The SCPU is a holy grail item for the hard core C64 scene, and I had much to learn. Back in the late 90s, CMD made two versions of the SCPU: the original 64 version, and later a SCPU128 version.
- The SCPU 128.
They also made some updates to both along the way.
First: the SCPU64. What is it?
At a high level, it is an accelerator that can transform your C64 from 1Mhz to a shocking 20Mhz. The program this most tightly targeted back in the day was GEOS - a notoriously slow GUI-based operating system for C64/128 machines. Once you pop a SCPU into the cartridge slot, however, you get both JiffyDOS (no need to install it into your computer) and the accelerator. It also has a cartridge pass-through, so you can add an REU cartridge to it - like CMD's 2MB 1750XL - and now you had a machine that honestly felt faster than your 1998 Windows PC in many respects. It was kind of crazy. For me personally, it meant I could attach my CMD RAMlink to the computer in addition to the SCPU, which meant near-instant loading of software I had saved on the RAMlink and blazing performance of said programs when loaded.
The Version 1 SCPU64 came with bugs on the DOS chip, though. I didn't encounter them personally for a long time. But after getting the SCPU I eventually started to experiment heavily with C64OS, which frankly works REALLY well with JiffyDOS alone. But adding the SCPU gave it even just a bit more "oomph" and silkiness. C64OS really shines when you add an REU, too, for "fast app switching" and that's where I saw the bugs finally emerge.
Memory corruption.
Note also that my v1 SCPU was running the very earliest original 1.32 DOS that shipped with my v1 back in 1996. Today in 2025, there is actually an official CMD repair person named Wes Weise, based in Maine. I contacted him and he was permitted to create a brand new 2.04 DOS chip replacement for me. That brought my v1 up to as far as it could go based on its hardware design.
- Smell the freshness.
It was an easy swaparoo, and I fired up the Commodore + newly updated SCPU. Voila!
- 2.04 in the house! That was the furthest CMD ever took the DOS.
Next I fired up the SCPU + 1750XL and C64OS and this time...
- No more memory corruption!
I let the machine run for 90 minutes, then started using other applications like the application viewer. No issues. It appeared my DOS upgrade solved the problem. Stoked!
- Flying high...
Now, earlier in the year I always kept my radar on full alert for the unobtanium SCPU128 version. The SCPU128 is almost 1) never for sale, because anyone who has one never wants to let go of it and 2) if they do show up they cost more than a blinged out Amiga 3000 (i.e. somewhere in the $3,000 US range). That's just a fact. But, I kept my eyes and ears open because, well, you never know.
The CMD SCPU128 is unique in a few ways.
1) If you have a SCPU, you must also have an MMU board upgrade. CMD shipped these with the devices back in the day. What's brutal is that over the years, many SCPU128s would become separated from their original machines. Pulled off the back like a cartridge and set on a shelf. Now let's say that owner died and the family sold off his stuff. The SCPU128 would go to some new home, but the MMU board would still be inside the C128Dcr and usually wind up in a completely different place! Thus, making the SCPU128 lose ½ of it powers. A lot of those loose MMU boards would simply get removed and tossed.
- Original photo by Charles Hawn. Originally CMD made tiny little labels on each wire so you knew which pin to pin them to. Later, since a lot of those labels came off, they switched to colored wires (but most manuals don't say which wire goes to what). It works like this: Red clip = 39; Yellow clip = 30; Blue clip = 29, Green clip = 28, Black clip = 24.
A SCPU128 if simply added to a C64 will function exactly like a SCPU64. It all works perfectly. But if you want it to work with the C128 natively, you have to have that MMU board. (Fun trivia: using a SCPU64 with a C128D is actually REALLY cool. If you have the SCPU active, it will auto-boot C64 mode. No more holding down the C= key! And if you need C128 mode? Just disable the SCPU by flipping a switch. Boom!)
Also, the SCPU128 had a different PCB design, which allowed it to be upgraded with an internal RAM card. That RAM card could hold up to 16 MB of RAM.
This is where the C64/128 really gets screwy.
For every RAM addition made for the C64 and C128 (neoRAM, geoRAM, official Commodore expansion RAM, RAMlink, CMD RAM (1750xl or SCPU), etc.) every single one would have to be coded independently by developers in order to support each type. Standards schmandards. As a result, most of these RAM upgrades only work - realistically - with one or two commercial programs. And the SCPU RAM? Considering how rare the devices are, we're talking about half a dozen software programs that were written to take advantage of it. Things like: gateWay, Wheels, Metal Dust (made in the 2000's) and maybe a tiny handful of others. The accelerator works with tons of things and in particular productivity software is off the chain with one of these, which is great, but not the internal RAM. So, not a lot. But if you are hard core into the GEOS/gateWay/Wheels world (we exist!) the SCPU128 was the tippy top of the promised land, especially when fully decked out and combined with some other CMD hardware.
Anyway, I bought the loose MMU from the online friend who found one left behind in his C128Dcr and gave him a very fair price. He had no reason to keep it as he believed he'd never find a SCPU128 nor be able to justify buying it if he ever did find one. If they ever appear on Ebay, the bidding can get really tough to say the least. Each year the bids seem to increase - by a lot.
So, I put the weird looking thing in an anti-static bag and put it in my drawer of rare Commodore chips and doodads. It felt like I'd bought a really expensive lottery ticket. The scratch-off kind.
In the meantime, I enjoyed my SCPU64 with various programs.
Fun fact: some games REALLY fly when you have the accelerator on, and fly in a good (and sometimes hilarious) way. Example: Space Rogue. Instead of flying through outer space where the stars go past your ship 1 second per movement, now they would zoom past the cockpit as if you were Luke Skywalker commanding an actual X-Wing. Really trippy and fun in its own way to see and experience.
Months went by.
Now we're near the end of 2024 and I got word that a guy in Indiana was in a Discord (that I'm a member of but mute because it's too distracting) saying he had some CMD hardware he was wanting to unload outside of Ebay. I immediately reached out to him, and to my surprise no one else did. I think it was because the Discord was heavily Amiga focused and they really don't care about Commodore 8-bits for the most part at the same level.
Regardless, we hit up a conversation. And the dude was COOL. Due to the cost of these things he was willing to hold it for me while I tried to sell my C64 version on Ebay first so I could use those funds towards the purchase of his device.
Here's where the slow trombone starts to play. These days, the fees Ebay charges are NUTS. They really are. Let's say you sell something on Ebay for an even $3000. When done, Ebay will take approximately $390 of that for themselves. So you're left with $2600.
And from the buyer's side here in the USA we generally have to pay state sales tax (which is usually around 10%), which never fails to piss me off. "Paying tax on garage sale items!?" I scream into the wind. Anyway, a buyer would wind up paying $3,300 due to added tax and then shipping on top of that (items at this level require extra insurance, too)! Everyone loses. Note: someone sold a SCPU128 on Ebay on December 22, 2024, for a staggering $3,600. That means the buyer paid over $4,000 total! And the seller got closer to $3,100.
I sold my SCPU64 for less than $3,000 on Ebay because as sucky as it is it's still generally the fastest and safest route. So in order to make up the difference after the fees I also sold one of my CMD HD drives. I have one left in mint condition so it didn't affect me personally to let one go. Nervous about doing a deal like this, though, "off the grid" from any buyer protections I accepted the fact that this guy I was talking to was very well-connected and known in the community. So I felt safe. A touch nervous (this was a lot of money!) but safe enough. As much if not more than Ebay, to be honest.
So I took a massive gamble. And it all worked out!
The SCPU came with a new MMU board from Corei64, and also the internal RAM card with 16MB! It was fully loaded.
I hooked everything up. Didn't work.
The screen would just stay solid black and the light on the SCPU128 would briefly flash and nothing would happen.
I'm fortunate to have 3 different C128Dcrs. (Don't ask.) So I began a methodical process of swapping machines, then swapping to the original MMU, over and over and over mixing and matching. And I finally got one setup to work!
The variables that seem to matter:
- When you have this much hardware plugging into a C128Dcr (+RAMlink + SCPU + REU) it takes a lot of power. Two of the three systems I have simply have PSUs that are too weak to drive all of the hardware. Only one of the three could do it.
- The MMU by Corei64 ... I could NEVER get it to work under any circumstances. My old original MMU board with 5 janky pin clips worked, though. So using the old original + the machine with stronger power gave me a setup that actually worked. And it is as solid as a rock! The seller recommended I send the MMU back to Canada for analysis, but at this point with all of their post office strikes and whatnot I imagine I'll just sit on it.
- Note the GFD-2000 in the background, which is an FD-2000 drive with a GoTek upgrade. This allows me to mount and run D81 or FD disk images right off a thumb drive, which is how I am running this game. I downloaded it off itch.io, moved the file over to the USB drive and boom! Fired right up like a physical disk. This drive has changed my C= life forever for the better! I love it.
- Metal Dust requires a SCPU + 4MB minimum internal RAM card to even load. The game itself is SO hard, but the music and graphics... hard to believe it's possible to have a game like this on a Commodore 8-bit machine (runs in 64 mode).
I can't wait.
UPDATE:
After talking to the seller back and forth, he confided in me that he used a brand new internal PSU build by Ray Carlsen in his C128Dcr. Drop-in replacement!
I contacted Ray before the holidays and ordered two, since I knew 2 of my 3 machines had weak PSUs. I got them a couple of days ago, and have already installed one of them in my daily driver. It works beautifully, of course. This PSU should outlive me now at this point.
- Ray's brand new PSU on the left, the old original on the right. I am going to have to get used to fan noise! It's been a while...
. I will however look forward to the videos show casing it all.
Nice write up too.