Rekindling Forth With A Propeller Jupiter Ace


The Jupiter Ace was a small membrane keyboard, cassette tape drive computer akin to the ZX Spectrum released in 1982. Priced at £90, it was a little more expensive than its home computer contemporaries, but had a very interesting feature: instead of BASIC, the Ace ran Forth. This interpreted stack-based language is far more capable than the BASIC variants found on home computers of the day, but unfortunately the Ace failed simply because Forth was so foreign to most consumers.

Not wanting to let a good idea die, [prof_braino] is bringing Forth back into the modern age. He’s using a Parallax Propeller to emulate a simple home computer running Forth. Instead of a book-sized computer, the new Propeller version runs on a single chip, with 8 CPU cores running 24 times faster than the original, with 32 times more RAM and an SD card for basically unlimited storage.

26 thoughts on “Rekindling Forth With A Propeller Jupiter Ace

  1. It was more of a ZX81 than a Spectrum. Black and white, no hi-res graphics, no sound, crappy non-moving membrane keyboard. These are much more to blame than the traditional scapegoat for the Jupiter Ace, which is it’s Forth.

    Most kids who owned 8-bits never learned more BASIC than LOAD “” anyway. It’s the games that made it for most kids, tho of course a few appreciated the BASIC, and a few even the machine code.

    I think manufacturers gave BASIC as default simply because everyone else did. In the very early days that was something reviewers would review, something they thought computers would compete on. In the end though of course it was the games. After the first good games came out, a year or two later, everyone realised that.

    1. Sorry, as a kid who grows with a ZX81, I could tell you that games I’ve played are games that I have programmed repeatedly copying them from books or magazines. Games on cassette are too expensive at this time.

      1. Games were from 2 quid upwards. The ZX81 was the “miracle” it was for being the first computer under 100 pounds. Yes, I read those books too, and typed in the odd, mostly useless game (AND debugged it!). Magazines did type-ins until someone had the bright idea of sticking tapes to the cover. But the ZX81 had a large games library, the Spectrum had at least 5,000 games. Most people bought games. Which were CHEAP, 2 quid a tape compared to 30 or 40 for a ROM cartridge for one of the terrible consoles. An expensive game might be 9.99.

        1. Just remembered when the “midi system” was invented around 1985. Twin tapes, record player, radio, often very cheap, in a medium-small box. For the first time, dubbing tapes was easy! That caused a freakout in the software industry on the same scale as when CD-R burners got cheap enough for ordinary people to own. Same impending doom, too. AND then worse when STs and Amigas came with their 3.5″ disks.

          Meanwhile the software industry’s bigger than cinema, the biggest entertainment industry there is. Poor software publishers have had to resort to spending tens of millions on endless sequels to whatever today’s popular format is (still FPSes, or “Doom clones” as we called ’em!).

          You can buy “Home taping is killing music” on T-shirts now, an amusing out-of-time joke. If only SOMETHING would kill everything in the top 40 music charts. I heard poor Britney Spears had to get the private jet WITHOUT the built-in swimming pool, poor hotbed of talent that she is.

    2. Thumbs up the post above. He nailed it. I did learn BASIC in one of these simple machines but the games is what attracted me to the machines in the first place. Heck… even my first inuendos into BASIC were modifying simple games written in BASIC as well.

    3. I agree that the whole “it failed because it had Forth” is a bit overblown because the hardware was definitely a step backwards: it was more ZX81 when the kids were wanting Spectrums. However, just to be pedantic, the Ace did actually have sound but it was just a small speaker driven from a single digital output (like the Speccy) and the keyboard was more of a ‘chiclet’ keyboard than a membrane: it was like the rubber mat of the Spectrum but with a blob of conductive rubber under each key that shorted some traces on the PCB. Finally, it could do a kind of hi-res as the character set was stored in RAM and could be redefined.

      The Ace is actually a very interesting machine to look at if you are interested in 80’s machines: do a search for the schematic and you’ll see that it’s built from standard 74LS TTL chips with not a ULA in sight!

    4. I agree. I started on a VIC-20 that came with a nice little introduction to the built-in BASIC. Nothing fancy, simple programs (like a flying bird animation in PETSCI to illustrate loops, and other such small programs for most program stuctures) and in the back of the book was a list of keywords for you to go on experimenting with. If it had been Forth then I would simply have learned that instead (assuming it came with said book – or i would probably have gone to the library to get more info – as i did with C64 especially, both books and the magazines at the time with program listings were available at the library)

    5. The keyboard was definitely Spectrum style dead flesh, not ZX81 style flat membrane. The screen was B&W only but allowed you to reprogram the character code maps to give you 256×192 pixel, which was about par for the course in those days.

      The real problem was that the FORTH language was hard to get to grips with and, on paper at least, the standard 1K memory looked small compared to the competition. If they’d chosen to go with 16k of DRAM they may have had a chance, more so if the promised colour adaptor had also turned up.

      1. I found the forth language easy to get to grips with, and I liked teh fact that forth/rpn made you define your task up-front rather than just bolting on extra bits ad hoc like basic did.
        The end came for me when I could not get the floppy disc interface to work reliably, because the machine code that ran that part was UFU. It took me twenty years of intermittent pestering of the company who made it, to even get the offer of a credit note out of them for the defective disc interface! (I didn’t use it, I just wanted some acknowledgement of responsibilty, also why would I trust anything else that MPE (in southampton IIRC) might sell me?). I paid 129 quid for that piece of junk, and risked criminal sanctions when I stole the required cumana external disc drive, and it just wouldn’t work!
        I did however get two aces to talk to each other over distance using my own code and a pair of proprietory serial interfaces, two acoustic modems and two cb sets…
        I also managed to code (in z80 M/c) a real time counter that ran at the top of the screen irrespective of anything else you did in the background, and was trying to write a routine to allow mult threaded forth code to be run. it ran off the end of their screen refresh interrupt routine… (I wanted the radio connected aces to communicate in the back ground…)
        Ace was more reminiscient of the ZX80 than the 81 in it’s hardware, but at least they had got the tape interface to work pretty reliably!
        What I think the world really needs is an ARDUINO centric design that emulates the ace at the machine code level if such a thing is possible, Forth is absolutely a hardware tinkerers environment. I have also long carried the idea thought that anyone who wants to try and make a digital “brain” of some sort could do worse than use a shedload of simple forth boxes sharing data over a common bus, like i2c maybe?

        1. I’ve never used FORTH, but back when there was much talk of it, and the Jupiter Ace a new thing, I thought it would make sense to put it in ROM.

          You get a programming language in a small space, it gives monitor functions, and you can build on it.

    1. We don’t? For constrained environments Forth is very powerful- the ability to easy mix (fast) interpreted code with assembly means that both speed and extreme compactness is possible.

  2. Not wanting to be too critical, but prof_braino’s Forth computer doesn’t remotely emulate a Jupiter Ace: the version of Forth, the display characteristics, its serial terminal interaction and program loading/saving are nothing like a Jupiter Ace. It’s a nice project in itself, and of course it’s at an Alpha stage, but in this respect I’m afraid he’s merely using the name.

    If you want to see a computer much closer to a Jupiter Ace, then check out FIGnition: it runs a version of Forth closer to Ace Forth; has a comparable, composite video memory mapped display; interacts via a built-in keyboard; supports UDGs. It’s not trying to emulate the Jupiter Ace: FIGnition supports a 160×160 bitmapped video mode with sprite support and loads/saves via 512Kb of external Flash, but it’s close enough to convert Ace programs relatively easily. Best of all, it comes as a kit and is very easy to build.

    Check it out at

    Sorry about the shameless plug.

      1. Hi t-bone. Along with the Hackaday title; most of the primary page gives the impression that this is an emulated Jupiter Ace built on a parallax Propeller demo board. For example “Using PropForth we can make something similar to the JupiterACE. It won’t run Z80 code, but can do everything and more than the original… Enjoy a modern JupiterACE standalone computer.. The initial priomary goal for my joining the Propforth effort was to obtain a useable JupiterACE. This goal has been achieved… Any PropForth system can be a JupitierACE (sic)”

        Yet, this system can’t “do everything and more than the original” – it can do different things; faster things and more complex things, but not much like a Jupiter Ace. It isn’t a “standalone computer”, because it needs a terminal, a host computer (e.g an Android device), for it to work. A ‘usable JupiterACE’ has not been achieved.

        Yet, it is probably possible for the propeller Forth system to implement a true Jupiter Ace – like computer. For example, it’s possible for a propeller chip to implement a 6502-based laptop….

        (except, oops, the Propeller chip is only used for I/O, the 6502 is separate). But a propeller chip can emulate classic 8-bit video games:

        So, it’s not a crazy idea, and he does qualify his term at the very end, but his liberal claim of Jupiter Ace emulation is somewhat misleading.

        1. Since Forth is apparently so powerful and everything, I wonder how hard it’d be to port games across to the Prof’s new machine. I’m thinking that if it was so fast, games wouldn’t need to be in machine code. Tho pretty quickly games programmers on all formats picked up machine code, so they’d have used it out of preference.

          Looking up on it seems like the Ace didn’t get ANY of the mainstream games or publishers. So I wonder how much of it all was written in Forth by Ace owners and enthusiasts.

          AIUI Forth is quite small and simple to implement, the whole point being that you build from simple blocks in a powerful way. So I wonder. Even without the UDGs you’d get something recognisable, if the professor added those to a custom terminal emulator of his own, maybe he could do that. Altho certainly the Wyse 60 (from experience) and a few other glass terminals had a few definable characters of their own.

          Used to love tweaking those old Wyses on the Honeywell Unix server. I was >< far from getting hi-res graphics on the thing, though I don't how much of the screen it'd cover. I still dream about it regularly! Aah! Shame they're not cheap on Ebay, people must still have a few hanging round on their networks, attached to serial port servers on the network.

          But I digress…

    1. Ooooh, nice! Still the page says it’s basically just the AVR and a few passives and connectors. You could probably wire-wrap it if you didn’t want to get PCBs made. Dunno if he has a PCB file available to download, doesn’t seem so, but it’s a simple looking circuit, all the brains are in the AVR. If you could get or make a PCB design, have a few run up for you and the nippers.

      You could try one yourself, then once you’ve mastered it bring the kids in. I’d be really pleased to bring a computer into school that I’D MADE MYSELF! Especially with a built-in LCD and batteries, and some games on the SD card, there were lots of fun games on the ZX81 (as the guy who owns the page knows, he’s playing 3D Monster Maze, the Doom of the ZX81!). Your kids could probably get extra marks for it somewhere or other.

  3. I desperately tried and tried to get one of these at the time BECAUSE IT HAD FORTH. I had a ZX80 and a ZX81(Timex Sinclair 1000) and the Jupiter Ace was promoted through adverts in US magazines, but alas it never arrived.

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