From: danh@quantum.on.ca (Dan Hildebrand)
Subject: Re: Interactive and the 486
Date: Thu, 09 Apr 92 14:14:51 GMT
Organization: Quantum Software Systems, Ltd.
Keywords: Interactive 486 DX SX EISA ISA

In article <1992Apr8.010735.8238@news.rn.com> larry@news.rn.com (Larry Snyder) writes:
>dvb@emisle.uucp (David Van Beveren) writes:
>
>>1. EISA not ISA
>>   There are several reasons for this. First, 16 Meg is not enough when
>>running X, especially if you are a developer. The disk subsystems, even with
>>EISA are not all that fast (but far faster then ISA) and swapping really
>>kills you. I recommend 32MB or more if you can afford it. Second, Disk
>
>ok -- I'm running ISA with 16 megs -- I was considering EISA but the
>cost doesn't justify the performance increase (according to information
>I've been told here -- please correct me if I'm wrong) --

I agree with you. With QNX4.0 running on a ISA bus 33 MHz 486, Adaptec
1542 SCSI controller and a Micropolis 340 Mbyte SCSI drive ( which
delivers about 1.65 Mbytes / second physical read rate ), I get about 1.6
Mbytes / second at the application read() level ( reading from disk, not
from cache ). Since that's nearly the full speed of the drive, putting an
EISA bus and a 32 bit EISA SCSI controller in the box won't make the hard
drive deliver blocks to the application any faster. What EISA might do is
minimize the time during which bus cycles are being arbitrated away to
the SCSI controller card, thus giving a few more cycles to the processor.
I think you'd have to work pretty hard to come up with a benchmark that
could measure a difference, though.

On the other hand, if you were to put a few SCSI drives on the controller
and run a dual ethernet out of that bus, ISA simply runs out of
bandwidth, justifying the use of EISA or MCA.

>running a 16 bit 1542B on a ISA machine requires an additional
>fetch when accessing memory above 16 megs -- which is barely
>noticable (so I've been told) on a 486/50

A 486/33 can memcpy() at about 20 Mbytes / second, so this shouldn't add
significantly to the overhead in disk I/O.

>few OS's have drivers to take advantage of the enhanced mode on
>the 1742 -- most of them run the 1742 in the 1540 emulation mode
>(which I've been told is slightly slower than a real 1542)

From the Adaptec AHA1740/1744 Version 2 Installation Guide, Page 12:

"The Standard Mode allows software drivers for the AHA-1542 or AHA-1640
families to run unaltered on the AHA-1740 family. There is no performance
penalty for this -- the AHA-1740/1744 performs 32-bit transfers at up to
33 Mbytes / second and can operate 10 Mbyte / second Fast SCSI
transfers."

>>3. 486/50 is the only way to go.
>>   Look at the costs. A 50MHz 486 is well worth the extra few hundred over a
>>486/33. That cost will be recovered in faster compiles over a few months.
>
>I agree 100%

I've heard of people quoting $850 for 486/50 ISA motherboards.

>-- 
> =*= Larry Snyder, Public Access Unix Site - 219-289-0777 - 10 lines =*=
>      internet: larry@gator.rn.com        uucp: uunet!trauma!larry

Dan Hildebrand                     email: danh@quantum.on.ca
Quantum Software Systems, Ltd.     QUICS: danh     (613) 591-0934 (data)
(613) 591-0931 (voice)             mail:  175 Terrence Matthews          
(613) 591-3579 (fax)                      Kanata, Ontario, Canada K2M 1W8