Path: utzoo!mnetor!uunet!cbmvax!daveh
From: daveh@cbmvax.UUCP (Dave Haynie)
Newsgroups: comp.sys.amiga
Subject: Re: 68030 Questions
Message-ID: <3507@cbmvax.UUCP>
Date: 24 Mar 88 18:14:31 GMT
References: <4890@videovax.Tek.COM>
Organization: Commodore Technology, West Chester, PA
Lines: 49

in article <4890@videovax.Tek.COM>, stever@videovax.Tek.COM (Steven E. Rice, P.E.) says:
> 
> Dave Haynie's (daveh@cbmvax) most recent article was number
> <3394@cbmvax.UUCP>.  In it, he cast aspersions on the poor, struggling
> LANCE and suggested that real systems do 32-bit DMA.  Well, maybe --
> but if you want to use Ethernet, the LANCE is about the only way to
> go, slow or no!

Calm down!  That's not what I said.  I said that in very high 
bandwidth-consuming operations, such as hard disk interfacing, where the
transfer between an I/O device and CPU addressable main memory can be sent
in large atoms, is best served by DMA, even in a 68020 or 68030 system. I
also said that in systems where transfers must occur in small atoms or at
relatively slow speed (like perhaps networks or things which must be
highly interactive), the I/O scheme to shared CPU memory was a good idea.

> In a perfect world, 32-bit DMA with a 512-byte assembly buffer and 
> fast-as-a-speeding-bullet burst transfers would be possible.  In real
> life, we have to make do with what we can buy.  (Commodore can build
> what it needs; the economics in the Television Test and Measurement
> market are different than those in the personal computer market.)

That's true, Commodore can build what it needs for those cases.  The 16 bit 
wide DMA driven hard disk controller on the 16 bit bus delivers around 625K
bytes/second with the Fast FileSystem.  Fast FileSystem allows DMA from the
hard disk directly to the target memory, not intermediate buffers used.  I
believe that any peripheral going this fast wants DMA.  It's fully extensible
to a 32 bit machine, though a _conservative_ 32 bit machine rates that's 
2.5 megabytes/second thoughput (not even getting to things like burst 
transfers, which are ideally suited to DMA transfers).  If you're LAN is only
going 2.5 megabits/sec, that's certainly overkill and extra cost.

Which seems to make sense even today; most Amiga hard drives are DMA driven,
most Amiga LANs are CPU driven via shared RAM DMA.

> There is another thought, too -- if you have only one DMA device, you
> could argue that it shouldn't make much difference if it DMAs into
> system RAM or into a dual-ported buffer.  If you have more than one
> device contending for the system bus, however, multiple dual-ported
> buffers are a clear win.

Not unless you have multiple CPUs to read them.


> 					Steve Rice
-- 
Dave Haynie  "The B2000 Guy"     Commodore-Amiga  "The Crew That Never Rests"
   {ihnp4|uunet|rutgers}!cbmvax!daveh      PLINK: D-DAVE H     BIX: hazy
		"I can't relax, 'cause I'm a Boinger!"