Need explanation of the CPU and FPU technology on turbo boards

[shoggoth]

rkauer wrote:
Just a memo: you could NOT use a faster FPU than your CPU. If you bought a faster FPU, simply put a cristal the same clock of CPU.

kidkoala wrote:
I think it's like this: if you have a cpu@30mhz and a fpu@50mhz, then the fpu would have to wait for the cpu all the time, because it's so much faster.
it's better if they have about the same Mhz to cooperate better, meaning there's no point in having a really fast fpu if the cpu is lagging behind.

Dudes, no offence, but you have it all wrong. The FPU is faster at floating point calculations (naturally), but it still uses a lot of cycles to perform it's calculations. The CPU waits for the FPU, not the other way around. Speed up the FPU, and the CPU spends less time waiting.

This has been tested and verified on other architectures.

-- Peter

[kidkoala]

@donnyemu

great you come up with this kind of post, it's just what i want

"The point I am making is it's more worthwhile to get an accelerator that combines the FPU and CPU with memory. That's where you'll get your speed increase and programs that really take advantage of the special math operations.. " what do you mean by this, where does memory fit into this?



@shoggoth

it's great with you're explanation to, but after hearing every meaning possible on just the matter you speak of, i'm having problems trusting anyone hehe, any kind of proof, documentation etc.? (i don't suppose you have this, it's just a question).

[shoggoth]

kidkoala wrote:
it's great with you're explanation to, but after hearing every meaning possible on just the matter you speak of, i'm having problems trusting anyone hehe, any kind of proof, documentation etc.? (i don't suppose you have this, it's just a question).

Don't wish to offend you in any way, but... It's not like you have a well defined technical reason not to believe me (prove me wrong here, and please do so with real arguments on a technical level). There's a big difference between sayong "no" because you know better, and saying "no" just because you can. In case of the former you shouldn't have a problem presenting your arguments.

Overclocking the FPU is a well known DIY hack, at least on other 68k platforms. Consider the following. The program counter resides in the CPU - not in the FPU. This means that the FPU isn't "pushing" program execution forward - the CPU is. When the CPU reaches a point where it needs data from the FPU and the FPU hasn't finished processing the current FPU instruction
, the CPU will wait. If the FPU is finished, the CPU will continue with the next instruction.

If you really need proof, consult the official docs from Motorola. Or find some tables with cycle times for FPU and CPU instructions. Do the maths.

Note that CPUs with built-in FPUs works a bit differently from the 020/030+68881/2 combo, but it's really not applicable here.

-- Peter

[shoggoth]

Ah, sorry kidkoala. I didn't realize it was you who started the thread.

1. It's asynchronous. It doesn't have to be the exact same speed. Generally it's better to have a faster FPU clock to improve the throughput.

2. Yes you can. You'll also need to get a dedicated oscillator for it, of you want it to run at some higher speeds. There is no connection between the CPU and FPU speed, hence the word "asynchronous".

3. (first part - dunno). Second part - overclock using a dedicated oscillator connected to the clock input of the FPU. The 68881/2 overclocks very well, and can often run at 2x-2.5x of the original speed.

4. Floating point stuff, naturally. Usually no games from that era uses it, and it won't speed up your workbench. Raytracers often came with FPU versions, and modern stuff can often be compiled to take advantage of FPUs.

Hack for the Atari Falcon which can probably be adapted for the Amiga:
http://atari.nvg.org/fpu/

-- Peter

[kidkoala]

@shoggoth

thanks a lot! you've already been to great help, you seem to have a good understandig og the matter.

the atari-page (written by a norwegian i see, like me, interesting) was really something i've been eager to find, but never found. the overclocking-info is very valuable to me.

i have a cd32 w/sx32 (40mhz?) and a fpu @33mhz and a osc.@33mhz (the osc. has a dedicated slot on the accelerator,i bought the fpu and osc. together to upgrade my accelerator, i already knew before that, that an fpu wasn't needed for everyday use, it's just for the fun of putting one on:). what i didn't know is that you could put on any oscillator you want, and it will be overclocked! fun.
but where do you get osc. of up to 100mhz?? when i bought the 33mhz one, the biggest one was at 50mhz or something..
second, won't the fpu have a very shortened lifetime when overclocking it to 2-2.5x? i guess the cooling will help but anyway..

about the proof i wanted, it was just to see if you had any documentation or anything, i didn't recon you had any cause it's kinda weird old stuff to be tinkering with hehe, no offense intended towards you either..

EDIT: just found 64mhz oscillators on amigakit, they can just be fitted? i have to activly cool the fpu itself with a standard cooler also then?

what programs do i use to becnhmark this thing after being overclocked?

and also, how do i overclock the cpu on a sx32@030/40?
can i just replace the one i have with a 50mhz one? or should i just overclock the one i have? (i've read that you can often overclock, say a 33mhz to a higher level than a 50Mhz?

[shoggoth]

but where do you get osc. of up to 100mhz?? when i bought the 33mhz one, the biggest one was at 50mhz or something..

100Mhz oscillators are rare, but they exist. I wouldn't go that high to start with. Try common ones first, say 50 and 66Mhz.

second, won't the fpu have a very shortened lifetime when overclocking it to 2-2.5x? i guess the cooling will help but anyway..

Probably, but keep it within sane temperatures and it won't be a big deal.

EDIT: just found 64mhz oscillators on amigakit, they can just be fitted? i have to activly cool the fpu itself with a standard cooler also then?

Yes. And yes. Try with a heatsink and some thermal glue first, it could be enough. Use something FPU-intensitive when testing, such as a raytracer compiled with FPU support.

what programs do i use to becnhmark this thing after being overclocked?

Dunno.

and also, how do i overclock the cpu on a sx32@030/40?
can i just replace the one i have with a 50mhz one? or should i just overclock the one i have? (i've read that you can often overclock, say a 33mhz to a higher level than a 50Mhz?

Dunno. I've used a 33MHz 030 for years at 50MHz w.o. problems. Needs a heatsink and a fan though.

-- Peter

[kidkoala]

thank you for all the answers..appreciated..

one more, if you care to answer:

do you know anything about overclocking the 68ec020 14mhz cpu on the standard cd32? does it have a oscillator or is it oscillated by the motherboards own clock?

[Hyperspeed]

Overclocking a Megadrive:
"It is possible to overclock the Motorola 68000 CPU in some cases in excess of 300% (the current known world record is 25.4 MHz), though it may not be completely stable beyond a certain point on each console.[9] The result of overclocking the CPU doesn't speed up the games any, but actually eliminates slowdown that some games are plagued by.

Another curious modification is to replace the stock 68000 processor with a 68010. Since the CPU isn't socketed, this requires the removal of the old CPU, and soldering in of the new. The 68010 is a pin-compatible, 'enhanced' version of the 68000, which is a bit more efficient internally and offers some new features. According to modder Robert Ivy, upgrading the CPU to a 68010 does not necessarily make the games run faster, it just reduces how much they slow down in intensive situations. Also, for some reason, the DAC's digital audio output sounds cleaner and less distorted. However, the 68010 is not 100% object code-compatible with the 68000, so machines modified with a 68010 processor are not able to run certain games properly; such as Sonic 3, Sonic and Knuckles, Street Fighter II, Red Zone, and a few others."

I wonder if you could use a toggle switch and alternate between a full 68020 and a 68EC020 or even different clock speeds...

[Piru]

does it have a oscillator or is it oscillated by the motherboards own clock?

It uses the motherboard clock (~28MHz/2), so it's difficult (but probably not impossible) to overclock. You could possibly use external clock to drive the CPU, but I've never seen such hack myself (also: would it cause problems with the CPU accessing the system bus?).

Overclocking the motherboard clock is out of the question as it will skew all chipset timing.

[Hyperspeed]

by Piru:
You could possibly use external clock to drive the CPU, but I've never seen such hack myself

Doesn't the Apollo 630 piggy back a 68030 onto the A600's 68000? This is going even further than a seperate clock I suppose... also shoggoth's link re Falcon FPU has some kind of bypassed oscillator.

It's a shame compatibility went out the window with the 68040/68060. If there had been a super '030 things could have been so much more fun!

Still, overclocking that 40Mhz '030 would be cool. As long as it's the more tolerant MC68882 being overclocked and you take cooling into account you could have an interesting end result there - Leirbag28 has a cooling fan cut into the front of his CD32!

[kidkoala]

@hyperspeed

i know, this whole thing is very interesting and sounds like a lot of fun even trying and not succeeding would be quite fun

"the DAC's digital audio output sounds cleaner and less distorted." the 68010 sounds like a cool thing, if you could use a switch, i'm a little audiophile myself so getting even better sound from the beautiful sound coming from the amiga sounds like a dream:)

the fpu i have is the 33mhz 68882 so i'm probably going to buy it a 64mhz oscillator from amigakit and cool the beast down to normal temp. with some silver-paste, a big ol' cooling-grill(?) and a nice quite fan..

so the performance in games are only remarkable when a game is going slower than it should, but in raw performance in wb it should speed things up a bit?

[Piru]

Overclocking the FPU will have very little effect at all. It certainly won't make any difference in Workbench. Heavy FPU number crunching should speed up, say rendering some 3d scene or calculating mandelbrots etc.

In my experience FPU doesn't overclock very well, it'll easily begin to generate errors (this was with 33MHz chip at 50MHz).

[kidkoala]

@piru

i know, the performance in wb was cpu wise

overclocking anything that old would anyway be just for the fun of it, i remember overclocking my old pc's to the max (before watercooling and stuff like that, in the diy-overclocking days when buying silver-coolingpaste and polish down the cpu surface to the metal, to get better contact, that actually helped several degrees sometimes)

and i'm not sure that overclocking the fpu to the double won't have any effect, at least not in the benchmarks, wich have been documented to show great differences..

http://atari.nvg.org/fpu/ here's the link to the guy who has tried iverclocking the 68882 with atari's for a while, he says he ran his 20mhz at 50mhz for a long time (ok, he might not have used it all the time, but i guess he knows what he's doing). ofcourse you have to cool it down, and you have to use a 68882 not 68881, and you have to know the outer limits of your architecture (maybe the cd32 or the 600 can't handle more than 33mhz through the components though you can overclock your fpu/cpu as much as you want).

but this is the purpose of this for me, i want to know as much about it as possible and then i'm going to try it all out in practice ...for fun

this is a guy that's been overclocking since 1975 or something, i guess he knows what he's doing to, and he has some interesting things to say:

http://members.iinet.net.au/~davem2/amiga.html


"I have also found over the years that more recently made versions of a particular chip will often overclock to a higher frequency. For example you would be more likely to get better results from a 4 year old 68030 compared to one that is 7 years old. Likewise you will usually get less from a chip at the top of it's range compared to a lesser rated one. For example you will usually be able to overclock a 25MHz 68030 further than a 50MHz 68030.

Thirdly you need to bear in mind that the accelerator card itself will also limit maximum possible overclocking speeds. A good example is the Commodore A2630 which cannot be clocked past 29MHz - even if you install a 50MHz 68030 in it."

[shoggoth]

Piru wrote:
In my experience FPU doesn't overclock very well, it'll easily begin to generate errors (this was with 33MHz chip at 50MHz).

You either got A: A bad FPU, B: an unstable clock, C: EMI trouble or D: all three.
Motorola chips from that era overclocks extremely well, especcially the 68882. Dunno about the 68881 though, but that's not an option anyway.

I've used a 33Mhz 68882 at 50Mhz for many years without cooling. No problems whatsoever - but I wouldn't be surprised if a bad clock could give the symptoms you describe.

-- Peter

[Piru]

This was 68882 on Blizzard 1230-III.

Well, I guess I was just unlucky.

[kidkoala]

@piru, shoggoth

another problem could be cooling? if his fpu gets to hot it could possibly create bad data?

another thing: why does the cd32 have so many oscillators?
the one that's used for the 14mhz cpu i guess is the 28mhz oscillator (running at half speed)? and there's a 16,9mhz osc. around the area of all the video output, that's because video runs at 32fps and that the double of 16mhz?

wouldn't it be possible to overclock the cpu osc. or something then?

just asking..