The magazine of the Melbourne PC User Group Hard Disk Drives Morris Tobias

WHAT IS A HARD DISK DRIVE? 

Not unlike a Floppy Diskette, a Hard Disk Drive is a mass storage device for data held as magnetic media flux reversals, commonly in binary form as ones and zeros. The essential differences between floppy and hard drives are:

1. Speed of writing to and reading from the drive.
   
2. Capacity to store data. DIFFERING TYPES OF HARD DISK DRIVES have appeared but the commonly accepted types are:
   
   1. Fixed hard drive.. such as the 1OMb or 20Mb unit that is common in the XT style of Machine.
      
   2. Hard Card. This integrated drive and controller takes the form of a full length peripheral card and plugs into the XT Bus as would any other card. It is ideally suited to machines with limited space for drive placements such as the Olivetti M24. It has the added advantage of lower current consumption. At present, these hard cards are available in capacities ranging from 10 Mb to 45Mb.
      
   3. Removable Media hard drive. Units such as Syquest, Bernoulli and DMA have manifested themselves in the marketplace but with limited success for various reasons. The concept of having 5Mb to 30Mb of removable magnetic media is, in theory, great from the aspect of security and backup, but reliability has hampered some of these products. Experience showed that in many instances, data written to a removable pack in one drive, cannot be retrieved from a different drive of the same type.
      
   4. CD ROM (Compact Disk Read Only Memory) These units are for high speed management of bulk Fixed data such as Encyclopaedia, Dictionaries and Directories. No data write facility exists in this particular system, as CD ROMS such as Groliers' Dictionary are purchased "ready to run."
      
   5. WORM (Write Once Read Multiple) drives are the newest technology, allowing the user to write ONCE via a laser beam altered Dye Line. After this one time write has been effected, retrieval is by the same method as for the CD ROM.
      

The Major Factor for the development of hard disk drives was a need to be able to handle bulk storage, that is, the ability to store large, contiguous files containing perhaps a database. For example, if you needed to maintain employee records for say 5000 employees in a large company, and then retrieve information from that data base concerning the employees' income level in relation to their academic standard, and further relate it to absenteeism, you could not effectively or efficiently perform this task on a floppy diskette based system. 

Further to this aspect came the need for fast access. If it were necessary to access such a large database, the time taken on a floppy based system to retrieve such data would be agonizingly slow. Hence the evolution of the high speed Winchester technology fixed media disk drive. 

This perhaps has over-simplified the facts, especially concerning the use of large disk packs in the mainframe environment, but as our topic here deals with our needs in the PC arena, we can take some license. 

WHEN DID HARD DISKS BECOME A VIABLE MICROCOMPUTER PRODUCT? 

Small Hard disks have been with us for well over a decade now, but in the PC area, they really came to the fore in 1982 with products like the almost forgotten Davong 5Mb external disk drive, and the other products such as Tallgrass and Tecmar. Until late in '83 or early '84, these drives could not be used as "bootstrap" drives; they required floppy boot diskettes to initialise them. 

In May 1983, IBM released its XT with a 10Mb Bootable hard disk, and in a relatively short time many other system manufacturers followed. 

There were some problems in those times, most of the available drives needed to be externally mounted because of the smallish power supplies fitted to PCs. With the fitting of the earlier Davong internal drives to PC-1 systems, it was quite common to see a PC-1 with a 64Kb motherboard running a 10Mb internal hard disk (only bootable from floppy) with an external power supply cabinet attached. Then came the Tallgrass machines with their 12Mb disk and 20Mb streamer tape backup units. Once again these models required a boot diskette to start them, and formatting was at best, an overnight exercise. 

So here we are today with bootable hard disk drives of practically any size and priced in such a way that no PC should be without one. Do you know that the RRP for a second full height 360Kb diskette drive for an IBM PC in 1983 was about $830.00? Today a 20Mb Hard disk, controller and cable kit retails for as little as $700 including sales tax, and might even become cheaper! 

WHERE ARE WE GOING WITH THIS TECHNOLOGY? 

If you are the owner of an original 1983 IBM XT, when you look at the hard disk therein, you see a Full Height 5  1/4" 10 Megabyte disk drive. This drive in its day was considered to be enormous in storage capacity, but today that drive could be easily replaced by a 3 1/2" low power drive with a net capacity of 45 Megabytes. So what are we seeing? With miniaturization and more effective processor control within disk drive circuitry we are gaining greater capacities on small drives and gaining even bigger capacities than we would have imagined 5 years ago on the standard 5 1/4" formats. 

For Example, in 3 1/2" format, Rodime have a 55Mb drive with low power consumption and Voice coil head positioning. This drive is standard equipment in the Apricot Xen-i and is renowned for its performance. 

But it will appear relatively small in the shadow of the recently announced Maxtor LXT170 series drives which are 3 1/2" units of 170 Mb capacity. But what of the 5 1/4" drives?... These are becoming higher and higher in capacity to the point that they have encroached on what once was only within the realms of 8" drives or larger to manage. For 3 years, we have had the Maxtor XT1000 range of drives giving us capacities ranging from 65Mb through to 140Mb which we could attach to our XT or AT systems (although device drivers are required if we use DOS). Other manufacturers have followed along these lines and now there is a move toward different interfacing.

The ST506 standard with which we are familiar from our PC, XT and AT computers is apparently becoming superseded by 2 alternative standards, namely SCSI (Small Computer System Interface) and ESDI (Enhanced Small Device Interface) IBM have adapted both of these standards, SCSI in the model 30 and ESDI in the larger PS/2 systems. 

Maxtor and others have been producing SCSI and ESDI drives for some time now and capacities of up to 380Mb are readily available in both interface standards. These very large storage devices are being sold in Australia into the QBUS market and are commonly sold in pairs. The Webster Computer Corp's QBUS ESDI controller can support 4 X 380Mb drives for a formatted capacity of more than a Gigabyte. 

What of newer technology? Maxtor have also announced their XT8000 series giving capacities of 760 Mb. (We will have a limited number of these drives in Australia in this quarter.) 

Rodime have already released a half height 51/4" higher capacity series, namely the 805000 series. in Australia they have a 125 Mb SCSI drive and a 90 Mb ST506. Shortly they will release their 180 Mb SCSI drive.

WHAT OF THE 32MB DRIVE SIZE LIMIT IMPOSED BY DOS 3.20 AND LOWER? 

A problem with larger drives has always been how to have DOS recognize the larger format. This has been dealt with by several clever people to provide alternatives. 

The controller manufacturers have come up with a controller BIOS that allows multiple "Logical Drives" to appear on the one physical drive; for example C: and D: 

Another way that this has been dealt with by Rodime with their 805000 formatter creates a device driver in the config.sys file to enable a DOS bootable partition and several other drive partitions which are alluded to as "Non DOS Partitions" by FDisk but still function as DOS Disk drives. 

Specialist Software houses such as Golden Bow have produced some real gems like "V-Feature Deluxe" which allows for up to 24 logical drives to be allocated (C: through to Z:). V-Feature also has the ability to format large drives as one logical drive; for example a Maxtor XT1140 (140Mb unformatted) yields a bootable DOS drive C: of 133 Mb. But, thats not all! This program allows the installation of TWO Hard Disks which it will span as one volume and call it drive C: With 2 XT1140 drives, your Chkdsk program will show total capacity of 266Mb. This will go out to some 400Mb on drive C: if an RLL. controller is used with suitable drives. 

RLL is a relatively new encoding technology for the PC area which has previously been strictly MFM (Modified Frequency Modulation). 

RLL stands for Run Length Limited and simply, it allows higher capacity to be gained from Disk Media by the use of more sectors per cylinder; typically 25 sectors vs. 17 sectors per cylinder under MFM. To clarify the terms Cylinder (or track) Head and sector which are commonly referred to whilst formatting disk drives, let us consider the following:

1. Hard disk drives generally have between one and eight platters on one driven spindle, and each platter may have magnetic storage media c6ating on one or both sides.
   
2. The media is formatted in cylinders or tracks, which are concentric rings not unlike the rings of an archery target only many more of them. Generally, there are between 305 and 1024 which are acceptable within thQ DOS environment, but some drives have 1224 cylinders or tracks.
   
3. Sectors are arc positions on the cylinder similar to the "double" or "triple" positions on any given number position on a standard dartboard.
   
4. As there is usually more than one surface within a disk drive that is, probably two sides of each platter each surface is written to or read from by a "head" somewhat similar to that in a cassette recorder, but much smaller and lighter.

1. there is no stylus to come in contact with the media and
2. the heads are usually stacked up in a comb like structure so that one head can service each platter surface.

As up to date as we are here in Melbourne, tomorrow or next week will probably bring us yet more outstanding technology to make disk drives store yet more in-formation with more speed and at lower costs per Megabyte.

What next?

Reprinted from the November 1987 issue of PC Update, the magazine of Melbourne PC User Group, Australia

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