How to choose a new 386 UNIX PC...
Cliff C Heyer
cliffhanger at cup.portal.com
Fri Sep 8 12:13:42 AEST 1989
(The HARDWARE Q's are intermixed below.)
I'm planning to buy a 80386 PC for use with UNIX, MSDOS, OS/2,
and WINDOWS/386. After studying the trade papers and marketing
literature, I've made the following conclusions: (feel free to
1. Price: 33MHz hardware about same ballpark as 25MHz hardware.
2. 33MHz hardware not yet reviewed in key areas of:
bus speed, paged/interleaved memory, shadow
(BIOS/video) RAM, disk cache(memory or controller),
extended memory speed, wait states.
3. 100% of 33MHz hardware gives 10-20% better MIPS than 25MHz.
4. 33MHz hardware disk I/O only 0-5% better than 25MHz. In
other words, it might as well be the same.
5. 80386 portables are about the same price as 33MHz
desk hardware but are 50% slower in CPU and 70% slower in
QUESTIONS about 33MHz 80386 PCs...
(This is where I need the help!)
1. UNIX (or any multitasking OS) and the effects of
the on-board cache:
While multitasking, does flushing the cash waste a
measurable amount of run time or is it
insignificant compared to swapping, paging, and/or
other overhead? In other words, is the cache still
beneficial even though it is being flushed? (I
assume "yes" since minicomputers such as all VAX
models have them.)
2. Is memory technology (cost/speed ) lagging behind
microprocessor technology? All the newest 33MHz
80386 PCs are using 70+ ns DRAMs when the 386 is
running at 30 ns and the on-board caches are rated
at 25 ns. You can't get 0 wait states 100% of the
time with this approach.
3. Is it impractical (cost and/or size) to put 40 256KB
25ns SRAMs (no refresh overhead and cycle
time=access time) up for main memory? In other
words, is it cheaper to implement paged (PMRAM,
SCRAM) or interleaved schemes to reduce wait
states rather than use 40 SRAMs? It seems like
alot of trouble to go to...but how much do SRAMs cost?
4. Are any board makers making (or have made)
motherboards with ESDI and/or SCSI interfaces ON
BOARD to bypass the 8MHz AT bus? Also hopefully
this mfg. would include shadow RAM (BIOS & video)
and extended/expanded memory that is as fast as
main memory. (eg. add on memory boards have same
cycle time as the first 2MB.)
5. I assume the ONLY thing that makes the 33MHz PCs
faster is the 25 ns cache. Otherwise, with 70 ns
DRAM the BEST you could do would be run as fast as
a 16MHz 80386 PC (62 ns) but with lots of wait
states. In other words, memory cycle time limits
non-cache CPU performance to that of a 16MHz 80386.
6. If you whipped out your trusty soldering gun and
anti-static gear and changed all your memory chips
to 25 ns SRAMS(on a 33MHz machine w/no cache) would
the wait states go away? OR is the timing part of the
hardware architecture? (Pardon me for this seemingly
naive question, but I've been doing software for 8
7. The PC manufacturers never talk about parity error
checked memory, ECC memory, Harvard A.(separate
data/instruction cache), data write-thru cache,
write buffers (CPU can go on after issuing write
instruction only), and multi-word memory
transfers. Are PC mfgs behind the times? Or is this
because of all the canned stuff from the east.
8. Is there ANY manufacturer who has fully exploited
the power of the 80386 chip? That is, at 33MHz is
there any hardware that...
>can support sustained disk I/O >1MB/sec by
bypassing the AT bus via on-board controllers, or using VME, etc.,
>has "real" 100% zero wait state memory (probably SRAM), AND
expanded/extended memory boards (no wait states 100% of the time),
>(for PCs) has shadow RAM (BIOS & video),
>gives you several "real" 32-bit "backplane" slots
and controllers for them (Intel or Zeos?),
>operates FCC class B.
Please POST your comments.
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