Need buying advice for 386 and Unix

[Wm E. Davidsen Jr]

In article <1026 at gistdev.gist.com> flint at gistdev.gist.com (Flint Pellett) writes:
jca at pnet01.cts.com (John C. Archambeau) writes:


>A 486/33 motherboard will yield about 14 to 15 MIPS.  But keep in mind the
>bottleneck going across the ISA bus.  A 64-bit processor running on a 16-bit
>bus.  Sort of reminds me of a traffic jam when a highway suddenly goes from 4
>lanes to 1 (which is the correct ratio).  Do NOT get a 486 unless you're going
>to go EISA or MCA.  It's a waste of CPU bus bandwidth if you don't.

  I'm sure somewhere there's someone who is running memory on the AT bus
instead of the motherboard or a memory bus, but in general that's not
the case. What is going to be backlogged by not using EISA?

  Not the 8 bit serial ports, or the eight bit parallel ports, or the
10-20Mbit disk drives, or the 16 bit video cards... The bandwidth is
about 8MHz clock / 4 clocks per xfer, times 16 bits per xfer, or 32Mbit.
Any device using less than 16 bit xfer is a bottleneck on an EISA bus,
too.

  You are technically correct that if you have an EISA bus and EISA
peripherals running in 32 bit mode, you can save a few bus cycles, but
the AT bus is easily fast enough to keep up with normal peripherals. To
imply that you will notice (or even be able to measure) the performance
loss due to the slow bus is probably incorrect. The only thing you might
be able to actually see would be a 32 bit video card, and then only with
a custom driver, since current unix drivers usually work on a single
byte most of the time (yes they might use string copy for bitblt).

  Having experience with many AT bus machines at work, and a small
number (several dozen?) of EISA machines, I can tell you that several
manufacturers do not present the same bus timing to AT cards on the
EISA bus as the accepted AT bus timing for ISA, and that some AT cards
won't work. If you are doing industrial control and use a *lot* of
special cards which are finicky, you might well find that the ISA bus
is still desirable. If you just want to buy commodity cards off the
shelf and be sure they will work, you still might make a trade-off of
incremental performance for reliability.

  I am *not* disagreeing that there is some performance loss, just
saying that running benchmarks on the same manufacturer's ISA and EISA
486, that I didn't see it using typical 16 bit video and cached disk
controller w/ 15Mbit disk. I don't think this is a serious issue, and I
believe that people should choose the bus based on intended peripherals,
and stay with ISA if the EISA is not going to be used.
-- 
bill davidsen - davidsen at sixhub.uucp (uunet!crdgw1!sixhub!davidsen)
    sysop *IX BBS and Public Access UNIX
    moderator of comp.binaries.ibm.pc and 80386 mailing list
"Stupidity, like virtue, is its own reward" -me