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 with minimal overhead, to maximize performance.

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 get more information on this system.)

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?)

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 someone posted in the "SS50 & SS30 Computer Bus Chat" Facebook group page that he had a very complete Gimix system containing not only the GMX 6809 CPU III, but also a hard drive that probably contains the OS-9 GMX III boot file, and a floppy disk also containing it, along with various other software, and a complete set of original documentation! A few days later he sold this system to a former employee of Gimix, who says he will scan all the documentation, capture all the floppy disk contents, as well as the hard drive contents. He will then make it all available to the community in the coming months!

Also in October 2025, I was able to contact the family of one of the co-founders of Gimix, Richard Don. I have now created a page containing a short history of Gimix, which also mentions many of the other products that Gimix produced over the years.

(Click on a picture for a larger version.)


B&W ad photo (1.7MB .png) and color ad photo (525KB .jpg)



Pictures of Gene Buckle's GMX system (155KB-855KB .jpg)

Gene Buckle gave me permission to post pictures of his GMX system. The six pictures are:

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 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.

	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)

"OS-9" is a registered trademark of Microware LP.

Last revised 2025-Oct-25 20:41 PDT.
Copyright 2019- David C. Wiens.

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