The Gemini machines were intended, from the beginning, to be CP/M systems. This is why they were supplied with the RP/M control program, which would boot from the first floppy drive that it found or default to a monitor if there was none present. It is also why it was designed to run (some – with limitations) CP/M programs from cassette or EPROM.

Over the life of the Gemini system there were several changes in the equipment available. I’ve attempted to document them here.

Initially the floppy disk system used the Henelec package, originally designed for the Nascom systems and now named G805 when brought under the Gemini umbrella. This had an internal power supply (G804) and control board using the WD1771 controller with no external data separator and was supplied a box containing either one or two single density double sided drives (G805-1 or G805-2). It connected via the PIO rather than the bus. Up to 3 drives could be used. Single density only, double sided, 35 tracks.

The GM809 was the first Gemini floppy disk controller. This was quite an upgrade, using the very new WD1797 control chip.

The GM815 was the next stage, the same drives that were used for the G805 but this time without the internal control board, driving the disks in double density format using the GM809 board (not included) instead. Once again, it was available with either one or two drives.

GM825 was an upgrade to the GM815. The same case and internal power supply (G804) were used, but the drive capacity was increased. Four versions were available:

  • GM825-15 – Single drive – 400k
  • GM825-25 – Double drive – 800k
  • GM825-10 – Single drive – 800k
  • GM825-20 – Double drive – 1.6M

The GM829 was an upgrade of the GM809, using the WD1797 controller again. It supported faster stepping drives and included support for SASI hard disk drives (“Winchester” drives). It could handle up to four mixed 5-1/4″ and 8″ drives in single/double sided, single/double density with all switching under software control. When used with 5-1/4″ drives it was completely compatible with the GM809.

GM835 was a “Winchester” drive to accompany the GM809. It had a formatted capacity of 5.4MB and cost a massive £1450 in November 1982. The case was the same as that of the G805 & GM815.

GM835/10 As above, but with 10.8MB capacity for £1850.

The GM849 was the final disk control board for the 80-Bus. This time the control chip was changed for the WD2793. It now included support for up to eight mixed 3″, 3.5″, 5-1/4″ (up to 1.2MB) and 8″ disks. It was generally backwards compatible (not identical) with the GM809 and GM829.  A later version, the GM849A, added support for tape streamers. A SCSI port was used for connection to hard disk drive & tape streamer controllers.

The versions of SIMON and the CP/M BIOS varied with the disk control card. The cards weren’t quite software compatible! On single-sided floppy only systems there was no problem. There was a problem though when it came to selecting the second side of double sided disks. The WD1771 controller had been used until the GM829. This produced a side select signal for the drives. The WD2793, used on later boards, has no such signal, so the side was selected using bit 3 of port E4.

GM809      SIMON 3.1 or later      BIOS 3.2 or later

GM829      SIMON 3.1 or later      BIOS 3.2 or later

GM849      SIMON 3.3 or later      BIOS 3.4 or later

GM849A   SIMON 4.2 or later      BIOS 3.5 or later



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