Hardware FAQ Part 1

This FAQ was compiled and written by Willie Lim and Ralph Valentino
with numerous contributions by others.  Acknowledgements are listed at
end of this FAQ.

Copyright notice:

The comp.sys.ibm.pc.hardware.* Frequently Asked Questions is
distributed in the hope that it will be useful, but WITHOUT ANY
WARRANTY.  No author or distributor accepts responsibility to anyone
for the consequences of using it or for whether it serves any
particular purpose or works at all, unless he says so in writing.
Refer to the GNU General Public License for full details.

Everyone is granted permission to copy, modify and redistribute this
FAQ, but only under the conditions described in the GNU General Public
License.  Among other things, the copyright notice and this notice
must be preserved on all copies.

Where section authors are noted, the copyright is held by that author.
Where no author is noted, the copyright is held by the FAQ editors
Willie Lim (wlim@lehman.com) and Ralph Valentino (ralf@wpi.wpi.edu).

Changes, additions, comments, suggestions and questions to:
Ralph Valentino ralf@wpi.edu

Table of Contents:
S) 1.0 Introduction
 Q) 1.1  What does this FAQ cover?
 Q) 1.2  Where can I find the latest copy of this FAQ?
 Q) 1.3  Is it ok to (sell/buy/job-offer/advertise) things here?
 Q) 1.4  Where should I post?
 Q) 1.5  How come no one answers my questions?
 Q) 1.6  What are the going prices for...?
 Q) 1.7  Who makes/Where can I find [some obscure piece of hardware]?
 Q) 1.8  What is the history of the IBM PC?
S) 2.0 Motherboards
 Q) 2.1  What are the differences between the 80x86 CPUs?
 Q) 2.2  How do I pick the right processor?
 Q) 2.3  What is the difference between the 386SX/386DX and 486SX/486DX?
 Q) 2.4  What is a ZIF socket?
 Q) 2.5  What is over clocking and should I do it?
 Q) 2.6  Which is faster, a DX-50 or DX2-66
 Q) 2.7  *What is the P24T/Overdrive?
 Q) 2.8  What are the differences between the 80x87 co-processors?
 Q) 2.9  Would a math co-processor speed up my machine?
 Q) 2.10  Can I use a x387 with my 486?
 Q) 2.11  Memory terminology, what does it mean?
 Q) 2.12  What happen to my 384k?
 Q) 2.13  How do I tell how big/fast my SIMMs are?
 Q) 2.14  What speed SIMMs do I need?
 Q) 2.15  Will 9 chip and 3 chip SIMMs work together?
 Q) 2.16  What are "single-sided" and "double-sided" 72-pin SIMMs?
 Q) 2.17  Can I use Mac or PS/2 SIMMs in my PC?
 Q) 2.18  What do wait states and burst rates in my BIOS mean?
 Q) 2.19  Cache terminology, what does it mean?
 Q) 2.20  How do I upgrade the size of my cache?
 Q) 2.21  Do I need to fill the "dirty tag" RAM socket on my motherboard?
 Q) 2.22  How fast do my cache RAMs have to be?
 Q) 2.23  Which is the best cache policy, write-through or "write-back?"
 Q) 2.24  What about an n-way set associative cache, isn't it better?
 Q) 2.25  Which is better, ISA/EISA/VLB/PCI/etc?
 Q) 2.26  *What are the (dis)advantages of ISA/VLB/EISA SCSI?
 Q) 2.27  Will an ISA card work in an MCA (PS/2) machine?
 Q) 2.28  What does the "chip set" do?
 Q) 2.29  How do I enter the CMOS configuration menu?
 Q) 2.30  What is bus mastering and how do I know if I have it?
 Q) 2.31  Can I put an ISA cards in EISA or VLB slots?
 Q) 2.32  How should I configure ISA/VLB cards in the EISA config utility?
 Q) 2.33  What is the difference between EISA Standard and Enhanced modes?
 Q) 2.34  Is there any point in putting more than 16M in an ISA machine?
 Q) 2.35  What disadvantages are there to the HiNT EISA chip set?
 Q) 2.36  *Should I change the ISA bus speed?
 Q) 2.37  Why is my PC's clock so inaccurate?
 Q) 2.38  How can I automatically set my PC's clock to the correct time?
 Q) 2.39  What is the battery for and how do I replace it?
 Q) 2.40  Can I use IRQ2 or is it special?
 Q) 2.41  Where do all the IRQ's go?
S) 3.0 IO controllers/interfaces
 Q) 3.1  *How do IDE/MFM/RLL/ESDI/SCSI interfaces work?
 Q) 3.2  +How can I tell if I have MFM/RLL/ESDI/IDE/SCSI?
 Q) 3.3  Do caching controllers really help?
 Q) 3.4  Do IDE controllers use DMA?
 Q) 3.5  Why won't my two IDE drives work together?
 Q) 3.6  Which is better, VLB or ISA IDE?
 Q) 3.7  How do I install a second controller?
 Q) 3.8  Which is better, SCSI or IDE?
 Q) 3.9  Can MFM/RLL/ESDI/IDE and SCSI coexist?
 Q) 3.10  What's the difference between SCSI and SCSI-2? Are they compatible?
 Q) 3.11  Can I share SCSI devices between computers?
 Q) 3.12  What is Thermal Recalibration?
 Q) 3.13  +Can I mount my hard drive sideways/upside down?
 Q) 3.14  How do I swap A: and B:
 Q) 3.15  My floppy drive doesn't work and the light remains on, why?
 Q) 3.16  What is a 16550 and do I need one?
 Q) 3.17  *Are there any >4 serial port cards?
 Q) 3.18  Should I buy an internal or external modem?
 Q) 3.19  What do all of the modem terms mean?
 Q) 3.20  What kinds of sound cards are avalable?
 Q) 3.21  Where can I find EISA/VLB sound and IO cards?
 Q) 3.22  How does the keyboard interface work?
 Q) 3.23  Can I fake a keyboard so my computer will boot without it?
S) 4.0 Storage/Retrieval Devices
 Q) 4.1  Why do I lose x Meg on my hard drive?
 Q) 4.2  *Should I get an IDE/floppy/SCSI/parallel port tape drive?
 Q) 4.3  I have two floppies. Can I add a floppy based tape drive?
 Q) 4.4  How fast is a tape drive? Will a dedicated controller improve this?
 Q) 4.5  What is QIC80, QIC40?
 Q) 4.6  How come I can't fit as much stuff on my tape drive as they claim?
 Q) 4.7  Are Colorado/Conner/Archive/... tapes compatible with each other?
 Q) 4.8  How does the drive/software know how long the tape is?
 Q) 4.9  What are all those QICs?
 Q) 4.10  Which QICs are read/write compatible?
 Q) 4.11  What is the CMOS/jumper setting for my hard drive?
S) 5.0 Video
 Q) 5.1  Can I use two video cards in the same system?
 Q) 5.2  *What kinds of monitors are available?
 Q) 5.3  +How can I get a fixed frequency monitor to work on my PC?
 Q) 5.4  *Can I get an RGB monitor to work with my PC?
 Q) 5.5  *How can I hook more than one monitor to my video card?
 Q) 5.6  *Which video card is best for DOS/Windows/X11/OS2?
 Q) 5.7  *What is the black horizontal line on my monitor?
S) 6.0 Systems
 Q) 6.1  *What should I upgrade first?
 Q) 6.2  Do I need a CPU fan / heat sink
 Q) 6.3  What does the turbo switch do?
 Q) 6.4  How does the front panel LED display measure the system's speed?
 Q) 6.5  Should I turn my computer/monitor off?
 Q) 6.6  Are there any manufacturers/distributers who read the net?
S) 7.0 Diagnostics
 Q) 7.1  What do the POST beeps mean?
 Q) 7.2  What do the POST codes mean?
 Q) 7.3  *I think my cache is bad. What's a good diagnostic?
S) 8.0 Misc
 Q) 8.1  What is the pin out for ...?
 Q) 8.2  *Where are benchmark programs located. What do they mean?
 Q) 8.3  What is Plug and Play?
 Q) 8.4  What size should I set my DOS partitions to be?
 Q) 8.5  Why won't my system boot from the hard drive?
 Q) 8.6  How do I clean my computer?
 Q) 8.7  *What OS's are available for the PC? Which are free?
 Q) 8.8  *How can I transfer files between my PC and a Unix system?
 Q) 8.9  What tape backup software is available?
 Q) 8.10  Why doesn't my new device work as fast as it should?
 Q) 8.11  My drive lists a MTBF of 300,000 hours. Will it really last 34 years?
 Q) 8.12  How do I find pin 1 on my chip/card/cable/connector?
 Q) 8.13  I've run out of power connectors, what can I do?
S) 9.0 References
 Q) 9.1  What other FAQ's are out there?
 Q) 9.2  What do the industry acronyms stand for?
 Q) 9.3  Where can I get the ISA/EISA/VLB/PCI/etc specs?
 Q) 9.4  What books are available for the PC architecture?
 Q) 9.5  What books are available on network programming?
 Q) 9.6  +Which companies have ftp sites?
 Q) 9.7  +Which companies have WWW sites?
 Q) 9.8  What's the phone number for...
S) 10.0 Acknowledgments

* = incomplete
+ = new or significant changes since last post

S) 1.0 Introduction

Q) 1.1  What does this FAQ cover?

This FAQ covers Frequently Asked Questions from all groups in the
comp.sys.ibm.pc.hardware.* hierarchy.  Software topics are only
included if they are directly related to hardware or hardware

Q) 1.2  Where can I find the latest copy of this FAQ?

If you haven't done so, new users on the net should read
news.announce.newusers. In particular, the following posts are a good
	A Primer on How to Work With The Usenet Community
	Answers to Frequently Asked Questions About Usenet
	Hints on Writing Style for Usenet
	Introduction to The *.answers Groups

This FAQ is currently posted to news.answers, comp.answers,
comp.sys.ibm.pc.hardware.cd-rom, comp.sys.ibm.pc.hardware.chips,
comp.sys.ibm.pc.hardware.comm, comp.sys.ibm.pc.hardware.misc,
comp.sys.ibm.pc.hardware.networking, comp.sys.ibm.pc.hardware.storage,
comp.sys.ibm.pc.hardware.systems, and comp.sys.ibm.pc.hardware.video.
All posts to news.answers are archived and are available via anonymous
FTP, uucp and e-mail from the following locations:

	FTP is a way of copying file between networked computers. If
	you need help in using or getting started with FTP, send
	e-mail to mail-server@rtfm.mit.edu with
		send usenet/news.answers/ftp-list/faq
	as the body of the message.

	location:  rtfm.mit.edu []
	directory: /pub/usenet/news.answers/pc-hardware-faq
	filenames: part1 to part5

	location:  ftp.uu.net []
	directory: /archive/usenet/news.answers/pc-hardware-faq
	filenames: part1.Z to part5.Z [use uncompress]

	location:  nic.switch.ch []
	directory: info_service/Usenet/periodic-postings
	filenames: [Check info_service/Usenet/00index]

	location:  uunet!/archive/usenet/news.answers/pc-hardware-faq/
	filenames: part1.Z to part5.Z

	Send email to mail-server@rtfm.mit.edu containing these lines:
	send usenet/news.answers/pc-hardware-faq/part1
	send usenet/news.answers/pc-hardware-faq/part5

You can find a dozen or more sites in the US, Europe and Japan that
store the FAQ and archives for this various newsgroups by using the
Internet search programs, Archie or Wais.

Q) 1.3  Is it ok to (sell/buy/job-offer/advertise) things here?

No, none of the above fit within the charter of the
comp.sys.ibm.pc.hardware.* hierarchy, therefore such posts are
considered unacceptable.  For buying/selling things, use groups with
the words 'wanted' or 'forsale', and for job offers, use groups with
the words 'jobs'.  All of these can be found in the misc.* hierarchy.
For commercial advertisements, use only the biz.* hierarchy as per the
guidelines of USENET.  (refer to the news.* groups for more

Q) 1.4  Where should I post?
[From: grohol@alpha.acast.nova.edu (John M. Grohol)]

PC-Clone Hardware Newsgroup Pointer                   By: John M. Grohol

This Pointer will help you find the information you need and get your
questions answered much quicker than if you were to simply crosspost to
every hardware newsgroup in existence. It is provided as a public service.
Post your article in the most appropriate newsgroup according to its topic.
Please do not post your hardware questions to software newsgroups,
and vice-a-versa. "For Sale" articles are never appropriate to
either the hardware or software newsgroups.

Comments & suggestions are always welcome! 

Question on...                       Post to...
-----------------------------------  ----------------------------------
Networking/networks                  comp.sys.novell
				     comp.dcom.lans.* (where * equals:
				     ethernet; fddi; misc; token-ring)
PC Networking hardware/cards/cables  comp.sys.ibm.pc.hardware.networking
-----------------------------------  ----------------------------------
Laptops & notebooks (over 3 lbs.)    comp.sys.laptops
Palmtops (under 3 lbs.)              comp.sys.palmtops
-----------------------------------  ----------------------------------
Servers                              comp.dcom.servers
Modems                               comp.dcom.modems
Printers                             comp.periphs.printers
SCSI devices                         comp.periphs.scsi
Other peripherals                    comp.periphs
PCMCIA devices                       alt.periphs.pcmcia
-----------------------------------  ----------------------------------
Dell users & support                 alt.sys.pc-clone.dell
Gateway 2000 users & support         alt.sys.pc-clone.gateway2000
Micron users & support               alt.sys.pc-clone.micron
Zeos users & support                 alt.sys.pc-clone.zeos
-----------------------------------  ----------------------------------
Technical topics on PC soundcards    comp.sys.ibm.pc.soundcard.tech
Advocacy for a particular soundcard  comp.sys.ibm.pc.soundcard.advocacy
Using soundcards with games          comp.sys.ibm.pc.soundcard.games
Music & sound using soundcards       comp.sys.ibm.pc.soundcard.music
Soundcards in general                comp.sys.ibm.pc.soundcard.misc
-----------------------------------  ----------------------------------
Discussion of items, forsale groups  misc.forsale.computers.discussion
Mac-specific sale of items           misc.forsale.computers.mac-specific.*
				     (where * equals: cards.misc;misc; 
Sale of all computer memory,         misc.forsale.computers.*
   modems, monitors, net-hardware,   (where * equals: memory;modems;
   printers, storage devices         monitors;net-hardware;printers;storage)
Sale of other computer items         misc.forsale.computers.other.*
				     (where * equals: misc;software;systems)
PC-specific sale of items            misc.forsale.computers.pc-specific.*
				     (where * equals: audio;cards.misc;
Commercial sale of hardware          biz.comp.hardware
-----------------------------------  ----------------------------------
Monitors/video cards                 comp.sys.ibm.pc.hardware.video
Modems/fax cards/communication       comp.sys.ibm.pc.hardware.comm
Hard/floppy/tape drives & media      comp.sys.ibm.pc.hardware.storage 
CD-ROM drives & interfaces           comp.sys.ibm.pc.hardware.cd-rom
Computer vendors & specific systems  comp.sys.ibm.pc.hardware.systems
System chips/RAM chips/cache         comp.sys.ibm.pc.hardware.chips
-----------------------------------  ----------------------------------
Other hardware questions             comp.sys.ibm.pc.hardware.misc
-----------------------------------  ----------------------------------

This Pointer is freely distributable to any other mailing list, newsgroup,
     or network service provider as long as it remains fully intact.
	  Copyright 1994 John M. Grohol. All rights reserved.

Send comments/questions/suggestions regarding this newsgroup Pointer
to the author (replying to this message should work). Do *not* include
this Pointer in your reply, or it may not be read.

Q) 1.5  How come no one answers my questions?

If you don't give enough information when asking your question, then
people will not be able to answer it.  If you're not willing to take
the time to look up the necessary information, then why should you
expect people to take the time to answer your question?  For instance,
if you're asking a question about SCSI, it is very important to know
what type of SCSI host adapter (controller) you have.  Some other
important things to mention are which device drivers/tsr's you are
loading, what other similar devices you have in your system, and
exactly what in your setup has changed since it last worked.

Q) 1.6  What are the going prices for...?

If you're looking for new equipment, pick up a copy of Computer
Shopper.  This is the "bible" for buying new equipment.  Skim through
it for the best prices and give these distributers a call.  In most
cases, the advertisements must be placed months in advance; the actual
price may be even lower than the advertised price!  Two other things
to note are the warranty, return policy and location of the company
(companies within the same state as you may be required to add extra
sales taxes).

If you're looking for the expected price of used equipment, then scan
the newsgroup misc.forsale.computers.pc-clone for similar items.  This
will give you the best idea as what to expect.  Don't make assumptions
that the price of used equipment will follow the market trends of new
equipment.  For instance, when new memory prices nearly doubled, the
used prices were barely effected.

Q) 1.7  Who makes/Where can I find [some obscure piece of hardware]?
[From: uwvax!astroatc!nicmad!madnix!zaphod (Ron Bean)]

You can ask on the net, but you'll get a better response if you do
some investigating on your own first. Try calling vendors who
advertise similar or related hardware, they often have things that
aren't in the ads. Vendors who specialize in parts rather than
complete systems are a good bet. You can also ask local dealers to
check their wholesale sources.

Q) 1.8  What is the history of the IBM PC?

Around 1978 and '79, the market served by IBM's Data Entry Systems
division began to change.  Instead of terminals and minicomputers or
mainframes, customers began demanding autonomous, low cost,
single-user computers with minimal compute power or connectivity, but
compliance to standards like the ASCII alphabet and the BASIC
programming language.  The closest product in IBM's line was the 5110,
a closed, BASIC-in-ROM machine with a tiny built-in character display.
The 5110 was uncompetitive, and IBM started losing bids from key
customers, mostly government agencies.

Data Entry commissioned a consulting firm (Boca Associates?) to design
a stop-gap machine to fill what was perceived within IBM as a
short-lived, specialized niche.  It was intended that the stop-gap
machine would only be offered for a couple of years until it would be
replaced in "The Product Line" by an internal IBM design.  Some IBM
executives believed the single-user desktop system was a fad which
would die out when the shortcomings of such systems became

The motherboard design was based very closely on a single-board
computer described in a 1978 (?) Intel application note.  (Anybody got
an original copy of this collector's item?  Among other things, Intel
argues that 640KB is more memory than single-user applications will
ever need, because of the efficiency of segmented memory
"management"!)  The expansion slot "bus" is based on an Intel bus
called Multibus 1, which Intel introduced in its microprocessor
software development equipment in the mid '70s.  The Monochrome and
Color Graphics Display Adapters are based on application notes for the
Motorola 6845 video controller chip, except that the strangely
interlaced pixel addresses in the CGA appears to have been extremely
short sighted.  The "event driven" keyboard is an original design, but
the concept is from the Xerox Alto and Star graphics workstations.
The keyboard noise and "feel" are intended to emulate those of the IBM
Selectric typewriter.  The Cassette Interface design is original, but
similar in concept to the one on the Radio Shack TRS-80.

Data Entry Division approached Digital Research Inc. to offer its
popular CP/M-86 operating system on the machine, but DRI rebuffed
them.  IBM's second choice was BASIC-in-ROM vendor Microsoft, which
had no OS product at the time but quickly purchased a crude disk
operating system called 86-DOS from Seattle Computer Products to offer
it to IBM.  Its command interpreter was an imitation of Unix' Bourne
Shell, with the special characters changed to avoid infringing AT&T's

Data Entry Division began bidding this system in various State
procurements, without any plan to offer it to the public.

It became obvious that the Cassette Interface and optional 360KB
Flexible Disk Drive were inadequate.  The Cassette Interface was
dropped, and an optional Fixed Disk Drive offered on a revised model
known as the IBM Personal Computer XT.  (A fixed, or "hard" disk had
been offered on the PC by special order, with a Xebec controller, but
few were sold.)  The disk controller was designed around the Western
Digital 1010 chip, and its design is taken directly from a WD
application note.

The XT succeeded beyond all expectations.  IBM offered the system to
the public after it became clear that no other division was going to
come up with anything timely.  IBM published complete schematics and
ROM listings, encouraging clones.

In 1984, IBM introduced an upwardly compatible model based on the
Intel 80286.  The expansion slot "bus" was extended to 16-bit data
path width the same way Intel had extended Multibus: by adding data
and address bits, a signal for boards to announce their capability to
perform 16-bit transfers, and byte swapping on the motherboard to
support the 8-bit boards.

S) 2.0 Motherboards

Q) 2.1  What are the differences between the 80x86 CPUs?

This section is posted separately as the "Personal Computer Chiplist"
and archived along side this FAQ.  Refer to section one for
instructions on retrieving this file.

Newsgroups: comp.sys.ibm.pc.hardware.chips,comp.sys.ibm.pc.hardware.systems,
Subject: Personal Computer CHIPLIST 7.0 part * of *
From: offerman@einstein.et.tudelft.nl (Aad Offerman)
Summary: This list contains the various CPU's and NPX's and their features,
	 used in the IBM PC, IBM PC/XT, IBM PC/AT, IBM PS/2 and compatbles,
	 and the differences between them.
Archive-name: pc-hardware-faq/chiplist

Q) 2.2  How do I pick the right processor?
[From: jabram@ichips.intel.com (Jeff Abramson)]

This is a hard question.  You have tradeoffs between price,
performance, compatibility, upgradebility, and power consumption.
As a desktop unit owner, you probably have less concerns about
power, but as a laptop owner, this is very important.

The frequency of the CPU defines how fast its internal clock runs.
This defines how fast instructions are executed.  In many ways, this
is meaningless, because a RISC machine (MIPS) running at 100MHz may
in reality be slower than a 50Mhz i486 because a RISC system must
execute more instructions to perform the same function (in some
cases).  Even when comparing processors in the same family, this
info can be misleading.  For example, an Intel486-25 is faster than
an AMD386-40, since the 486 has microarchitectural advancements over
the 386.  The same can be said for the Pentium, where a 66Mhz
Pentium is twice as fast as a 66MHz 486.

For compatibility, keep in mind that the Intel parts are the basis
for all of these processors.  Therefore you always run the risk that
an imitator's part may not be compatible.  AMD chips are compatible
because they are copied.  For some of you, these factors may be

As far as upgradability goes, this depends on both your motherboard
and the processor.  If you purchase a 486DX, then you can upgrade to
a DX2 and double your internal clock simply by buying an overdrive
chip if your motherboard has the ZIF socket.  If it doesn't then you
can replace the CPU with a DX2.  Many new 486 motherboards contain
overdrive sockets for the not-yet-released Pentium chip that is pin

Q) 2.3  What is the difference between the 386SX/386DX and 486SX/486DX?

[From: jabram@ichips.intel.com (Jeff Abramson)]

The Intel386DX contains full 32 bit buses for external data,
internal data, and address.  The Intel386SX contains a smaller 16
bit external data bus, and a smaller 24 bit address bus.

The Intel486DX contains a floating point unit, the Intel486SX does
not.  A common rumor is that the 486SX is simply a DX part that has
a failure in the floating point unit, so it has been disabled and
the part has been produces as an SX.  This was true for early
production parts and samples, but not for the mass produces SX parts
that we see today.  

Q) 2.4  What is a ZIF socket?

[From: jabram@ichips.intel.com (Jeff Abramson)]

ZIF stands for Zero Insertion Force, and describes a socket on your
motherboard that supports an upgrade processor (overdrive
processor).  In general, an overdrive upgrade works in conjunction
with your original processor so you cannot remove the original
processor after upgrade.  NOTE: Some motherboards do not have a ZIF
socket so you must replace the existing processor to upgrade.

Q) 2.5  What is over clocking and should I do it?
[From: jabram@ichips.intel.com (Jeff Abramson)]

Overclocking is a term generally used to describe how you have
increased the clock frequency on your board to run your system at a
higher speed.  For example, if you plug a 25MHz i486 into a board
that is configured to run a 33MHz i486, then you are overclocking
your CPU.  Most boards allow you to configure your clocking via
jumpers, and others require a new clock oscillator.

Although users have had success with overclocking, it is a dangerous
practice for two reasons.  First, the chip has been designed to meet
a certain speed.  Therefore, some circuits do not have the margin to
operate at a higher frequency.  The chips coming from a wafer have
various speed specs (statistical distribution), so you may be lucky
and own a CPU that has the circuit margins you need to overclock.
But you don't know - and if you overclock, you may get data failure.
The data failure may be reproducable - and therefore avoidable, but
most likely not.

Second, you have reliability concerns when overclocking.
Overclocking means faster frequency, which means more current and
power.  This can lead to real failures in your CPU.
Electromigration is one such failure where metal lines in your CPU
will actually break or connect if they get too much current.  This
is irreversable, and most likely not covered under warranty.

So when can you overclock?  Really only if you don't care about
burning out your CPU and you don't care if you get wrong data every
now and then.  If you own a machine and you use it just for games,
then overclocking may be something to try - and you simply upgrade
to a new CPU when you burn out the current one.  Otherwise, it's not
worth the small performance gain.

Q) 2.6  Which is faster, a DX-50 or DX2-66

The two processors are relatively close for overall usage.  The DX-50
has more I/O bandwidth and the DX2-66 has more computational power.

Q) 2.7  *What is the P24T/Overdrive?
Q) 2.8  What are the differences between the 80x87 co-processors?

See reference in: "What are the differences between the 80x86 CPUs?"

Q) 2.9  Would a math co-processor speed up my machine?

[From: jruchak@mtmis1.mis.semi.harris.com (John Anthony Ruchak)]

If you do a lot of number-crunching with CAD/CAM applications,
spreadsheets, and the like, a math co-processor is likely to increase
performance.  If on the other hand, your primary work is word
processing, a math co-processor will have barely any effect at all.
Also, a math co-processor will not provide any benefit if your CPU
already has one built-in (486/586-DX chips).  In addition, a math
co-processor is not likely to improve the over-all performance of
Microsoft Windows, except when you are running the afore-mentioned
number-crunching programs.

Q) 2.10  Can I use a x387 with my 486?

[From: Shaun Burnett (burnesa@cat.com)]

No, they are not pin compatible.  The 486DX and above contain an
on-chip floating point unit. Therefore, a 387 (SX or DX) math
coprocessor is not needed.  All software written for a 387 coprocessor
will run on your 486.
If you want a math coprocessor for a 486SX, you need to purchase the
487SX or a 486 Overdrive processor.
While we're talking about math coprocessors, I'll make a brief note
about the Weitek.  Some motherboards may have a socket for a Weitek
math coprocessor.  These coprocessors are not compatible with the
Intel 387 math coprocessor and should only be used if your software
requires it.  The Weitek 3167 replaced the Weitek 1167 and is for the
386 while the Weitek 4167 is for a 486.

Q) 2.11  Memory terminology, what does it mean?
[From: cls@truffula.sj.ca.us (Cameron L. Spitzer)]

Read/write memory in computers is implemented using Random Access Memory
chips (RAMs).  RAMs are also used to store the displayed image in a video
board, to buffer frames in a network controller or sectors in a disk
controller, etc.  RAMs are sold by their size (in bits), word width (how
many bits can you access in one cycle), and access time (how fast you
can read a location), among other characteristics.

RAMs can be classified into two types: "static" and "dynamic."

In a static RAM, each bit is represented by the state of a circuit
with two stable states.  Such a "bistable" circuit can be built with four
transistors (for maximum density) or six (for highest speed and lowest
power).  Static RAMs (SRAMs) are available in many configurations.
(Almost) all SRAMs have one pin per address line, and all of them
are able to store data for as long as power is applied, without any
external circuit activity.

In a dynamic RAM (DRAM), each bit is represented by the charge on a
*very* small (30-50 femptofarads) capacitor, which is built into a
single, specialized transistor.  DRAM storage cells take only about
a quarter of the silicon area that SRAM cells take, and silicon
area translates into cost.
The cells in a DRAM are organized into rows and columns.  To access
a bit, you first select its row, and then you select its column.
Unfortunately, the charge leaks off the capacitor over time,
so each cell must be periodically "refreshed" by reading it and
writing it back.  This happens automatically whenever a row is accessed.
After you're finished accessing a row, you have to give the DRAM time
to copy the row of bits back to the cells: the "precharge" time.

Because the row and column addresses are not needed at the same
time, they share the same pins.  This makes the DRAM package smaller
and cheaper, but it makes the problem of distributing the signals
in the memory array difficult, because the timing becomes so
critical.  Signal integrity in the memory array is one of the
things that differentiate a lousy motherboard from a high quality

Through the 1970s, RAMs were shipped in tubes, and the board makers
soldered them into boards or plugged them into sockets on boards.
This became a problem when end-users started installing their own
RAMs, because the leads ("pins") were too delicate.  Also, the
individual dual in-line package (DIP) sockets took up too much board
In the early 1980s, DRAM manufacturers began offering DRAMs on tiny
circuit boards which snap into special sockets, and by the late '80s
these "single in-line memory modules" (SIMMs) had become the most popular
DRAM packaging.  Board vendors who didn't trust the new SIMM sockets
used modules with pins: single inline pinned packages (SIPPs),
which plug into sockets with more traditional pin receptacles.

PC-compatibles store each byte in main memory with an associated
check bit, or "parity bit."  That's why you add memory in multiples
of nine bits.  The most common SIMMs present nine bits of data at
each cycle (we say they're "nine bits wide") and have thirty contact
pads, or "leads."  (The leads are commonly called "pins" in the trade,
although "pads" is a more appropriate term.  SIMMs don't *have* pins!)

At the high end of the PC market, "36 bit wide" SIMMs with 72 pads
are gaining popularity.  Because of their wide data path, 36-bit SIMMs
give the motherboard designer more configuration options (you can
upgrade in smaller chunks) and allow bandwidth-enhancing tricks
(i.e. interleaving) which were once reserved for larger machines.
Another advantage of 72-lead SIMMs is that four of the leads are used
to tell the motherboard how fast the RAMs are, so it can configure
itself automatically.  (I do not know whether the current crop of
motherboards takes advantage of this feature.)

"3-chip" and "9-chip" SIMMs

In 1988 and '89, when 1 megabit (1Mb) DRAMs were new, manufacturers
had to pack nine RAMs onto a 1 megabyte (1MB) SIMM.  Now (1993) 4Mb DRAMs
are the most cost-effective size.  So a 1MB SIMM can be built with
two 4Mb DRAMs (configured 1M x4) plus a 1Mb (x1) for the check-bit.

In graphics-capable video boards, the displayed image is almost
always stored in DRAMs.  Access to this data must be shared between
the hardware which continuously copies it to the display device (this
process is called "display refresh" or "video refresh") and
the CPU.  Most boards do it by time-sharing ordinary, single-port
DRAMs.  But the faster, more expensive boards use specialized DRAMs
which are equipped with a second data port whose function is tailored
to the display refresh operation.  These "Video DRAMs" (VRAMs) 
have a few extra pins and command a price premium.  They nearly double
the bandwidth available to the CPU or graphics engine.

(As far as I know, the first dual-ported DRAMs were built by Four-
Phase Systems Inc., in 1970, for use in their "IV-70" minicomputers, which
had integrated video.  The major DRAM vendors started offering VRAMs
in about 1983 [Texas Instruments was first], and workstation vendors
snapped them up.  They made it to the PC trade in the late '80s.)

DRAMs are characterized by the time it takes to read a word,
measured from the row address becoming valid to the data coming out.
This parameter is called Row Access Time, or tRAC.  There are many
other timing parameters to a DRAM, but they scale with tRAC
remarkably well.  tRAC is measured in nanoseconds (ns).
A nanosecond is one billionth (10 e-9) of a second.

It's so difficult to control the semiconductor fabrication processes,
that the parts don't all come out the same.  Instead, their performance
varies widely, depending on many factors.  A RAM design which would
yield 50 ns tRAC parts if the fab were always tuned perfectly, instead 
yields a distribution of parts from 80 to 50.  When the plant is new,
it may turn out mostly nominal 70 ns parts, which may actually deliver 
tRAC between 60.1 ns and 70.0 ns, at 70 or 85 degrees Celcius and
4.5 volts power supply.  As it gets tuned up, it may turn out mostly 60 
ns parts and a few 50s and 70s.  When it wears out it may get less 
accurate and start yielding more 70s again.

RAM vendors have to test each part off the line to see how fast it is.  
An accurate, at-speed DRAM tester can cost several million dollars, and 
testing can be a quarter of the cost of the parts.  The finished parts 
are not marked until they are tested and their speed is known.

Q) 2.12  What happen to my 384k?

The memory between 640k and 1Meg is used for the BIOS, the video
aperture, and a number of other things.  With the proper memory
manager, DOS can take advantage of it.  Many systems, however, won't
identify its existence on boot.  This does not mean it isn't there.

Q) 2.13  How do I tell how big/fast my SIMMs are?

Individual DRAMs are marked with their speed after they are tested.
The mark is usually a suffix to the part number, representing tens of
nanoseconds.  Thus, a 511024-7 on a SIMM is very likely a 70 ns DRAM.
(vendor numbering scheme table to be added)

Q) 2.14  What speed SIMMs do I need?

[From: cls@truffula.sj.ca.us (Cameron L. Spitzer)]

There is no reliable formula for deriving the required RAM speed from
the clock rate or wait states on the motherboard.  Do not buy a
motherboard that doesn't come with a manual that clearly specifies
what speed SIMMs are required at each clock rate.  You can always
substitute *faster* SIMMs for the ones that were called out in the
manual.  If you are investing in a substantial quantity of RAM,
consider buying faster than you need on the chance you can keep it
when you get a faster CPU.

That said, most 25 MHz and slower motherboards work fine with 80 ns
parts, most 33 MHz boards and some 40 MHz boards were designed for
70 ns parts, and some 40 MHz boards and everything faster require
60 ns or faster.  Some motherboards allow programming extra wait states
to allow for slower parts, but some of these designs do not really relax
all the critical timing requirements by doing that.  It's much
safer to use DRAMs that are fast enough for the no-wait or one-wait
cycles at the top end of the motherboard's capabilities.

Q) 2.15  Will 9 chip and 3 chip SIMMs work together?

[From: cls@truffula.sj.ca.us (Cameron L. Spitzer)]

Almost always.  But there are exceptions.

1.  Some motherboards do not supply enough refresh address bits for a
4Mb x1 or a 1Mb x4 DRAM.  These old motherboards will not work with 4
MB 9-chip SIMMs or 1 MB 3-chip SIMMS.

2.  Some EL CHEAPO motherboards do not have proper terminations on the
lines which drive the DRAM array.  These boards may show only marginal
compatibility with various SIMMs, not working with all prefectly good
SIMMs you try, favoring SIMMs with parameters skewed towards one end
or another of the allowed ranges.  In some cases, most of the SIMMs
you happen to try might be 9-chip modules, and in other cases they
might be 3-chip modules.  A random selection of a dozen SIMMs might
lead you to conclude the motherboard doesn't "work" with 3-chip
modules, or with a "mixture" of 3-chip and 9-chip modules.

You might find the real solution is to use SIMMs one speed faster
than the manual calls for, because the particular motherboard design
just cuts too many things too close.

Q) 2.16  What are "single-sided" and "double-sided" 72-pin SIMMs?

[From: uwvax!astroatc!nicmad!madnix!zaphod (Ron Bean)]
   All 72-pin SIMMs are 32 bits wide (36 with parity), but
double-sided SIMMs have four RAS (Row Address Strobe) lines instead of
two. This can be thought of as two single-sided SIMMs wired in
parallel. But since there is only one set of data lines, you can only
access one "side" at a time.
   Usually, 1Mb, 4Mb, and 16Mb 72-pin SIMMs are single-sided, and 2Mb,
8Mb, and 32Mb SIMMs are double-sided. This only refers to how the
chips are wired-- SIMMs that are electrically "single-sided" may have
chips on both sides of the board.
   Most 486 motherboards use memory in banks of 32 bits (plus parity),
and may treat a double-sided SIMM as "two banks" (see your
motherboard's manual for details). Some can take four SIMMs if they're
single-sided, but only two if they're double-sided. Others can take
four of either type.
   Pentium (and some 486) motherboards use pairs of 72-pin SIMMs for
64-bit memory. Since double-sided SIMMs can only access 32 bits at a
time, you still need to use them in pairs to make 64 bits.

Q) 2.17  Can I use Mac or PS/2 SIMMs in my PC?

Yes, just about all SIMMs are compatible, be they from another
personal computer, a mainframe, or even a laser printer, though are a
few some odd systems out there.  There are three significant issues:
speed, parity and number of pins (data width).  Speed is obvious,
check the rating, ie: 70ns, to make sure they meet the minimum
requirements of your system.  Parity either exists or doesn't exist
and can be identified by an extra bit per byte, ie: 9 bits or 36 bits.
If your system does not require parity, you can still use SIMMs with
parity.  If, however, your system does require parity, you can't use
SIMMs without parity.  For this case, many PC's have an option to
disable the parity requirement via a jumper or BIOS setting; refer to
your motherboard manual.  The final issue is the number of pins on the
SIMM; the two most common are 30 pins (8 or 9 bit SIMMs) and 72 pins
(32 or 36 bit SIMMs); the second is physically larger thus the one can
not be used in the other.  A few motherboards have both types of

Q) 2.18  What do wait states and burst rates in my BIOS mean?

[From: cls@truffula.sj.ca.us (Cameron L. Spitzer)]

Modern motherboards are equipped with variable clocks and features
for tuning board performance at each speed.  The BIOS knows how to
program the register bits which control these options.

1.  Wait states may be adjustable to allow for slower DRAMs or
cache RAMs.  If you don't have a motherboard manual, or it doesn't
say, then you will just have to experiment.
2.  Sometimes a wait or two on a write is required with write-through
cache.  The programming allows for slower DRAMs.  The extra wait
state may cost you enough time that you would do better running at a
slower clock rate where the wait state is not required.
3.  Burst rates refer to the number of wait states inserted for
each longword access in the cache fill cycle.

Bob Nichols (rnichols@ihlpm.ih.att.com) adds:
These numbers refer to the number of clock cycles for each access of a
"burst mode" memory read.  The fastest a 486 can access memory is 2 clock
cycles for the first word and 1 cycle for each subsequent word, so
"2-1-1-1" corresponds to "zero wait states."  Anything else is slower.

How fast you can go depends on the external clock speed of your CPU, the
access time of your cache SRAMs, and the design of the cache controller.
It can also be affected by the amount of cache equipped, since "x-1-1-1"
is generally dependent on having 2 banks of cache SRAMs so that the
accesses can be interleaved.  With a 50MHz bus (486DX-50), few
motherboards can manage "2-1-1-1" no matter how fast the SRAMs are.  At
33MHz or less (486DX-33, 486DX2-66), many motherboards can achieve
"2-1-1-1" if the cache SRAMs are fast enough and there are 2 banks
equipped (cache sizes of 64KB or 256KB, typically).

Q) 2.19  Cache terminology, what does it mean?
[From: cls@truffula.sj.ca.us (Cameron L. Spitzer)]

Why cache improves performance
Today's microprocessors ("uPs") need a faster memory than can be made
with economical DRAMs.  So we provide a fast SRAM buffer
between the DRAM and the uP.  The most popular way to set it up is
by constructing a "direct mapped cache," which is the only setup
I'll describe here.

Generic motherboard cache architecture
The direct mapped cache has three big features:
1. a "data store" made with fast SRAMs,
2  a "tag store" made with even faster SRAMs, and
3. a comparator.

The data store is the chunk of RAM you see in the motherboard price
lists.  It holds "blocks" or "lines" of data recently used by the CPU.
Lines are almost always 16 bytes.  The address feeding the cache is
simply the least significant part of the address feeding main memory.
Each memory location can be cached in only one location in the data

There are two "policies" for managing the data store.  Under the
"write-back" (or "copy-back") policy, the master copy of the data is
in cache, and main memory locations may be "stale" at times.  Under
"write-through", writes go immediately to main memory as well as to
cache and memory is never "stale."

The tag store mantains one "word" of information about each line of
data in the data store.
In a "write-back" or "copy-back" cache, the tag word contains two items:
1.  the part of the main memory address that was *not*
    fed to the data store, and
2.  a "dirty" bit.

A write-through cache doesn't need a dirty bit.  The tag store is
addressed with the most significant address bits that are being fed to
the data store.  The tag is only concerned with the address bits that
are used to select a line.  With a 16 byte line, address bits 0
through 3 are irrelevant to the tag.

An example: The motherboard has 32 MB main memory and 256 KB cache.
To specify a byte in main memory, 25 bits of address are required: A0
through A24.  To specify a byte in data store, 18 bits (A0 through
A17) are required.  Lines in cache are 16 bytes on 16 byte boundaries,
so only A3 through A17 are required to specify a line.  The tag word
for this system would represent A18 through A24 (plus dirty bit).  The
tag store in this system would be addressed by A3 through A17,
therefore the tag store would require 16 K tag words seven bits wide.
The dirty bit is written at different times than the rest of the tag,
so it might be housed separately, and this tag store might be built in
three 16K x4 SRAMs.

What happens when it runs
Each motherboard memory cycle begins when the uP puts out a memory
address.  The data store begins fetching, and simultaneously the
tag begins fetching.  When the tag word is ready, the Comparator
compares the tag word to the current address.

If they match, a cache hit is declared and the uP reads or writes
the data store location.  If the hit is a write, the copy-back
cache marks the line "dirty" by setting its dirty-bit in the line's
tag word.  The write-through motherboard simultaneously stores the
write data in data store and begins a DRAM write cycle.  The uP
moves on.

If the tag word doesn't match, what a bummer, it's a cache miss.
If the line in cache is dirty, double bummer, the line must be
copied back to main memory before anything else can happen.  All
16 bytes are copied back, even if the hit was a one-byte write.
This data transfer is called a "dirty write flush."

On a read-miss, the motherboard has to copy a line from main memory
to cache (and update the tag, the whole operation is called a "cache
fill"), and the uP can stop waiting as soon as the bytes it wants
go by.  On a write-miss, the caches I've worked with ignore the
event (that's an oversimplification) and the main memory performs
a write cycle.  I've heard of systems that fill on a write-miss,
that is they replace the cache line whenever it misses, read or
write, dirty or not.  I've never seen such a system.

The 486, the 68020, and their descendants have caches on chip.
We call the on-chip cache "primary" and the cache on the
motherboard "secondary."  The 386 has no cache, therefore the cache
on a 386 motherboard is "primary."  I like to call the DRAM array
"core" for brevity.  Motherboard = "mb."  Megabyte = "MB."

I added "core" and I had to disable my secondary cache to
get the board running.  Or, I added core and performance took a dive.
Disabling secondary cache improved it, but still real slow.
What happened?

 Whenever you are adding memory and you cross a power-of-2
address boundary, another address bit becomes interesting to the
tag.  That is, the tag does not care when you add your 8th MB
(MB) but it cares a lot about the new address bit 24 when you add
your 9th MB, or your 17th (bit 25).  Evidently, at the low-price
end of the mb market there are boards with not enough tag RAM
sockets to support all the core they can hold.  Most of these EL
CHEAPO mbs don't even try to use cache in the region beyond the
tag's coverage.  Some of them don't have the logic to stay out or
the BIOS doesn't know to enable it.  These boards just don't run

Do not buy a mb if you are not sure it can cache all of core.  The
worst case is with core fully stuffed with whatever the board claims
to hold, and the smallest cache configuration.  Some motherboards
ask you to add cache when you add core, so that they don't have to
provide for that worst case tag width.  These motherboards may ask
you to move some jumpers in the tag area.  The jumpers control
which address bits the tag looks at.  Do not buy a motherboard if
you don't know how to set all the jumpers.

Q) 2.20  How do I upgrade the size of my cache?

Look in your motherboard manual.  Each motherboard is different.
You will have to add or replace cache RAMs and move jumpers.

Q) 2.21  Do I need to fill the "dirty tag" RAM socket on my motherboard?

[From: cls@truffula.sj.ca.us (Cameron L. Spitzer)]

Perhaps you don't *have* to for the board to run, but the missing RAM
will cost you performance.  Most "write-back" mbs cope with the
missing RAM by treating all lines as dirty.  You get a lot of
unneccessary write cycles; you might even do better with

Your bargain-basement no-documentation no-brand mb might not have the
pullup resistor on that socket, and it might run for a second, ten
minutes, or ten years with that pin not driven.  I think it's a
pointless risk to leave the socket empty.

Q) 2.22  How fast do my cache RAMs have to be?

[From: cls@truffula.sj.ca.us (Cameron L. Spitzer)]

Only the person who designed your mb knows for sure.  There is
no simple formula related to clock rate.  However, most people tell
me their 33 MHz mbs' manuals call for 25 ns data store and 20 ns
tag store, and their 40 and 50 MHz mbs want 20 ns data store and
15 or 12 ns tag.  Tqhe tag has to be faster than data store to make
time for the comparator to work.  Do not buy a motherboard if you do
not know what speed and size of cache RAMs it requires in all its
speeds and configurations.

If you're not sure, it doesn't hurt to use faster RAMs than your
manual calls for.  If your manual says 20 ns for location x and you
happen to have 15 ns parts, it's ok to "mix" the speeds.  It's ok to
"mix" RAMs from more than one manufacturer.  However, the faster RAMs
will not buy you more performance.

Q) 2.23  Which is the best cache policy, write-through or "write-back?"

[From: cls@truffula.sj.ca.us (Cameron L. Spitzer)]

For most applications, copy-back gives better performance than
write-through.  The amount of win will depend on your application and
may not be significant.  Write-through is simpler, but not by much any

Q) 2.24  What about an n-way set associative cache, isn't it better?

[From: cls@truffula.sj.ca.us (Cameron L. Spitzer)]

At the high end of the mb market, caches are available with more than
one set.  In these caches, the data store is broken into two or four
parts, or sets, with a separate tag for each.  On a miss, clever
algorithms (such as Least Recently Used) can be used to pick which set
will be filled, because each set has a candidate location.  The result
is a higher hit rate than a direct mapped (single set) cache the same
size can offer.

The primary cache on the 486 is four-way set associative.

Q) 2.25  Which is better, ISA/EISA/VLB/PCI/etc?
[From: ralf@wpi.wpi.edu (Ralph Valentino)]

Here is a quick overview of the various bus architectures available
for the PC and some of the strengths and weaknesses of each.  Some
terms are described in more detail at the bottom.

XT bus:
	8 data bits, 20 address bits
	4.77 MHz
   Comments: Obsolete, very similar to ISA bus, many XT cards will
	     work in ISA slots.
ISA bus: Industry Standard Architecture bus (aka. AT bus)
	8/16 data bits, 24 address bits (16Meg addressable)
	8-8.33MHz, asynchronous
	5.55M/s burst
	bus master support
	edge triggered TTL interrupts (IRQs) - no sharing
	low cost
   Comments: ideal for low to mid bandwidth cards, though lack of
	     IRQs can quickly become annoying.
MCA bus: Micro Channel Architecture bus
	16/32 data bit, 32 address bits
	80M/s burst, synchronous
	full bus master capability
	good bus arbitration
	auto configurable
	IBM proprietary (not ISA/EISA/VLB compatible)
   Comments: Since MCA was proprietary, EISA was formed to compete with
	     it.  EISA gained much more acceptance; MCA is all but dead.
EISA bus: Enhanced Industry Standard Architecture bus
	32 data bits, 32 address bits
	8-8.33MHz, synchronous
	32M/s burst (sustained)
	full bus master capability
	good bus arbitration
	auto configurable
	sharable IRQs, DMA channels
	backward compatible with ISA
	some acceptance outside of the PC architecture
	high cost
   Comments: EISA is great for high bandwidth bus mastering cards
	     such as SCSI host adaptors, but its high cost limits
	     its usefulness for other types of cards.

P-EISA: Pragmatic EISA (also Super-ISA)
	(see the description of the HiNT chipset elsewhere in this FAQ)

VLB: VESA Local Bus
	32 data bits, 32 address bits
	25-40MHz, asynchronous
	130M/s burst (sustained is closer to 32M/s)
	bus master capability
	will coexist with ISA/EISA
	slot limited to 2 or 3 cards typical
	backward compatible with ISA
	moderate cost
   Comments: VLB is great for video cards, but its lack of a good bus
	     arbiter limits its usefulness for bus mastering cards and
	     its moderate cost limits its usefulness for low to mid 
	     bandwidth cards.  Since it can coexist with EISA/ISA, a
	     combination of all three types of cards usually works best.
PCI:  Peripheral Component Interconnect local bus
	32 data bits (64 bit option), 32 address bits (64 bit option)
	up to 33MHz, synchronous
	132M/s burst (sustained) (264M/s with 64 bit option)
	full bus master capability
	good bus arbitration
	slot limited to 3 or 4 cards typical
	auto configurable
	will coexist with ISA/EISA/MCA as well as another PCI bus
	strong acceptance outside of the PC architecture
	moderate cost
   Comments: The newest of the buses, combining the speed of VLB with
	     the advanced arbitration of EISA.  Great for both video
	     cards and bus mastering SCSI/network cards.


Auto configurable:  Allows software to identify the board's
requirements and resolve any potential resource conflicts

Bus master support:  Capable of First Party DMA transfers.

Full bus master capability:  Can support any First Party cycle from any
device, including another CPU.

Good bus arbitration:  Fair bus access during conflicts, no need to
back off unless another device needs the bus.  This prevents CPU
starvation while allowing a single device to use 100% of the available
bandwidth.  Other buses let a card hold the bus until it decides to
release it and attempts to prevent starvation by having an active card
voluntarily release the bus periodically ("bus on time") and remain
off the bus for a period of time ("bus off time") to give other
devices, including the CPU, a chance even if they don't want it.

16Meg addressable:  This limits first party DMA transfers to the lower
16 Meg of address space.  There are various software methods to
overcome this problem when more than 16 Megs of main memory are
available.  This has no effect on the ability of the processor to
reach all of main memory.

Backward compatible with ISA:  Allows you to place an ISA card in the
slot of a more advanced bus.  Note, however, that the ISA card does
not get any benefit from being in an advanced slot, instead, the slot
reverts to an ISA slot.  Other slots are unaffected.

Q) 2.26  *What are the (dis)advantages of ISA/VLB/EISA SCSI?

Q) 2.27  Will an ISA card work in an MCA (PS/2) machine?

No, they will not.  MCA, unlike EISA and VLB, is not backward
compatible with ISA.

Q) 2.28  What does the "chip set" do?
[From: cls@truffula.sj.ca.us (Cameron L. Spitzer)]

The motherboard "chip set" contains all the logic that's not in
the microprocessor ("uP") and its coprocessor, or the memory.
These functions always include:
 *  Address decoding and "memory mapping"
 *  keyboard interface controller (which includes reset generator)
 *  Direct Memory Access (DMA) channels
 *  interrupt controller
 *  bus controller(s)
 *  battery-powered "real time" clock/calendar circuit
 *  crystal-controlled clock oscillator(s)
 *  main memory controller
They almost always include:
 *  controller for cache external to the uP
 *  "turbo" switch logic
 *  programmable "wait state" logic

and some of them include:
 *  controller(s) for PCMCIA slots
 *  "green" power-conservation logic
 *  video display logic for CRT, LCD, or both
 *  serial ports, parallel ports, floppy, SCSI and/or IDE, etc. controllers
 *  network interface controllers (for Ethernet)

Some people consider the BIOS ROM part of the "chip set."

Sometimes part of an EISA or VLB bus controller is implemented
in an optional, socketed integrated circuit.  A motherboard like that
can be sold with the socket empty, and you have to go back and
buy the "bus mastering option" later when you find out you need it.

"Chip sets" are usually a set of highly integrated, special purpose
integrated circuits.  The keyboard interface controller is usually
in a 40-pin dual-inline pin (DIP) package compatible with the Intel
8048 single-chip microcomputer which was used for that function in the
IBMPC-AT.  The rest of the logic often fits in a single IC.  In the trade,
you may see this single IC referred to as "the chipset," even though
the keyboard interface and other logic is external.  The Asian data sheets
often call the high-integration chips "LSIs."

The word "ChipSet" is a trademark of Chips and Technologies
Inc. (San Jose, California), which introduced a 5-chip set of LSIs
for AT-clone motherboards in early 1985.  CTI may also own "chipset"
and "Chipset"; I don't know.
CTI was very successful at promoting the term "ChipSet," but less
successful at associating it in the public mind with their particular
brand.  People use the word to refer to any high integration chip
used in PCs.  For example, you'll hear people talk about the "ET4000
video chipset."  The ET4000 is a single chip which integrates most
of an SVGA controller.  The word "ASIC" (application-specific integrated
circuit) would be more appropriate.

Single, high-integration ICs are not very good at driving heavily
loaded signals, like the ones in the memory array and the expansion
slots.  Better motherboards use buffer chips external to the LSI
for this electrical function.  It may not show up in "WinMark"
comparisons, but it shows in electrical compatibility.  Well-buffered
motherboards are less likely to require SIMM "cherry-picking,"
and are more likely to work at high ambient temperatures.
The 74F245, which costs about 15 cents in high volume, is often used for
this electrical buffering.

Q) 2.29  How do I enter the CMOS configuration menu?

[From: burnesa@cat.com (Shaun Burnet)]

	AMI BIOS        Del key during the POST
	Award BIOS      Ctrl-Alt-Esc
	DTK BIOS        Esc key during the POST
	IBM PS/2 BIOS   Ctrl-Alt-Ins after Ctrl-Alt-Del
	Phoenix BIOS    Ctrl-Alt-Esc or Ctrl-Alt-S

Q) 2.30  What is bus mastering and how do I know if I have it?

Bus mastering is the ability of an expansion (ISA/EISA/VLB/MCA/etc)
card to directly read and write to main memory.  This allows the CPU
do delegate I/O work out to the cards, freeing it to do other things.
For all of the above busses, bus mastering capability is assumed.
Unless specifically stated otherwise, you should assume each slot has
this capability.  For cards, this is not assumed.  If you want a bus
mastering card, you should specifically request it and expect to pay
more.  Note that some cards (RLL/MFM/IDE/com) are not available in bus
mastering versions.

Q) 2.31  Can I put an ISA cards in EISA or VLB slots?

Yes, you can put ISA cards in both EISA slots and VLB slots, as both
buses were specifically designed to be 100% ISA compatible.  ISA cards
will not directly effect the performance of EISA/VLB cards; a well
balanced system will have both.  Note, however, that the total
bandwidth of the bus will be split between all cards, so there is a
strong advantage to using EISA/VLB cards for the high bandwidth
devices (disk/video).

Q) 2.32  How should I configure ISA/VLB cards in the EISA config utility?

Only EISA cards matter in the ECU; ISA and VLB entries are only place
markers.  While this is a good way to keep track of IRQ, DMA and BIOS
conflictions, ISA and VLB need not be placed in the configuration at
all, nor should it be assumed that the settings for them match the
actual card settings.  If you wish to add them, you can use the
"Generic ISA Card" configuration file for either.  Do not expect card
vendors to supply them.

Q) 2.33  What is the difference between EISA Standard and Enhanced modes?

Many EISA cards support both Standard (ISA) and Enhanced (EISA) modes.
In Standard mode, the card will appear to be an ISA card to the OS; it
will generate edge triggered interrupts and only accept ISA addressing
(for bus mastering cards), for instance.  An important thing to note
is that the card may still do EISA specific things like 32-bit data
bus mastering and EISA configuration setup as this functionality is
hidden from the OS.

Q) 2.34  Is there any point in putting more than 16M in an ISA machine?

[From: cls@truffula.sj.ca.us (Cameron L. Spitzer)]

Sure.  Even inferior operating systems can use it for something.
The question is how much performance it buys.  In ISA, the DMA
channels and bus-mastering IO cards can only address the first 16 MB.  
Therefore the device drivers have to copy data up and down or just
not use the space.  I am told the Linux SCSI drivers know how to
do this.  I don't know about OS/2 or MSWindows.

Q) 2.35  What disadvantages are there to the HiNT EISA chip set?
[From: ralf@wpi.wpi.edu (Ralph Valentino)]

The HiNT Caesar Chip Set (CS8001 & CS8002) can come in three different
configurations.  All three of these configurations have EISA style
connectors and are (sometimes incorrectly) sold as EISA motherboards.
The differences should be carefully noted, though.

The rarest of these configuration uses a combination of the first HiNT
chip (CS8001) and the Intel chip set.  This configuration can support
the full EISA functionality: 32 address bits, 32 data bits, level
sensitive (sharable) interrupts, full EISA DMA, watch dog (sanity)
timer, and so forth.

The second configuration is called Super-ISA, which uses both of the
HiNT chips.  This configuration is very common in low-end models.  It
supports a very limited functionality: 24 address bits, 32 data bits,
edge triggered (non-sharable) interrupts, ISA (16 data, 24 address)
DMA, and no watch dog timer.  Some EISA boards, such as the Adaptec
1742A EISA Fast SCSI-2 host adapter, can be configured to work in this
mode by hacking their EISA configuration file (.CFG) to turn off these
features.  Other EISA cards require these features and are therefore
unusable in these systems.

The final configuration is called Pragmatic EISA, or P-EISA.  Like
Super-ISA, both HiNT chips are used but external support logic
(buffers and such) are added to provide a somewhat increased
functionality: 32 address bits, 32 data bits, edge triggered
(non-sharable) interrupts, ISA (16 data, 24 address) DMA, and no watch
dog timer.  The full 32 bits for address and data allow bus mastering
devices access to the complete range of main memory.  As with
Super-ISA, there may be incompatibilities with some EISA cards.

Q) 2.36  *Should I change the ISA bus speed?

Q) 2.37  Why is my PC's clock so inaccurate?
[From: uwvax!astroatc!nicmad!madnix!zaphod (Ron Bean)]
Usually this means that the clock crystal is not adjusted properly, or
that it needs a new battery. The interrupt-based "DOS time" can also
be affected by programs that disable interrupts for too long.
   If your clock is just running at the wrong speed, you can
compensate for it in software by measuring the drift rate and applying
a correction factor. In the long run, this can be *very* accurate. One
such program for DOS is adclk100.zip. An alternative is to run a
program that periodically sets your clock to another one that's known
to be accurate (see the question on setting your clock).
   If your clock is more erratic (eg, it stops when the machine is
turned off, or the date gets scrambled), try replacing the battery
(but remember to write down your CMOS settings first!).  The CMOS RAM
takes considerably less power than the clock, so it may keep working
even though the battery is too weak to run the clock (see the question
on replacing the battery for details).
   A typical cheap quartz watch is rated at +/- 15 sec per month (3
minutes per year) which is about 5.7 ppm (parts per million).  Dallas
Semiconductor rates their encapsulated clock modules for +/- 1 min per
month, or 22.8 ppm. Clocks with external crystals can be "fine-tuned"
with a trimmer capacitor-- if the motherboard designer included one
(Note: there may be more than one crystal on the motherboard). The
crystal's frequency will change slightly over time as the crystal
   The original IBM AT used the Motorola MC146818, which is a
real-time clock plus 50 bytes of CMOS RAM. This chip is discussed in
the book "The Undocumented PC", from Addison-Welsey. The Dallas
Semiconductor DS1285 is a drop-in replacement for the MC146818, and
the DS1287 is the same chip encapsulated with its own battery and
clock crystal. Other variants include larger amounts of CMOS RAM. Most
software accesses the clock through the BIOS.
   Before the AT, there were various types of aftermarket clock cards,
and some of them may still be available (even though every PC sold in
the last several years "already has a clock").
   Given the number of times this comes up on the net, there should be
a market for accurate clock cards. On the other hand, most people
don't seem to care whether their computers have anything close to the
correct time. If your machine doesn't keep time correctly, and you
think it should, you might consider complaining to the manufacturer
about it.

Q) 2.38  How can I automatically set my PC's clock to the correct time?
[From: uwvax!astroatc!nicmad!madnix!zaphod (Ron Bean)]

If you're on the internet, you can use ntp (network time protocol) to
set your clock to another machine that's known to be accurate (see RFC
1129, Internet Time Synchronization).
   ACTS, the Automated Computer Time Service, is available by modem at
(303)494-4774 (note this is NOT a toll-free call). There are programs
that will dial this number and set your clock automatically. This
service is run by NIST (the National Institute of Standards and
Technology, formerly the National Bureau of Standards).
   In Germany, the Physikalisch-Technische Bundesanstalt (PTB)
broadcasts a coded time signal on 77.5 kHz from a transmitter near
Frankfurt, and inexpensive receivers are available that can plug into
a serial port (this signal should cover most of Europe). In the US,
NIST runs a similar station (WWVB) on 60 kHz, but the data is encoded
differently and receivers are expensive and hard to find.
   Other coded time signals are available from WWV and WWVH on 2.5, 5,
10, 15, and 20 MHz (shortwave), and from the GOES and GPS
satellites. CHU Canada on 3.33, 7.335, and 14.670 MHz broadcasts time
codes using Bell 103 FSK (300 baud modem tones) during seconds 31
through 39 of each minute. If you don't have a shortwave receiver, the
WWV audio time announcement can be heard by dialing (303)499-7111
(again, this is NOT a toll-free call).
   NIST publishes a 30-page booklet (NIST Special Publication 432)
that explains all of their time services in detail. They also have a
partial list of companies that make receivers, but it appears to be
somewhat out of date. Both are available from:
	  NIST/Radio Station WWV
	  2000 East County Road 58
	  Fort Collins, CO  80524-9499
   You may have heard about Heathkit's "Most Accurate Clock", which
decodes the time signal from WWV and has a serial port.  Heath no
longer sells kits, but they still sell the factory-built version of
the clock (it's not cheap, though). Their address is P.O. Box 1288,
Benton Harbor, MI 49023.

Q) 2.39  What is the battery for and how do I replace it?

[From: uwvax!astroatc!nicmad!madnix!zaphod (Ron Bean)]

   The battery maintains power to the CMOS RAM and the real-time clock
when your PC is turned off. You may have a small lithium "coin"
battery soldered to the motherboard, or a larger external one plugged
into a connector. Some motherboards have a jumper to select either
type, and a few have a NiCd battery that recharges automatically, or a
lithium battery encapsulated in the clock chip.
   NOTE: Always write down your CMOS settings before you mess with the
battery! In fact, you should write them down now anyway, in case the
battery fails later.
   The batteries that are soldered in or encapsulated with the clock
chip are supposed to last 10 years or more, but your mileage may
vary. Some people find that the external type has to be replaced every
couple of years. Self-recharging NiCds that get power from a disk
drive cable are available as aftermarket items.  A few people have
tried to save money by substituting 4 alkaline AA batteries for the
expensive external lithium battery, but they have to be replaced more
   If you need to replace a soldered-in battery, have a repair shop
install a socket (you shouldn't attempt this yourself, unless you're
experienced at soldering on expensive multi-layer circuit boards). If
the battery is encapsulated in the chip, there is no way to replace it
without replacing the chip-- again, consult a repair shop if it's not
socketed. These chips can be "turned off" via software to extend
battery life during storage, and are shipped from the factory that

Q) 2.40  Can I use IRQ2 or is it special?

IRQ2 is used to cascade the second interrupt controller on AT
machines.  This has one and only one side effect: from a software
point of view, IRQ2 = IRQ9.  You can freely use IRQ2 on any hardware
device, provided you are not already using IRQ9.  Your associated
software driver can be set to IRQ2 or IRQ9, which ever it happens to
prefer.  Note that many video cards have an IRQ2 enable jumper for
very, very old backward compatibility reasons; you should disable this
before attempting to use the IRQ for something else.  There are no
unexpected side effects.

Q) 2.41  Where do all the IRQ's go?
[From: wlim@lehman.com (Willie Lim)]

Default settings.

 IRQ   DMA  IO BASE   Card or Device
 ===   ===  ========  ===============
  0     *      *      timer (reserved)
  1     *      *      keyboard (reserved)
  2     *      *      interrupt 8-15 (cascade)
  2     *     330     MPU-401 Emulation (PAS 16)      
  3     *     2F8     COM2:
  3     *     2E8     COM4:
  3     *     300     3Com Etherlink II, II/TP, II/16, II/16TP, 16/16TP
  3     *     300     Novell NE2000
  3     *     300     SMC/Western Digital 8003EP, 8013EWC, 8013WB
  4     *     3F8     COM1:
  4     *     3E8     COM3:
  5     *     278     LPT2:
  5     *     368     Ungermann-Bass Ethernet NIUpc (long), NIUpc/EOTP (short)
  5     *     ???     DEC etherWORKS LC, Turbo, Turbo/TP
  5     1     220     Sound Blaster Emulation (PAS 16)
  5     5     A20     Proteon P1390
  6     *      *      floppy disk controller
  7     *     378     LPT1: (PRN:)
  7     3      *      Pro Audio Spectrum 16 (PAS 16)  
  8     *      *      clock (reserved)
  9     *      *      reserved
  9     5     300     Boca Ethernet (BEN100, BEN102, BEN300)  
 10     *      *      unassigned
 11     *      *      unassigned
 12     *      *      unassigned
 13     *      *      math co-processor
 14     *      *      hard-disk
 15     *      *      secondary disk controller

Ralph Valentino  (ralf@wpi.wpi.edu)
Hardware Engineer,  Worcester  Polytechnic  Institute

Continued in part 2.