x 40 characters with standard & alternate character sets
  • 2 pages of 25 lines x 80 characters with standard & alternate character sets
  • single 256×256 graphical display
On-board processing used a dedicated 6MHz Z80B processor. Associated with this was an 8k operating system EPROM, 2k workspace RAM and 8k video space RAM. All 256 possible characters could be redefined in software. The video system used a HD46505 CRTC chip. Hardware features allowed for the board’s output to be blanked or inverted. Also blinking, half-tone background and half-intensity character attributes were available.
An interesting feature was the capability of connecting the system keyboard (Gemini parallel type) via this card. This provided a type-ahead buffer and could also provide programmable function keys. Also fitted was a “beeper” which could be controlled in software. A light-pen socket was fitted and supported in the control program – returning character cell co-ordinates when triggered.

Like the earlier G812, communication with the system processor was via three I/O ports. This meant that the 64k address space of the system processor was not taken up by video memory. The flipside of this was that some operations took longer – scrolling being noticeable due to screen updating not taking place during screen memory access by the CRTC. Due to inbuilt keyboard buffering on the video card this did not cause a real problem as the system could keep up with fast typists!

There was 1kB spare RAM available in the workspace area and it was quite possible to write short programs to run in there – independent of the main processor. Just a bit of early multi-tasking there…