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Dajgoro
 Post subject: 68881 FPU and other FPU
PostPosted: Wed Oct 12, 2011 3:22 pm 
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Hello!

Since i am also trying to build a 68000 sbc, i stumbled across this FPU, and i found out that it can be used with non 68020 cpu, but as a peripheral.
Has anyone tried to use the MC68881 in peripheral mode?


Last edited by Dajgoro on Fri Oct 14, 2011 4:09 pm, edited 1 time in total.

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BigDumbDinosaur
 Post subject: Re: 68881 FPU
PostPosted: Wed Oct 12, 2011 5:28 pm 
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Dajgoro wrote:
Hello!

Since i am also trying to build a 68000 sbc, i stumbled across this FPU, and i found out that it can be used with non 68020 cpu, but as a peripheral. Has anyone tried to use the MC68881 in peripheral mode?

I've yet to see anything from the Motorola 68K family connected to a 65xx system, not that it couldn't be done. The 68K interface is different than a typical 65xx bus setup. It probably wouldn't be too tough with the 65C816, especially since the 816 has better address qualifying signals (VDA and VPA) than its eight bit counterparts, but I think interfacing with a 65C02 would present some significant glue logic challenges.

That said, you'd have to write routines to convert to/from IEEE floating point format to use the 68881. The FPU doesn't do that for you. Also, you'd need some way to add the 68881 assembly language instructions to your 65xx source code. The data sheet doesn't list the equivalent machine instructions. This could get interesting.
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GARTHWILSON
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PostPosted: Wed Oct 12, 2011 6:14 pm 
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Although slower, I think one of these would probably be much easier to use, and it takes less board space. I will however comment on their initial statement though:

Quote:
What is 10 divided by 3? Your Basic Stamp thinks the answer is 3! Most other microcontrollers can't do any better because they all use integer math. Sometimes you can get by with integer math, but sometimes you need precision floating point results.

No, that's what fixed-point and scaled-integer is for. You scale the numbers to the range you need to work with. An example might be that you make the lsb to represent one 1000th, so when you divide 10 by 3, you're dividing 10,000 by 3 and you get 3333, which is the representation for 3.333, so you get three decimal places (four significant figures in this case) even though you're using integers. The scale factor can be whatever meets the need; so for trig funcions on a 6502, you normally make the lsb represent 1/65,536 of a circle, so you get a resolution of about .00549°. How's that for not using floating-point. A radian is represented by $28BE. A lot of digital signal processing is done without floating-point.

I started a series on fixed-point, scaled-integer, and rational-number arithmetic at viewtopic.php?t=716&start=8 . (This is the 8th post, where the meat starts. For the beginning of the topic, remove the "&start=8" from the URL.) Unfortunately I didn't get very far. Too many projects going at once! Hopefully I'll get back to it someday. [Edit: I have an article on my website that further discusses this on the front page of the section on using large look-up tables for hyperfast, accurate 16-Bit scaled-integer math, including trig & log functions. There are separate web pages linked there for how to use the tables and how they were calculated, plus .hex files for the tables themselves of course.]

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TMorita
 Post subject: Re: 68881 FPU
PostPosted: Wed Oct 12, 2011 7:55 pm 
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Dajgoro wrote:
Hello!

Since i am also trying to build a 68000 sbc, i stumbled across this FPU, and i found out that it can be used with non 68020 cpu, but as a peripheral.
Has anyone tried to use the MC68881 in peripheral mode?


Speaking of which....

It would be nice if 6502.org could diversify and support other microprocessors such as the 68000 by adding special subboards. I'm a general processor geek, and I'm interested not only in the 6502, but also in other processors as well.

Toshi
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BigDumbDinosaur
 Post subject: Re: 68881 FPU
PostPosted: Wed Oct 12, 2011 8:58 pm 
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TMorita wrote:
It would be nice if 6502.org could diversify and support other microprocessors such as the 68000 by adding special subboards. I'm a general processor geek, and I'm interested not only in the 6502, but also in other processors as well.

It wouldn't be "6502.org" if other processor families were included here. The number one reason why I come here is because of its relatively narrow focus on the 65xx family. If I want to see stuff about a 68K or (ugh) x86, I go to forums that specialize on those families, where I wouldn't expect (or want) to find 65xx stuff.
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Dajgoro
 Post subject: Re: 68881 FPU
PostPosted: Wed Oct 12, 2011 9:19 pm 
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BigDumbDinosaur wrote:
TMorita wrote:
It would be nice if 6502.org could diversify and support other microprocessors such as the 68000 by adding special subboards. I'm a general processor geek, and I'm interested not only in the 6502, but also in other processors as well.

It wouldn't be "6502.org" if other processor families were included here. The number one reason why I come here is because of its relatively narrow focus on the 65xx family. If I want to see stuff about a 68K or (ugh) x86, I go to forums that specialize on those families, where I wouldn't expect (or want) to find 65xx stuff.


I agree on both statements, it might be nice to have a small area to discuss non 6502 cpu and stuff. Yes, there are other forums, but what if someone would like to discuss such things but with this forum's users...

Anyway... Since i am building the 68000 sbc, it seems that the 68000 has to connect as a peripheral, just like any other cpu would have to...
And i found this: http://home.kpn.nl/pj.fondse/ide68k/, the ide supports 68000,68020... and the 68881, i hope when the 68000 cpu is selected, it will automatically compile the c code like it was a peripheral(which might help understanding it for non 68000 use) but i can't get the c compiled programs to work... :D
The fpu mentioned before is 21$ if i got it right, and the 68881 is 16.99$ on ebay...
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BigEd
 Post subject: Re: 68881 FPU
PostPosted: Thu Oct 13, 2011 7:40 am 
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Dajgoro wrote:
BigDumbDinosaur wrote:
TMorita wrote:
It would be nice if 6502.org could diversify and support other microprocessors such as the 68000 by adding special subboards. I'm a general processor geek, and I'm interested not only in the 6502, but also in other processors as well.

It wouldn't be "6502.org" if other processor families were included here. The number one reason why I come here is because of its relatively narrow focus on the 65xx family. If I want to see stuff about a 68K or (ugh) x86, I go to forums that specialize on those families, where I wouldn't expect (or want) to find 65xx stuff.


I agree on both statements, it might be nice to have a small area to discuss non 6502 cpu and stuff. Yes, there are other forums, but what if someone would like to discuss such things but with this forum's users...
Indeed - I'm not sure any other retro forum has quite the low-level focus that this one has, on building systems from chips, or with FPGAs. Pointers to such forums would be welcome.

The nice thing about a new top-level category is that anyone not interested can ignore it. I have mixed feelings about making the focus too wide, but I wouldn't want to see a veto just because a topic was not of interest to some subset of readers.

Cheers
Ed
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Dajgoro
 Post subject: Re: 68881 FPU
PostPosted: Thu Oct 13, 2011 2:05 pm 
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BigEd wrote:
Dajgoro wrote:
BigDumbDinosaur wrote:
TMorita wrote:
It would be nice if 6502.org could diversify and support other microprocessors such as the 68000 by adding special subboards. I'm a general processor geek, and I'm interested not only in the 6502, but also in other processors as well.

It wouldn't be "6502.org" if other processor families were included here. The number one reason why I come here is because of its relatively narrow focus on the 65xx family. If I want to see stuff about a 68K or (ugh) x86, I go to forums that specialize on those families, where I wouldn't expect (or want) to find 65xx stuff.


I agree on both statements, it might be nice to have a small area to discuss non 6502 cpu and stuff. Yes, there are other forums, but what if someone would like to discuss such things but with this forum's users...
Indeed - I'm not sure any other retro forum has quite the low-level focus that this one has, on building systems from chips, or with FPGAs. Pointers to such forums would be welcome.

The nice thing about a new top-level category is that anyone not interested can ignore it. I have mixed feelings about making the focus too wide, but I wouldn't want to see a veto just because a topic was not of interest to some subset of readers.

Cheers
Ed


I've been to the mc68000.org, but it seems bots have occupied it, and i can't find one good topic... And it is mac orientated...
Maybe non 6502 topics should just be allowed, they are not so common after all...

Anyway, does anyone actually know how this fpu works in peripheral mode?
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GARTHWILSON
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PostPosted: Thu Oct 13, 2011 7:07 pm 
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Quote:
I've been to the mc68000.org, but it seems bots have occupied it, and i can't find one good topic

which is why we're determined to keep this forum clean and be super hard on spammers.

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ElEctric_EyE
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PostPosted: Thu Oct 13, 2011 11:37 pm 
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GARTHWILSON wrote:
Quote:
I've been to the mc68000.org, but it seems bots have occupied it, and i can't find one good topic

which is why we're determined to keep this forum clean and be super hard on spammers.

I was tempted to say, what a more perfect reason to have a 68000 topic here on 6502.org, but this is 6502.org. I would like to see it, only because one can see a natural progression of the accumulation of signals on a 6500/6800 family of CPU's.
Start with the 6502, then 65816, then 68xxx. We do talk alot about the 65816, here on 6502.org...
I always had big dreams, when I was young, fantasizing about hooking up a 68020. Knowing that within that nice thick piece of ceramic was a screaming processor waiting for my commands. That was years ago, and as far as I got was a 68020 owner's manual, But...
Like BigEd said, if you don't like it , so what, then don't go there. I think we all could possibly learn from it. But I guess it depends if the traffic is still there and whether Admin would allow it...
Maybe, if there is a thumbs up here, the next step might be to contact the owner of the old 68K forum. Word would spread nice then!

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GARTHWILSON
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PostPosted: Fri Oct 14, 2011 12:08 am 
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Quote:
We do talk alot about the 65816, here on 6502.org...

The 65816, 65802, 65Org16, 65Org32, 65020, etc., are extensions of the 6502, whereas the 68K family is not. The 65816 outperforms the 68000 in some ways in spite of the 68000's wider registers and buses, although not the 68020. I know the 65c22 was used with the 68000 sometimes, but this is a 65xx forum. I wouldn't have any problem having a little PIC (or similar) traffic when it's in the context of supporting the 65xx processors like for reset control and pre-loading RAM with a reset routine before releasing the 65xx to start execution. Beyond that, I think you'd have to see if Mike wants to also have a sister forum. That would be a good way to maintain our focus.

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BigDumbDinosaur
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PostPosted: Fri Oct 14, 2011 12:20 am 
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GARTHWILSON wrote:
Quote:
We do talk alot about the 65816, here on 6502.org...

The 65816, 65802, 65Org16, 65Org32, 65020, etc., are extensions of the 6502, whereas the 68K family is not. The 65816 outperforms the 68000 in some ways in spite of the 68000's wider registers and buses, although not the 68020. I know the 65c22 was used with the 68000 sometimes, but this is a 65xx forum. I wouldn't have any problem having a little PIC (or similar) traffic when it's in the context of supporting the 65xx processors like for reset control and pre-loading RAM with a reset routine before releasing the 65xx to start execution. Beyond that, I think you'd have to see if Mike wants to also have a sister forum. That would be a good way to maintain our focus.

I concur with Garth. Let's keep the focus on the 65xx family.
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Dajgoro
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PostPosted: Fri Oct 14, 2011 12:50 am 
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Well the 68881 can be used in 65xx systems, so that is why i posted here...
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ElEctric_EyE
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PostPosted: Fri Oct 14, 2011 12:59 am 
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Dajgoro wrote:
Well the 68881 can be used in 65xx systems, so that is why i posted here...

I say go for it, keep posting! You got my ears... And it's only $20.

I assume you already have the datasheet?

What purpose do you have for it?

Also, you must consider the MC68882 and why is it better or maybe worse?

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Dajgoro
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PostPosted: Fri Oct 14, 2011 2:16 am 
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ElEctric_EyE wrote:
Dajgoro wrote:
Well the 68881 can be used in 65xx systems, so that is why i posted here...

I say go for it, keep posting! You got my ears... And it's only $20.

I assume you already have the datasheet?

What purpose do you have for it?

Also, you must consider the MC68882 and why is it better or maybe worse?


The 68882 is much more expensive, but it seems also faster...
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