by Lloyd Borrett
Today's Computers, PC Australia, September 1985
The glittering image of the IBM PC-AT has been
tarnished and the core of the problem has been the 20
megabyte fixed disk from Computer Memories Inc. (CMI) which
IBM puts in the enhanced model AT.
As one of the first AT
users in Australia, I initially doubted early reports of
problems. None of the early Australian AT owners I came into
contact with had problems with their fixed disk drives, and
initial reports from America were conflicting.
later it became clear that there were software bugs with IBM
DOS 3.0 which coupled with an unreliable fixed drive forced
many different symptoms to the surface. In the end most
problems were directly attributable to the poor reliability
of the CMI drives.
Because of their delicate design,
Winchester-type disk drives have certain inherent
limitations. A number of factors, including power failure,
shock, and even simple vibration, can produce a head crash
that results in partial or total loss of all stored data.
The CMI drives have proved to be particularly intolerant to
A number of US dealers reported a 25 to 30 per
cent failure rate with the CMI drives. At one stage IBM
curtailed shipments of the enhanced AT because it claimed it
could not get enough drives from CMI. In truth IBM could not
get enough reliable drives, and was desperately searching
for other suppliers.
PC Week magazine reported that it even
got to the stage where IBM began supplying selected PC AT
installations with a 34 page document outlining problems
reported with the AT's hard disk drive and DOS 3.0, called
"IBM PC-AT Tips and Techniques".
An IBM spokesman was
reported as saying that the information was merely gathered
from the PC AT Technical Reference Manual and The Guide to
Operations Manual to answer queries.
To be fair, many of the
tips are in the manuals, if you know where to find them and
indeed many are standard practice to the more experienced
users. Here is a sample of the issues dealt with by the IBM
Preparing fixed disks for moving.
complained that their fixed disk drives lose data after the
AT is moved or simply shut off. CMI say their drive's heads
automatically retract to a "dedicated landing zone" when the
AT is shut off.
The IBM document maintains that users must
run the "Prepare System for Moving" program on the AT
Diagnostic diskette. This utility should move the heads off
the data area and lock them in place. In fact you can copy
it from the diagnostic diskette onto your fixed disk. It's
called Shipdisk.Com. and the more careful and experienced
users of any fixed disk always run this or similar, programs
before shutting down their systems.
Failure to recognize
Reportedly, the AT sometimes has difficulty
recognizing its fixed disk when requesting a list of its
In response, the document states, "... remove all cards
from the system unit except the disk/ diskette attachment
card and the display attachment card, and re-try the load
operation, both by power on and soft load several times."
General failure reading drive C.
The document implies that
users' ATs have trouble reading certain areas of the fixed
disk. IBM recommends that users move data to a different
area of the disk by using the DOS Recover command.
Reformatting fixed disk with diagnostics.
complained that the AT fixed disk mysteriously develops bad
sectors causing users to lose data.
IBM recommends that
users periodically run the Conditional Format Procedure
part of the AT's Advanced Diagnostics Program — to reformat
the fixed disk. "The conditional format will not erase the
flags from previously flagged tracks," the document said.
Unfortunately you must first backup your data as this
procedure erases the entire disk. Also the Advanced Diagnostics Program comes with the expensive Hardware and
Maintenance Reference Manual.
The fact is that none of the
IBM diagnostic procedures test the fixed disk drive
carefully enough. Third party programs are available in the
US which address this problem and some even run without
harming the data already on your disk.
DOS 3.0 Backup
IBM has acknowledged a bug in the DOS file
allocation algorithm for the PC AT's high capacity floppy
drives. The document describes a "patch" — a segment of
code that corrects the bug. In Australia, IBM will only be
shipping DOS 3.1 which includes solutions to this and a few
While IBM is using fixed disk drives in the
enhanced AT from at least two other manufacturers, most of
the above issues are still relevant.
The simple and
unavoidable fact is that higher capacity fixed disk drives
jam more data onto the same surface, increasing the chance
of -a significant information loss. This higher density
makes the drive more susceptible to vibrations, and the
engineering techniques used in a low performance 10
megabyte drive will not necessarily work at double that
capacity and speed.
Another issue is the installation of
slow and/or low quality fixed disk drives in base ATs. These
are then sold by dealers as if they were enhanced ATs with
the IBM supplied drive.
Since the IBM PC was introduced, the
use of non-IBM hardware items to expand a base system unit
into a usable system has been common place. Most dealers can
buy IBM equivalent (sometimes identical) add-on boards,
diskette drives, memory chips, and other hardware items and
use them to build-up the system you buy.
This is usually
done because the dealer can get a better margin on such
items and he also often obtains better continuity of supply
from the distributors. In many cases the substitutes
correct minor faults in IBM's own product.
The common use by
dealers of certain preferred options has forced IBM to make
a counter move. For instance, the original PC only had
single-sided floppy disk drives. Many dealers offered third
party double-sided drives and eventually IBM made them
available too. However there are cases where such
substitutions are extremely questionable and none are more
so than the widespread use in the US of substitute fixed
disk drives in the AT.
IBM's specifications for the AT fixed disk drive describe
a high performance drive and the market is flooded with 10,
20 and 30 megabyte drives which are used to expand the PC
and XT models. While these can be physically installed and
made to run quite easily in the AT, most drives do not even
come close to matching IBM's specifications. Using them in
the AT will result in a performance typically associated
with a PC-XT or other lower cost computers and is a waste of
the AT's capabilities.
And do not be fooled into thinking it will not happen in
Australia. I already know of one importer who was too keen
to make some extra money importing ATs before IBM Australia
started delivery. His US supplier was putting low
performance fixed disk drives in base ATs and he could not
understand why his "enhanced" ATs did not have the same
increase in processing power he'd read the PC-AT was
supposed to deliver.
Already Australian dealers are being stalked by fixed
disk drive importers who assure them that their drive will
work in the PC-AT. But many of the importers and dealers do
not know how to accurately check the third party drives'
performance against the IBM specifications.
The following table shows the values obtained by properly
testing three of the drives on offer.
Average Access Time
Track to Track
Random & Track to Track
The values were not obtained from manufacturers'
brochures. They are the result of running a program
developed by CORE International, an IBM value added dealer,
which identifies the actual performance specifications of
fixed disk drives running in the PC-AT. It's being widely
distributed in the US through user groups and bulletin
First is a track-to-track test which measures access time
resulting from loading or reading contiguously stored data.
The second test is typical of most business applications
which perform a mixture of random and contiguous disk
accesses. The last test is the most strenuous, measuring a
true random access which is identified by drive
manufacturers as the "Average Access Time".
As an example, a drive with an average access time of
30ms is twice as fast as another drive reporting 60ms. IBM
specifications for the AT list a track to track access time
of 10ms and an average access time of 40ms.
Note that the Seagate is a typical example of the low
performance drives on offer. While its specifications are
more than adequate for use in the PC and PC-XT, the drive
does not come close to matching the PC-AT specifications.
Note also that it is possible to obtain fixed disk drives
which meet or exceed IBM's specifications.
All fixed disk drives are not created equal, and the
opportunity for deception is real and I'd be very careful to
quiz the dealer on the brand and performance of any fixed
disk drives being offered in an "enhanced" PC-AT.
Lloyd Borrett is support co-ordinator for HiSoft
Australia, president of the Australian PC User
Association, founder of the Melbourne PC User
Association, founder of the Melbourne PC User Group, and
system operator of the PC Connection bulletin board
Saturday, 15 October 2011