2023-07-04 — updated 2023-10-04

Classic (68k) Macintosh computers commonly supported (officially or otherwise) “accelerator” boards, which served to upgrade the CPU and it’s interfaces to newer, faster processors.

I own a Total Systems Gemini Ultra accelerator, currently installed into my Macintosh SE FDHD, boosting it from an 8MHz 68000 to a 40MHz 68030. I’ve collected a few useful notes here.

Note: This documentation applies to the Ultra model, specifically, and parts may or may be relevant to earlier editions (Gemini, Gemini II), or other boards with the same lineage (Quesse, Extreme Systems, etc).

Specs

Gemini Ultra boards are known to have been sold in 20, 25, 33, 40, and 50MHz configurations, all supporting up to 16MiB of onboard memory and an optional 68882 FPU.¹²

Documentation

No full manual scan exists for Gemini boards, although a single page documenting memory requirements has been archived here. A B&W cover, color cover, and copyright page can also be found, but aren’t too useful.

As a substitude, the Extreme Systems Vandal is basically an identical card and the manual is available on archive.org. Page 64 which is particularly useful as it documents the DIP switches on the system.

However, this does not appear to fully document the available DIP switches. Combining the silk screen information on the board and additional testing, we find the following additional information:

Notes

The Gemini has two speed modes, boot, which is available immediately at system POST, and run which is used only after the driver is loaded.
DIP switches 3 and 4 are used to configure which speeds will be used in each configuration by dividing the oscillator frequency. For instance, a 40MHz oscillator at /4 will run the system at 10MHz.

Switches

S1: On: Use memory on accelerator.
S1: Off: Use memory on logic board.³
S2: Unknown.
S3: Off / S4: Off: boot at frequency / 4, run at frequency / 2.
S3: Off / S4: On: boot at frequency / 4, run at frequency.
S3: On / S4: Off: boot at frequency / 2, run at frequency / 2.
S3: On / S4: On: boot at frequency / 2, run at frequency.
S5: Memory wait states as documented in manual.

Software

Accelerator Driver

Gemini boards do not require a driver, though one is recommended to unlock the complete features of the accelerator, including full speed in most cases. Crucially, the driver adds the following functionality:

Switching the board from the boot frequency to the run frequency. Theoretically by using an oscillator at twice the CPU speed and setting S3: On / S4: Off, this is a no-op and the driver is not required.
Using FPU acceleration for SANE (standard Apple floating point library).
Enabling or disabling the instruction cache for compatibiltiy with older applications.
Enabling or disabling the data cache for compatibiltiy with older applications.
Playing a “cool” animation at boot time showing off your speedy system.
Copying ROM to RAM to accelerate toolbox calls.

It is recommended to use GemStart 3.0. The driver seems to work through System 7.5.5.

Extended Memory

Secondly, to use the onboard extended memory, you must use Connectix Compact Virtual. This also adds support for using the original logic board memory as a small RAM disk. Without installing Virtual only the first 4MiB will be usable by the system, albeit at faster speeds than the stock memory on the logic board.

This does not work in Systems newer than 7.1.x.

“Field Upgradable”

Logic Board Removal

When inserting or removing the logic board with a Gemini Ultra installed, or most other accelerator boards, never remove the logic board by sliding it along it’s rails. You cannot follow the standard Apple maintinence procedures. This will cause serious, potentially unrepairable damage to your accelerator board and/or the logic board, as there is insufficient vertical clearance.

Instead you must gently pry the rails apart and rotate the logic board outwards. This is similar to the procedure for an SE/30, however older Macs lack the cutouts to allow this to happen without prying. There are good guides to doing this, I suggest reading a few before proceeding.

CPU

The Ultra advertises itself as “field upgradable”, as the 68030, 68882, and oscillator are all socketed making for simple swaps. In practice, this appears to be able to bring any 20-33MHz board to 40MHz. Reports⁴ indicate the 50MHz version has faster logic chips which means slower boards may not be upgradable beyond 40MHz. From my experience, a “slower” board will freeze immediately after switching to a 50MHz run frequency.

Upgrading to 40MHz is relatively simple, though at your own risk as it requires a modicum of electronics knowledge:

Parts

40MHz or 50MHz 68030 CPU. Typically you can find these on eBay. While relabeled chips are likely on the market, it’s far less of a problem for ‘030s than ‘040s.
Optional 40Mhz or 50MHz 68882 FPU. eBay is also your friend.
DIP-14 full size 4 lead standard oscillator, for instance the ECS-100X series, with the correct frequency (1x or 2x the CPU). A reminder that most boards can only support up to 40MHz run frequency.
Optional Wiha 268 10 or Wera 027456 chip lifter.
Optional PGA lifter like an Intel 309178 (these came in Pentium Overdrive kits, you’ll have to find one used) or a TE 58113-5, which you can still find new but are extremely expensive.

Steps

Make note of the oscillator orientation (one corner is cut).
Carefully remove the oscillator with the chip lifter. A screwdriver does in a pinch, but be careful. Insert the new oscillator in the correct orientation.
Note orientation (gold trace) of the 68030 and 68882.
Remove the 68030 and (if present) 68882 with the PGA lifter. Once again a standard chip lifter or a screwdriver will work, but require additional care.
Insert the new 68030 and (if you have one) 68882 in the correct orientations.
If necessary, increase the memory wait states according to the manual or insert new low-latency (70ns recommended) memory into the accelerator board.
Set S4 to On if you used a 1x matching frequency oscillator, or Off if you used a 2x.

Memory

The Gemini Ultra takes 30-pin memory, with at least 70ns or faster required for 40MHz+ operation. Remember that memory speeds are measured by latency, so lower numbers are better.

Up to 16MiB is supported as a set of 4x 4MiB SIMMs. These are quite cheap these days, both used items and new on both eBay and homebrew hardware sellers like Tindie, so upgrading to the full 16MiB is highly recommended.

Note: all 4 SIMM slots must be populated, unless S2 is set to Off, disabling the onboard memory entirely.

I’ve successfully used 2, 8, and 9 chip memory in my board. I would assume 3-chip work as well.

It’s possible some accelerators use plastic memory clips on the SIMM sockets. If this is the case, use caution and exert minimal pressure on the clips. Old plastic can be britle and it is extremely difficult to seat and lock SIMMs if these clips have broken.

Unknowns

Without the manual it’s unclear what the expansion headers on the board might be used for. A 4-pin, 26-pin, and 64-pin header are all provided. I’ve not seen these documented nor explored them myself. There is a report of a Lapis display card which was compatible with some Gemini accelerators, but it isn’t clear if these are compatible with an Ultra.
The Ultra and Integra models identify as the same to the driver, and both appear to be labeled as Ultra on the board. It’s unclear the differences between the two.