GMX 6809 CPU III
The GMX 6809 CPU III from GIMIX Inc. was a high-performance 6809 processor card for the SS-50 bus. It was designed specifically to run Microware's OS-9/6809 Level II operating system (and later also TSC 6809 UniFLEX) with minimal overhead, to maximize performance.
Features
GMX 6809 CPU III features:
- 2 MHz MC68B09 processor
- MMU/DAT (Memory Management Unit with Dynamic Address Translation) uses a 256x12 SRAM to store 8 task maps (1 system, 7 user), each addressing 32 2KB pages
- 20-bit physical address for 1MB range
- attributes for each memory segment: write protect, memory not allocated, hardware single-step
- full 64KB (65536 bytes) available for each user task
- separate Supervisor and User states
- automatic switching to Supervisor state in response to hardware and software interrupts
- I/O only accessible from supervisor state, not user tasks
- automatic interrupt masking
- DMA for high speed memory-to-memory and memory-to/from-I/O transfers via MMU/DAT, custom design using PAL chips
- watchdog timer for automatic recovery from certain illegal instructions
- calendar/clock (146818), with on-board battery backup
- programmable timer module (MC68B40), with precision time-base and connector for external signals
- 28-pin socket for 2KB or 4KB or 8KB EPROM
- 2KB SRAM, with on-board battery backup
- missing cycle detector monitors AC power input for loss of power
- undervoltage detection on +8VDC board supply
- gold-plated SS-50 bus connector
- 1984 price list showed the GMX 6809 CPU III with OS-9 GMX III costing U$1698.01
The GMX 6809 CPU III had similar capabilities as the CMS 9639 (for the EXORbus) from Creative Micro Systems. (Recently someone mentioned he has a "recreation of the cms9639/gimix CPU III" and is working on booting OS-9 Level II on it. I hope to soon get more information on this system.)
Support Boards
Intelligent serial I/O boards (and also intelligent parallel I/O boards) for the SS-30 bus allowed many users to simultaneously use the system without bogging down the main processor with interrupts for each character.
A separate SS-30 bus CRC Generator board was available to speed up calculating the 24-bit CRC that each OS-9 memory module contains. Using the memory-to-memory DMA on the GMX 6809 CPU III, an 8KB module could be checked in 8.5 msec. vs. 455 msec. or more using the software routine. (But it required masking interrupts for the duration of the DMA transfer to the CRC Generator because it wasn't reentrant. Why didn't they rather set a "busy" flag to prevent starting another transfer until the first one was completed, to avoid masking interrupts?)
Additional Information
Søren Roug has posted scanned copies of several user manuals relating to the GMX III system, such as the DMA III disk controller and the OS-9 GMX III Support ROM. Unfortunately, he hadn't found any boot disk image for the GMX III.
However, in October 2025 we heard the wonderful news that someone had kept a complete GMX III system, including lots of documentation and software. Go to the Mike Padgett Gimix System page and the Mike Padgett Archive page for more details.
In other good news, also in October 2025, I was able to contact the family of Richard Don, who was one of the co-founders of Gimix. I have created a page containing a short history of Gimix, which also mentions many of the other products that Gimix produced over the years.
Pictures
(Click on a picture for a larger version.)
 |
 |
|
| B&W ad photo (1.7MB .png) and color ad photo (525KB .jpg) | ||
Gene Buckle's System
(Click on a picture for a larger version.)
 |
 |
 |
||
 |
 |
 |
||
| Pictures of Gene Buckle's GMX system (155KB-855KB .jpg) | ||||
In November 2020 Gene Buckle posted on the "SS50 & SS30 Computer Bus Chat" Facebook group page that he has a GIMIX computer. He gave me permission to post pictures of his system:
- GMX 6809 CPU III - note the heatsink on the 6809, and 14 custom PAL chips!
- GMX DMA III - 5" and 8" floppy disk controller with DMA
- GMX 256KB SRAM - with on-board battery backup
- GMX motherboard - SS-50/SS-30 bus
- GMX SIOP3 - intelligent 3-port serial I/O card
- GMX baud rate generator card
The 2764 EPROM on the SIOP3 board says "OS-9 V2.00 (c) 1983 MSC". I wonder if that means it is actually running a separate copy of OS-9 Level I on this I/O board? The purple chip at the bottom is an AMD Z8038 Z-FIO, which is a dual-port 128-byte FIFO buffer that provides an interface between the 6809 on the SIOP3 and the 6809 on the GMX 6809 CPU III.
User Manuals
 |
GMX 6809 CPU III User's Manual. (scanned image, 2.9MB .pdf) |
 |
GIMIX CRC Generator Board User's Manual. (searchable scanned image, 207KB .pdf) |
 |
Microware OS-9 Level Two System Designer's Guide (handwritten annotations are mine). (scanned image, 5.2MB .pdf) |
"GIMIX" was a registered trademark of Gimix Inc.
"OS-9" is a registered trademark of Microware LP.
"UniFLEX" was a trademark of Technical System Consultants (TSC).
Last revised 2025-Oct-30 13:35 PDT.
Copyright 2019- David C. Wiens.
Home Other 6809 Contact Site map
