From: daveh@cbmvax.commodore.com (Dave Haynie)
Newsgroups: comp.sys.amiga.advocacy
Subject: Re: EISA vs. ZorroIII
Date: 9 Apr 92 15:59:56 GMT
Reply-To: daveh@cbmvax.commodore.com (Dave Haynie)
Organization: Commodore, West Chester, PA

In article <1992Apr4.192231.21218@ux1.cso.uiuc.edu> sjc28473@uxa.cso.uiuc.edu (Steven Joseph Chmura) writes:
>Just need ed some technical information to help settle a dispute.  A person
>claimed that EISA runs at 33MHZ and 32 bits wide.  

EISA is clocked by the normal AT bus clock, which is generally in the 
neighborhood of 8MHz.  The so-called "ISA Specification", some after the fact 
attempt at a real AT bus spec, says 8MHz is the standard clock, though there
are systems that run the bus faster, since that bus clock used to be the
motherboard/CPU clock.  C= did this in their early PC-40s, but lots of things
failed beyond 8MHz.  I suspect EISA is much less likely to support faster
clocks.  

Performance-wise, I've read that the EISA burst mode peaks at 33MB/s.  That's
based on the EISA specification -- no guarantee that any particular 
implementation can ever go that fast.  There's also always a question of
just how much burst any given device can use.  The Mac's NuBus has 32-33MB/s
burst, but at least up to the IIfx, nothing in the Mac system supported it.

>This would seem to be far faster than our 25mhz 32-bit wide ZorroIII.  

The A3000's Zorro III state machine is based on a 25MHz clock (if you want
to get technical, it uses both edges of both A3000 motherboard clocks, CPUCLK
and CLK90, so in some sense it's 25MHz, in other senses it's 50MHz).  Zorro III
doesn't use a bus clock, so it's not really correct to speak of "25MHz Zorro 
III", although the meaning is clear if you understand the basis for it.

Anyway, the A3000's Zorro III implementation can support a 20MB/s random 
access, or a 33MB/s burst.  These aren't necessarily easy to get; you'll always
eat up some performance going from a tightly controlled CPU bus to an general
expansion bus.  So it'll be a bit more expensive to hit those speeds on Zorro 
III versus the A3000 motherboard.  In theory, the Zorro III random access peaks
at 50MB/s, with 150MB/s for burst.

>Also, he claimes the EISA scsiII gives 33 MB/sec throughput under OS/2 2/0.  
>This again sound physically impossible given the 486/33 setup. 

I think people nearly always assume that something on a bus is going full
speed, especially if they're the kind who go around quoting spec-sheets in what
generally amounts to nothing more than a hi-tech version of "the dozens".  If
you had 32-bit SCSI-II going in fast synchronous mode, you could get 40MB/s
peaks out of a RAID box or something.  No single drive can give you peaks
anywhere near that.  Now, most PC add-in SCSI, especially the EISA flavor, are
pretty hefty boards with their own processors and all kinds of other goobers
on them.  They probably can get a decent transfer going, what with caching and
all, though you'll need OS/2 or UNIX to see any advantage in it.  In any
case, you can run EISA at a fraction of its theoretical limits and still
keep the disk going as fast as it can.  The '486 bus will have no problem 
with 33MB/s, but that still doesn't necessarily mean that the EISA to '486 
connection will run that fast.  

I really don't know how close to theoretical max some of these buses are
typically run, because no one likes to admit they aren't going full potential
speed, even when there's no obvious reason to believe they should be going
full speed.  I did hear some low end estimates on the Mac II, that the
original 16MHz II's would average around 5MB/s across NuBus, but of course
that improved in the 25MHz systems, again in the synchronous systems like
IIfx or IIsi, and probably yet again in the Quadras with their double-speed
NuBus implementation.
-- 
Dave Haynie Commodore-Amiga (Amiga 3000) "The Crew That Never Rests"
   {uunet|pyramid|rutgers}!cbmvax!daveh      BIX: hazy
   "We can't be silent, because they might be giants" - The Might Be Giants