Available open 68000 cores ?

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Available open 68000 cores ?

Postby exxos » Sun Feb 26, 2017 9:48 pm

I wonder what cores are open source and 100% instruction compatible with the proper 68000 CPU ? Not really talking about being cycle exact either. Though if there are cores which can run all the 68000 instructions in one clock cycle, then that would be the equivalent of something like a 100MHz 68000. Ideally would have to be a drop in replacement for the 68000.

AFAIK there are 68000 cores (I am sure I did a similar thread a couple years ago but can't find it) but from what I remember, they were either abandoned or never properly finished off or missing some features like VPA etc.

I also really wonder if a FPGA equivalent cache could also be coded into such a core. I mean 1MB of SRAM could cache the whole ST RAM area. Not saying that would be a advantage over a small cache, but adding caches to the 68000 is almost impossible these days.

With the 030 being interesting, and of course having data and instruction caches, each gives about 20% speed boost overall. Which isn't bad. Though I haven't looked at the instructions of the 68030, I assume it has some better instructions than the 68000 did for some tasks. Though with most of the software being 68000 coded, then no software is really going to take advantage of such instructions.

The 030 is nice, though as everyone knows, each CPU change starts to break software which I assume people don't really want. Though those who want speed, are really forced to go down the 030 route.

My only motivations in using a 030 is because of the caches and its 32bit, So it can at least access fast-ram and ROM in 32bit modes. If a FPGA core existed which could implement a cache and even offer 32bit transfers, then I think that would be a awesome CPU. At least it would offer the speed without losing to much of the original 68000 compatibility.

I doubt it will happen unless someone codes it all. Though unless there is some advantage over a generic 68000, then I don't think moving to generic 68000 FPGA core is going to offer much, if anything, over a super clocked 68000.
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Re: Available open 68000 cores ?

Postby rpineau » Sun Feb 26, 2017 10:10 pm

There are a few 68 core but I think the most complete one is the one from Suska : http://experiment-s.de/en/progress/
You can get the whole Suska III-C code there : http://download.experiment-s.de/Firmwar ... 13A.tar.gz
There is also the tg68 but I think it's missing a few signals : http://opencores.org/project,tg68
As well as this one more targeted toward Amiga apparently : http://opencores.org/project,ao68000

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Re: Available open 68000 cores ?

Postby ijor » Mon Feb 27, 2017 12:06 pm

exxos wrote:I wonder what cores are open source and 100% instruction compatible with the proper 68000 CPU ? Not really talking about being cycle exact either.


I'm not sure there is any core that was tested as a hardware drop in replacement. Most cores are being used just as a part of an entire system inside an FPGA (like Mist, Suska). The 68K signals are usually not exposed externally in those cases. Last time I checked Suska, years ago, it seems it supported 68K compatible signals, but don't know if it was actually tested.

I also really wonder if a FPGA equivalent cache could also be coded into such a core. I mean 1MB of SRAM could cache the whole ST RAM area. Not saying that would be a advantage over a small cache, but adding caches to the 68000 is almost impossible these days.


It is of course possible to add a cache inside the FPGA. But you would need a rather expensive FPGA if you want to fit 1MB RAM . Why you need it internal to the FPGA? Externally would be much cheaper, of course.

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Re: Available open 68000 cores ?

Postby exxos » Mon Feb 27, 2017 12:59 pm

ijor wrote:I'm not sure there is any core that was tested as a hardware drop in replacement. Most cores are being used just as a part of an entire system inside an FPGA (like Mist, Suska). The 68K signals are usually not exposed externally in those cases. Last time I checked Suska, years ago, it seems it supported 68K compatible signals, but don't know if it was actually tested.


AFAIK there is the only emulated cores, and I am not aware of any full 68000 drop in replacements. It's why I started this thread to see if there is any 68000 alternatives, I assume not, but no harm in checking what is about.

It is of course possible to add a cache inside the FPGA. But you would need a rather expensive FPGA if you want to fit 1MB RAM . Why you need it internal to the FPGA? Externally would be much cheaper, of course.


I didn't mean internal to the FPGA, I'm not even sure if thats even possible, maybe I don't know. I suggested 1MB as SRAM is easy to find in that size. If there is a FPGA with even a small amount of internal RAM (even 8K or whatever), then it could be used as a small cache like the 020 or 030 CPU.

I think a 68000 core with some internal cache would be really cool, Though I doubt any FPGA guys will ever be on board with it. I just don't have time to do everything and know nothing about FPGA myself.

I personally think it will never happen as all the FPGA guys seem to want big bucks or are in more commercial productions. So it's only likely going to be a hobbyist who would likely ever develop such a core IMO. The 68000 IC itself is still obtainable currently pretty easily, though there is nothing in place for the future of a better or more sustainable CPU.

If someone did develop such a core I could add it to possible future boosters instead of a 68000 chip. But this thread at least shows I am willing to integrate a FPGA core if its going to offer some advantage over a generic 68000, such as caches.
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Re: Available open 68000 cores ?

Postby vido » Mon Feb 27, 2017 1:02 pm

exxos wrote:AFAIK there is the only emulated cores, and I am not aware of any full 68000 drop in replacements. It's why I started this thread to see if there is any 68000 alternatives, I assume not, but no harm in checking what is about

If I am right Vampire 1 accelerator was full drop in replacement with tg68 core.

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Re: Available open 68000 cores ?

Postby exxos » Mon Feb 27, 2017 1:55 pm

vido wrote:If I am right Vampire 1 accelerator was full drop in replacement with tg68 core.


Is that a open source thing ? From what I read (or the impression I get) is that it's the apollo core which is closed source. I think I did contact the guy with all the info on his site about it all a while ago and didn't really get anyway in that respect so gave up asking about the vampire stuff.
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Re: Available open 68000 cores ?

Postby ijor » Mon Feb 27, 2017 1:56 pm

exxos wrote:I didn't mean internal to the FPGA, I'm not even sure if thats even possible, maybe I don't know. I suggested 1MB as SRAM is easy to find in that size. If there is a FPGA with even a small amount of internal RAM (even 8K or whatever), then it could be used as a small cache like the 020 or 030 CPU.


It is possible internally, no problems at all. Every FPGA has some internal memory. How much memory it is a matter of cost. The FPGA in the Mist has about 64K, and it is quite old. A modern replacement has at least twice as much. Of course that RAM is used for many other purposes at the core. Not all the RAM is free and available. But a small cache shouldn't be a problem.

But again, it might be much better to implement an external cache, caching the entire ST RAM.

vido wrote:If I am right Vampire 1 accelerator was full drop in replacement with tg68 core.


But I understand you don't need a full 68K drop in replacement for the Amiga. AFAIK you can't have a Bus Error on the Amiga, and possible there are no VPA and E signals? Of course, that could be added.

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Re: Available open 68000 cores ?

Postby vido » Mon Feb 27, 2017 2:18 pm

exxos wrote:
vido wrote:If I am right Vampire 1 accelerator was full drop in replacement with tg68 core.


Is that a open source thing ? From what I read (or the impression I get) is that it's the apollo core which is closed source. I think I did contact the guy with all the info on his site about it all a while ago and didn't really get anyway in that respect so gave up asking about the vampire stuff.

Vampire 1 accelerator wasnt part of the Apollo project. If it would be it would contain Apollo core.
But later, cripled Apollo core was adapted to fit into Vampire 1.

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Re: Available open 68000 cores ?

Postby vido » Mon Feb 27, 2017 2:19 pm

ijor wrote:
vido wrote:If I am right Vampire 1 accelerator was full drop in replacement with tg68 core.


But I understand you don't need a full 68K drop in replacement for the Amiga. AFAIK you can't have a Bus Error on the Amiga, and possible there are no VPA and E signals? Of course, that could be added.

I am not that much of hardware guy or familiar with Amiga. I just know it fited directly into CPU socket. Thats all I know :)

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Re: Available open 68000 cores ?

Postby joska » Mon Feb 27, 2017 2:22 pm

ijor wrote:But again, it might be much better to implement an external cache, caching the entire ST RAM.


Better to not have CPU cache at all, but use fast-RAM instead. Faster and more compatible.

ijor wrote:But I understand you don't need a full 68K drop in replacement for the Amiga. AFAIK you can't have a Bus Error on the Amiga, and possible there are no VPA and E signals? Of course, that could be added.


The Vampire 1 does not have the FC0, FC1 and FC2 signals, making it unusable in an ST.
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Re: Available open 68000 cores ?

Postby ijor » Mon Feb 27, 2017 5:50 pm

joska wrote:Better to not have CPU cache at all, but use fast-RAM instead. Faster and more compatible.


What's the difference between fast RAM and cache? Why it would be faster? Depending on the case it is more a semantic issue than something else.

If the FPGA has access to all the systems signals (including GLUE ones), which is quite reasonable in an accelerated system, you could implement a smart whole system cache. You could snoop the bus and cache everything, ROM, RAM written by DMA, Video RAM, etc.

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Re: Available open 68000 cores ?

Postby lp » Mon Feb 27, 2017 6:04 pm

Wolfgang does drop in testing, http://experiment-s.de/en/gallery/
Perhaps he tested his cpu core in a real ST motherboard, but I can't tell from the photos.

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Re: Available open 68000 cores ?

Postby exxos » Mon Feb 27, 2017 6:16 pm

lp wrote:Wolfgang does drop in testing, http://experiment-s.de/en/gallery/
Perhaps he tested his cpu core in a real ST motherboard, but I can't tell from the photos.


I did talk to Wolfgang a few years ago, and I am not sure he did direct replacements as such. I think he said a lot of signals were not used/needed with that setup.

Someone who knows VHDL in depth and knows the pins for the CPU would have to study the code. Even so, not sure what the suska effectively runs at speed wise.
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Re: Available open 68000 cores ?

Postby mfro » Mon Feb 27, 2017 8:21 pm

exxos wrote:Even so, not sure what the suska effectively runs at speed wise.


The Suska III-C core tries to resemble an STE as close as possible (16/8 MHz). No idea if (and when, how much) the clock frequency could be increased.

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Re: Available open 68000 cores ?

Postby joska » Mon Feb 27, 2017 9:04 pm

ijor wrote:What's the difference between fast RAM and cache? Why it would be faster?


The most obvious difference is of course that fast RAM /alt RAM adds more RAM to the system, which a cache does not. Also, a cache would have to be filled and flushed, fast RAM doesn't. If you have IDE you can transfer data between fast RAM and mass storage at full speed, with cache you still have to write to stock speed RAM.

Also, fast RAM is much easier to implement than a cache, which I think matters in a hobby project where time is limited. Of course, you can add both, especially if the cache can be disabled by software.

ijor wrote:If the FPGA has access to all the systems signals (including GLUE ones), which is quite reasonable in an accelerated system, you could implement a smart whole system cache. You could snoop the bus and cache everything, ROM, RAM written by DMA, Video RAM, etc.


True, if you have access to these signals. But typically you don't, because the accelerator replaces the CPU only.
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Re: Available open 68000 cores ?

Postby terriblefire » Mon Feb 27, 2017 11:11 pm

I'd recommend the TG68K model. The MiST and MiniMiG teams are keeping this up to date.

Personally I want a verilog model!!

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Re: Available open 68000 cores ?

Postby ijor » Tue Feb 28, 2017 1:14 am

joska wrote:The most obvious difference is of course that fast RAM /alt RAM adds more RAM to the system, which a cache does not. Also, a cache would have to be filled and flushed, fast RAM doesn't. If you have IDE you can transfer data between fast RAM and mass storage at full speed, with cache you still have to write to stock speed RAM.


External (to the CPU/accelerator) RAM can be accessed by other bus masters, that's true. But I don't think it would be generally faster.

External RAM is constrained by the system clock and limitations. OTOH, a close coupled RAM can be as fast as you want and it can be a low voltage part (which is a big plus!). Flushing the cache doesn't necessarily have to be slower. You can have logic for letting the CPU complete the bus cycle just by filling the cache, then flush it with a FIFO whenever the external bus is idle. You have an FPGA, remember, that kind of logic comes "almost" for free.

You can if you want make the memory support "dual" mode with some way to configure it. Be it fast RAM or cache, or even mixed. But then it might be difficult to implement and still retaining the benefits of a fast close coupled RAM ... Or may be not ... just thinking in loud voice.

ijor wrote:If the FPGA has access to all the systems signals (including GLUE ones), which is quite reasonable in an accelerated system, you could implement a smart whole system cache. You could snoop the bus and cache everything, ROM, RAM written by DMA, Video RAM, etc.

True, if you have access to these signals. But typically you don't, because the accelerator replaces the CPU only.


Well, you don't need that many additional signals. May be the "DMA" signal going from GLUE to MMU is all you need. And if there aren't any other bus masters (such as Blitter), may be you don't need even that. Because in that case you know that any bus cycle that you don't initiate yourself has to be a DMA cycle.

Although to be honest, if you go to that extent, it might be much more efficient to put the whole custom chipset inside the FPGA core. At least GLUE, MMU and SHIFTER. The whole chipset together is much smaller than the CPU in terms of logic. So it might not significantly affect the FPGA size required. And then you don't need "standard" ST RAM anymore!

But of course, besides that something like this would make installation more difficult, some might say that this is not an accelerator anymore. It is just an ST clone in an ST case. May be. Again, just thinking in loud voice :)

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Re: Available open 68000 cores ?

Postby stimpy » Tue Feb 28, 2017 10:55 am

exxos wrote:
lp wrote:Wolfgang does drop in testing, http://experiment-s.de/en/gallery/
Perhaps he tested his cpu core in a real ST motherboard, but I can't tell from the photos.


I did talk to Wolfgang a few years ago, and I am not sure he did direct replacements as such. I think he said a lot of signals were not used/needed with that setup.

Someone who knows VHDL in depth and knows the pins for the CPU would have to study the code. Even so, not sure what the suska effectively runs at speed wise.


I seem to remember back when I was working with that code thar every chip could be replaced one by one with FPGAs and I'm sure thats what Wolfgang did in many cases.
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Re: Available open 68000 cores ?

Postby tuxie » Mon Mar 06, 2017 11:38 am

Wolfgang made complete custom Chips replacements, look at his pictures. He replaced all chips on a ST with FPGA´s and his ip cores. So every single Chips is replaceable with FPGA´s. I talked to wolfgang and he said the same.


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