From: daveh@cbmvax.commodore.com (Dave Haynie)
Newsgroups: comp.sys.amiga.advocacy
Subject: Re: No AA machines in September -- NOT!
Date: 17 Aug 92 20:23:50 GMT
Reply-To: daveh@cbmvax.commodore.com (Dave Haynie)
Organization: Commodore, West Chester, PA

In article <1992Aug14.135735.18712@newshost.unh.edu> ggg@kepler.unh.edu (Gregory G Greene) writes:
>>But it's a *slow* interface...  Compare it to the 386/486 boards with IDE
>>interface: it's the bottleneck of these machines.  It's ashame that machines
>>with such processing power are released with the IDE interface in stead of
>>EISA or SCSI.

EISA is an expansion bus, not a periperhal interconnect like SCSI or IDE.  There
are plenty of PClones with both EISA and IDE.  Maybe you were thinking of EDSI?
Both IDE and SCSI are faster than EDSI.

>	I never seem to notice this awful bottleneck on my 486.  Maybe I'm
>   not running the right programs that show the IDE interface to be so slow.

You probably aren't.  The main problem with IDE isn't likely to be disk
performance, but system throughput.

Take the average run-of-the-mill decent hard disk these days, maybe a Quantum
LPS105 or something.  That disk spins fast enough to get maybe 1.5MB/s over
whatever interface you have, though because most drives have cache, you might
peak a bit faster.

SCSI-1 does about 5MB/s at best, SCSI-2 does about 10MB/s at best (at least
in 8-bit versions), IDE's base speed is 4MB/s at best, though there are three
speed classes now kind of supported, which can let you go faster.  The thing
to notice here is that any of these protocols far exceed what most drives
can do.  If you're hooking up something really fast, like that cool Maxtor
Magic drive that's been floating around our software group, you may have more
concern about the physial interface.

The interface isn't the only thing, though.  You have to consider the "host
adaptor", the bridge between the disk interface bus and your computer's system
bus.  A good many 32-bit computer buses these days go 33MB/s or faster.  Yet
virtually all IDE interfaces, as well as many SCSI interfaces, connect directly
to the CPU bus in a system and use the CPU to move data around.  This becomes
the real bottleneck -- this fast CPU system has to slow down to whatever the
disk interface bus runs at during disk transfers.  So in a PIO system, you can
spend as much as 100% of your CPU time transferring data around slowly.  Until
you get a preemptive multitasking OS, you aren't going to be able to use any
free CPU time during disk transfers anyway -- back to my assertion about "the
right software".

The Amiga has this preemptive multitasking OS.  So when we did the A2091 and
later the A3000, we built a SCSI interface that put a DMA controller with FIFO
between the SCSI chip and the main system bus.  This acts as a very efficient
funnel between the slow I/O bus and the fast local CPU bus.  Data is stored
in the FIFO until the FIFO's full or empty, depending on the direction of
transfer.  The SCSI bus may be busy all the time, but the A3000 local bus will
only be involved in SCSI transfers when the DMAC is mastering the bus, which
is does at full bus speed.  When the DMAC is not on, the CPU is, and it's
doing work not dependent on the disk request.

PClones generally use IDE because it's cheap and most users run MS-DOS or
Windows, and therefore wouldn't see an advantage to DMA or SCSI.  If you're 
using OS/2 or UNIX on a PC, a DMA driven hard disk adaptor would help out
considerably in system throughput.  It'll also cost more.


-- 
Dave Haynie Commodore-Amiga (Amiga 3000) "The Crew That Never Rests"
   {uunet|pyramid|rutgers}!cbmvax!daveh      BIX: hazy
	"Work like a horse, drink like a fish"	- Psychefunkapus
 
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