Gemini 80-Bus Resource


This page is designed to bring all of my Gemini software, firmware and documentation into a single resource. I have also included documents that, whilst not directly related to the Gemini system, I have referred to during the Gemini restoration (see The Gemini 80-Bus Saga – Part 1 and The Gemini 80-Bus Saga – Part 2). Naturally, this page will be updated as more items come to light.

Disk Images, Software, Firmware, Magazines, Documentation and Articles

The following repository contains all of the software, firmware, magazines and articles I have collected so far. The disk images are raw, GEMQDDS or GEMDDDS mages. Full details of the formats are shown in the respective README.md file(s) along with details of how these can be used with the Gotek floppy emulator (using FlashFloppy) and the xBeaver emulator. In addition utilities to manipulate the disk image files are sited there too.


The repository include many applications, programming languages and images of many Genuine Gemini disks including all CP/M Versions e.g. CP/M Bios’ 3.2, 3.4, 3.5, 3.6 including Multiformat Bios versions.

Gemini CP/M Bios

CP/M 2.2

The source code for the Gemini Multiboard Auto Density System (1.4) BIOS is located below. Whilst the BIOS is version 1.4, the CP/M version is 2.2.

Right click the following link and select “save link as…”.

The BIOS reports its version as shown below.

   Gemini Multiboard Auto Density System (1.4)   
   64k CP/M vers 2.2   

This can be built with the Microsoft M80 assembler and L80 linker.

SASI Implementation (GM829/GM849)

The Gemini GM829 Floppy controller also includes a SASI interface (SCSI-1) that can be used to connect via a 50 pin ribbon to a hard disk controller. There are two ports available to to access the interface. Depending upon the setting of LK5, these are either C5h and C6h or E5h and E6h. The latter pair is the normal configuration and will be assumed in this article.

Port E5h is a status/control port and allows software to read and write to the SASI control signals. Port E6h is the data/message port.

The control port (E5h) is implemented as follows;

On reading the port e.g IN A,(0E5h):

   Bit      Signal
    0        -REQ
    1         I/O
    2         C/D
    3        -MSG
    4        -BUSY

On writing to the port e.g. OUT (0E5h),A

   Bit      Signal
    0        -ATN
    1        -SEL
    2        -RST

Please note that at the time of writing I have been unable to get the GM829 controller to access a hard drive due to the timing of the select line (see The_Gemini_80-Bus_Saga_-_Part_1). I am told that a genuine Xebec controller may work but I have been unable to test this. The later GM8489 controller uses a latch to keep the select line active until de-activated by software.

For example to toggle the select line (ignoring any required delays etc.);

   LD  A,5          ; set bit 1 low (-SEL activated, -ATN and -RST deactivated)
   OUT (0E5h),A
   LD  A,7          ; set bit 1 high (-SEL, -ATN and -RST deactivated) 
   OUT (0E5h),A


The following is a list of articles that may be of particular use to the Gemini owner. Please note that the complete set of IMC News, 80-Bus News and Scorpio are available here.

Computer Press

  • Personal Computer World (Oct 1982)
    • Gemini Galaxy 1
  • Practical Computing (Nov 1982)
    • Galaxy 1
  • Computing Today (April 1983)
    • Gemini Galaxy System Review
  • Practical Computing (Jul 1983)
    • Galaxy 3
  • Personal Computer World (May 1986)
    • Galaxy Enigma

INMC 80 News

  • INMC 80 News (Issue 5)
    • Video Routine – Software for the Gemini Video Card Page 59
    • (More INMC Articles to be added shortly)

80 Bus News

  • 80-Bus News (Volume 1, Issue 4)
    • Gemini Multiboard Advert Page 27
    • Gemini GM822 RTC Review Page 29
    • Remote Terminals and the Galaxy Page 45
  • 80-Bus News (Volume 2, Issue 1)
    • Interrupt System of the Z80 Page 6
    • Understanding CP/M – Customising your BIOS Page 20
    • MAP 80 Ram board Page 29
    • RP/M, MAP256, SYS, Virtual DIR (R Beal Column) Page 37
    • 80-Bus + UCSD Page 50
  • 80-Bus News (Volume 2, Issue 2)
    • A Change of Tactics (CPZ, 40-80 conversion and more) Page 11
    • Gemini Multinet (Random Rumours) Page 51
  • 80-Bus News (Volume 2, Issue 3)
    • Wordstar for the Gemini IVC Page 11
    • 80-Bus Port Map Page 26
    • Large RAM Systems Using the MAP Card Page 37
    • Serial Interface Problems Made Easy Page 38
    • GM812 IVC Hardware Bug Page 50
  • 80-Bus News (Volume 2, Issue 4)
    • Colour Cards Page 5
    • SYS is Dead, Long Live? Page 20
    • 80-Bus IO Map Part 2 Page 21
    • RAM Disks Page 35
  • 80-Bus News (Volume 2, Issue 5)
    • Review of the Gemini Galaxy 2 Page 39
  • 80-Bus News (Volume 2, Issue 6)
    • Disk Drive Compatibility Page 14
    • Installing an 8 inch Drive with the GM829 Page 29
    • Video Output Stages Page 44
    • Mixed 5.25 inch and 8 inch Drives on the GM829 Page 45
  • 80-Bus News (Volume 3, Issue 1)
    • CCPZ and BDOSZ Page 9
    • SVC Board Page 20
    • GM809, GM829 Compatibility/Upgrading (using 8 inch mode on 5.25 inch drives) Page 35
    • Review of Compass, ZAP and Raven Assemblers Page 48
    • GM888 8080 Board Page 51
  • 80-Bus News (Volume 3, Issue 2)
    • Gemini GM860 EPROM Programmer Review Page 31
    • An insight into the Gemini IVC and SVC Part 1 Page 43
  • 80-Bus News (Volume 3, Issue 3)
    • Gempen/DiskPen a Review of the New Improved Version Page 4
    • An insight into the Gemini IVC and SVC Part 2 Page 16
    • The 80-Bus 800 Series Page 38
  • 80-Bus News (Volume 3, Issue 4)
    • Henry’s CCPZ, BDOSZ and Utility Disks Page 22
  • 80-Bus News (Volume 3, Issue 5)
    • Giant Intelligent Print Buffer for Gemini CPU Cards Page 9
    • GSX The Graphics interface Page 19
    • Lost Characters in CP/M Page 28
    • Gemini Cursor Page 31
  • 80-Bus News (Volume 3, Issue 6)
    • RP/M Version 2.3 Page 9
    • Interrupt Driven Printer for Gemini 2 Page 38
    • Put a Real Time Clock in your CBIOS Page 43
  • 80-Bus News (Volume 4, Issue 1)
    • An IVC Problem Page 4
    • Disk Skew and Deblocking (part of the DH Bits article) Page 9
    • Review of the Prestel Terminal Program Page 26
  • 80-Bus News (Volume 4, Issue 2)
    • The Gemini GM870 Modem Page 21
    • Gemini Bios Version 3.2 Page 25

Scorpio News

  • Scorpio News (Volume 1, Issue 1)
    • Disk Formats and CP/M Disk Routines Part 1 Page 25
    • Review of the 2Mb Upgrage for the Gemini GM833 Page 37
    • Prestel on Pluto Page 39
    • Review of the MAP-80 Video/Floppy Controller Page 42
    • Making CP/M More Friendly (CCPZ, ZCPR2, BDOSZ, CBIOS) Page 51
  • Scorpio News (Volume 1, Issue 2)
    • The ZCPR3 System PArt 1 Page 20
    • Disk Formats and CP/M Disk Routines Part 2 Page 40
    • MAP-80 Multipurpose Interface Review Page 44
    • GM809 Fast Steper Page 46
  • Scorpio News (Volume 1, Issue 3)
    • SVC Graphpac, Alias GM575 Page 8
    • Gemini Maintenance Memorandum Page 10
    • Disk Formats and CP/M Disk Routines Part 3 Page 11
    • The ZCPR3 System Part 2 Page 25
    • A Visit to IO Research Page 37
    • Upgrading a GM813 to 246K Page 42


The complete set of IMC News, 80-Bus News and Scorpio are available below.


At the moment this is just a list of my Gemini related bookmarks, I will sort these out and add some descriptions soon…