Lloyd Robert Borrett

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Megabyte Tarnish

by Lloyd Borrett
Today's Computers, PC Australia, September 1985

  Lloyd Borrett

Lloyd Borrett

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.

However, 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 such events.

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 document:

Preparing fixed disks for moving.
Users have 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 drive C.
Reportedly, the AT sometimes has difficulty recognizing its fixed disk when requesting a list of its files.

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.
Users have 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 patch fix.
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 other problems.

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
(in milliseconds)
20 MB
20 MB
Data 20
Track to Track 21.03 8.95 5.76
Random & Track to Track 53.77 23.83 21.64
Random 84.36 36.96 37.73
Total: 159.16 69.74 65.13

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 boards.

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 system.

Last modified: Saturday, 15 October 2011


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