Although my first post on this subject is not yet even a few days old at the time of writing, it seems like I kind of drifted from my original point a little and missed out a few things that I should mention in passing. Hopefully, this will fill in some blanks. And for that, I may as well start at the beginning, and here's what I know.
Around the mid to late 1990s, I was in regular contact with a Croydon-based guy named Russ Michaels, who was the sole proprietor of a new Commodore C64 start up called Electric Boys Entertainment Software. Russ was quite patient and put up with my C64 obsession quite well really. I stayed at his place a couple of times and that was the first time that I saw the mythical Flash8. Russ had set this hardware expansion up to quickly and reliably copy disks for his Public Domain library and his commercial software. And he had developed a Red Dwarf inspired demo for it too. With Future Publishing's Commodore Format magazine still plodding along through 1994 and I think until around 1997 (someone will correct me on that), he naturally assumed that there'd be enough interest in new C64 software to make a living from producing and publishing it, and maybe he'd also be able to develop his own games for his Electric Boys label.
Russ spoke of a Flashback port and a Defender clone (I think called Defensive). His Scene handle was Ironfist and he was interested in and very impressed by what the demo scene was pumping out. I'm not sure what his big plan was, but he was the official importer for Creative Micro Designs (CMD) Inc. to the UK (which if you don't know by now made the best C64 upgrades and hardware, unsurpassed until quite recently really). I imagine that in his head he thought that the fast CMD FD drives coupled with JiffyDOS would allow for bigger and better games with no disk swapping or waiting around for cassette multiload (even then the datasette was still the medium of choice in the UK). And because of the storage capacity even at the lowest end (~790K per disk), that would be enough to have some great demo-scene like cut scenes and then quickly into the action.
Anyway, Russ and I loved the potential of the Flash8, and around 1995 or early 1996, I think shortly after CMD launched its magazine Commodore World, he sent over (or arranged for CMD to receive) a Flash8 unit. For those of you who don't know, this device is an 8Mhz upgrade for the C64 that plugged into the expansion port on the C64 or C128 (in C64 mode), and it was powered by a WDC 65c816s CPU. It came with up to 1MB of RAM and I think there was a 256K RAM version too. If this is sounding familiar, then I think you may already know where this is going.
The problem with the Flash8 was compatibility and some general bugs that would cause unpredictable crashes. When it did work, it was great. And obviously copying from the 1MB RAM (essentially disk images) to real magnetic media was fast, efficient and reliable (as I saw for myself). The Flash8 worked with geoPaint - or at least there was a hacked/fixed version of geoPaint that you could use; with geoPaint being a GEOS package, and CMD being the official distributor of GEOS, this sparked some interest. From there, the guys at CMD had a base to work from, and to iron out the bugs and compatibility issues [of the Flash8], and maybe even to make an accelerator of their own, being fully GEOS compatible and integrating with all other CMD devices. Well, that's what happened. You'll note that early versions of the SuperCPU were to be 10 or 20Mhz, but the price difference is unlikely to have been enough of a draw for CMD's loyal customer base to the slower unit. And afterall, why would you want a 10Mhz upgrade when the same hardware was offered at twice that speed but not twice the price?
So how good is the compatibility of the SCPU then? Well in a sense, it's very close to 100% compatible, as even with the computer switched on you can switch out the whole unit as required. Simply and gently depress the reset button on the SCPU whilst holding down the CTRL key on the C64 or C128 keyboard, and switch off the unit on the top of the SCPU, let the reset button go and you're back to your stock C64 or C128 without having to turn off the computer and remove the hardware at all. I've never known this to cause any issues with any software, but also I haven't yet tried every single release on both the C64 and C128 to confirm this for sure (has anybody?) - but even with the unit enabled, the two other switches enable and disable JiffyDOS (again, I've never known a single program to fail or crash with JiffyDOS enabled), and to switch in real time between 1Mhz and 20Mhz (or 2Mhz and 20Mhz if you're in fast mode on the C128).
On the difference between SCPU 1Mhz and C64 1Mhz, there is not difference that I can tell for most of my Commodore uses. When at 1Mhz, the SCPU is synchronised with the C64. The one thing that it won't do is undocumented op codes of the 6510, and this is just how the 65816 CPU works. Whilst in "6502" mode (or more accurately, 65c02 mode), it accepts all documented instructions of the 6510. So there will be some games and demos that will hang or crash because the 65816 doesn't know what to do with the instructions that would otherwise work on a C64 or C128 when using undocumented features. But as you can just switch the unit out easily, it's not a big issue, at least after you know something doesn't work with the SCPU enabled.
Finally, those games and other programs that do work at 20Mhz. Some of them are really responsive and fast. Sometimes too fast. Sometimes about right. But just because you have a piece of entertainment software running at 20Mhz doesn't always mean that it will play any different than it does at 1Mhz. In this case, it is to do with how the game is coded and timed, which isn't a topic I'm currently too clear on. I'm sure that there'll be C64 coders out there who will know why this is.
For now, I'll leave it there. But just before I go, you may be interested to know that @dsp8bit has been mulling over a SCPU compatible accelerator, as you can see here. That's one of my old spare SCPUs that I kept for parts. Well I figured that it was no good to me if everything failed, and so at the same price I paid for this spare (not the crazy price that someone might pay for it), I sold it and sent it over to North America to hopefully help bring about a modern days SCPU alike device for the masses. Unfortunately, this labour of love has been slightly stalled, but I've heard that developments will be restarting in the (Northern Hemisphere) Spring. So whilst Russ played a small part in making the original SCPU a reality, I hope to play a small part in making a compatible (though not a 1:1 clone) and new SCPU like device a reality too, and soon. And I'm as excited to see what may be as I was when I first heard about this mythical Flash8 device!
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