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COM.POTER BITS By JENNIFER BONNITCHA A guide to hard disc drives The first law of computing says that software expands to fill the available storage space. Sooner or later, if you've been using a computer with floppy discs, you'll he wondering about the advantages of a hard disc drive. If you use your computer a lot, adding a hard disc to it will he a big step forward. A hard disc drive represents salvation for harassed floppy disc users. When you depend only on floppy discs, you soon find you need an ever increasing number of floppies to store your files. As their number increases, you will have trouble keeping track of them all and storing them for easy access. Wouldn't it be so much nicer to have it "all there" permanently, in the computer? Another important consideration these days is the amount of disc space programs take up. In the good old days you could happily run W ordStar on a single floppy disc machine and store your documents on the same disc. Today, the increased memory requirements of graphics-based programs, so-called "power-users" and lazy programmers has lead to the need for greatly increased storage. Many popular PC programs such as Excel and Ventura just won't work on a floppy disc machine; you must have a hard disc drive. And while many programs can be run on single or dual floppy disc drive machines, the speed advantage of a hard disc drive is quite dramatic. Typically you can expect the hard disc drive to read and Fig.1: this diagram shows the concept of interleaving although a hard disc has many more sectors than suggested here. HARD DISK INTERLEAVING write data at around five to 10 times the speed of a floppy disc drive. As well, you can't beat the convenience of having your word processing, database, spreadsheet, graphics and accounting software all readily at hand without having to boot them up from separate floppies. Just a couple of years ago, you could purchase a staggering 20 megabytes of hard disc storage for "just a few" thousand dollars. Today, the same hard disc with controller is likely to set you back $500 to $600 although most buyers would now tend to go for 40 megabytes, for around $700 or so. And if you shop carefully, 80 megabytes or so can be purchased for around $1200. As you can see, the extra capacity more than justifies the price difference. Hard disc standards The hard disc itself is just a magnetic recording device. What the disc receives, how fast and where it will be placed is determined by DOS (the Disc Operating System) and the controller board. So if you buy a hard disc, you must also have a hard disc controller. The hard disc controller handles the task of formatting the disc for later use, encoding the data it receives and instructing the hard disc where to place the read/write heads. A hard disc drive actually contains several aluminium discs which are coated with particles of ferric oxide which form the magnetic storage. The read/write heads record (write) and replay (read) data to and from the disc in SEPTEMBER1989 75 COMPUTER BITS - CTD much the same way as on an audio cassette deck or a video cassette recorder except that the signal format is different. The read/write head differs considerably for floppy disc drives, high capacity drives and fixed discs. In all cases though, it serves two basic functions: (1). It reads the data stored on the disc dependent on the polarity of the magnetic fields at a given point on the disc. (2). It stores or writes data onto the disc by magnetising the disc surface in one direction or the other (ie, north poles or south poles) for binary ls and 0s. Flying heads Read/write heads ride on a very thin layer of air and so don't normally come into contact with the disc surface. The 10Mb XT fixed disc consists of two 5.25-inch aluminium platters with each of the four sides capable of holding 2.5Mb of data. Both the platters and recording heads are sealed inside a container which includes an air filter to prevent dust particles coming into contact with the highly sensitive surface and recording head mechanisms. The platters are fixed on a spindle which rotates at approximately 3600 RPM. The rapidly spinning sealed disc creates sufficient air flow for the recording heads to "fly" or "float" at around 1.5µm above the platters' surfaces. The PC AT also has two platters and four surfaces but unlike the XT, the AT 20Mb hard disc has six read/write heads. So while the AT's rotational speed of 3573 RPM is not significantly different from the XT's, the AT can access its data at two times the speed of the XT's hard disc. Since the hard disc coating contains finer oxide particles than a floppy disc, information is packed more densely on its surfaces. They MAKE YOUR DREAMS COME TRUE LAPTOPS FROM s1995.oo *INTERNAL BATTERY, EXTERNAL AC ADAPTOR *LIGHT, FAST, 10MHz * COMPACT AND CONVENIENT * SUPER TWIST LCD DISPLAY *64OKB RAM * SERIAL AND PARALLEL PORT REAL TIME CLOCK * LASER PRINTER s2995.oo *6 PAGES PER MINUTE *SERIAL AND PARALLEL (DUAL HOST) * 6 BUil T-IN FONTS * HP LASER JET SERIES II EMULATION * OPTIONAL IBM PROPRINTER EMULATION * 4 ADDITIONAL SOFT FONTS INCLUDED * EXPANDABLE TO 4.5MB COMPUTER CARE KIT s115.oo * 6 CLEANING KITS IN ONE *SUITABLE FOR ALL COMPUTERS FREEPOST NO. 31 ALL PRICES INCLUDE SALES TAX FREE ALL PURPOSE CLEANING KIT WITH HARDWARE PURCHASES 76 SILICON CHIP ESCKAY PRINTERNET PO BOX 2591, NTH PARRAMATTA, 2151 TEL: (02) 891 1282 also spin about 10 times faster than a floppy disc and so a hard disc is able to pack in large amounts of information then access it very quickly. For example, data is stored on the IMB PC's 360Kb floppy disc at 48 tracks per inch while the XT's 10Mb hard disc contains 345 tracks per inch. A mechanism called an actuator finely positions the read/write heads across the disc to locate the required information. Interleaving The disc surface is divided into sectors by a magnetic numbering system. Since the surface of a hard disc moves so rapidly past the read/write heads, most computers can't transfer data to and from the disc fast enough to keep up with consecutively numbered sectors. Sectors are therefore not usually numbered sequentially to allow time for the controller and computer to transfer and process the data being read from the disc. Instead the sectors are interleaved, with the sequence of the sectors called the "interleave factor". For an AT computer, the sectors on the hard disc are often organised with two sectors separating the first and second sectors (1 **2**3 - an interleave of 1 to 3). The slower XT uses an interleave of 1 to 6 (1 *****2*****3). An interleave of 1:1 means data is placed in sequential sectors. If the interleave factor is incorrect, the files aren't transferred to and from the hard disc as fast as possible and the overall performance of the computer is slowed. If the interleave is set too fast, the computer can't process the data as rapidly as it appears under the read/write heads and so must wait for an entire revolution to read the remaining data. On the other hand, if the interleave is too slow, the computer is ready to read the next sector before it appears so time is lost waiting for the sector to appear. Head crash A "head crash" describes the loss of the air layer which brings the heads and the disc surface into contact, usually with disastrous Yet another option in the hard disc field is this completely removeable hard disc drive. Called the Portadisk, it is a standard 40Mb drive which fits into a normal 5¼-inch drive slot. It is available from Electronic Solutions. results. The hard disc surface is very delicate so a head crash can not only wipe out data stored at the particular location where contact is made, it can also tear a hole in the coating although this situation is very rare. Access times Look for information on the data access time when you consider purchasing a hard disc. Anything rated at 50 milliseconds is fairly fast, with 20ms or less very fast; either way, even the slowest hard disc operates much faster than a floppy disc. Installation Installing a hard disc in an existing PC is a relatively simple matter. The hard disc requires the installation of an adapter in one of the system expansion slots of the PC XT system unit. PC users may have to first upgrade their power supply because of the power requirements of the disc, although "low power" hard discs can be installed and used with the original supply included with the PC. You need to check this aspect before you purchase. Other alternatives are the removable hard disc, self-contained drives which fit into a flexible, removable cartridge, and removable hard cartridges. Removable hard discs can be taken out in seconds for transport. Since the platters are rigid, two or more can be packed closely together in one drive to give decent storage capacity. To counter the effects of moving, removable hard discs are typically protected to some extent by shock mounted assemblies. If the computer loses power unexpectedly or you neglect to "park" the read/write heads before turning the power off, the spinning disc no longer provides an air cushion for the heads. The potential for the heads crashing onto the discs increases markedly. To counter this, some makers provide "auto-parking" which may be timeactivated or triggered by the ejection of the hard disc from its receptacle. Flexible cartridges The flexible cartridge is typically enclosed in a plastic case to protect the disc surface. The beauty of this system is its ability to squeeze 20Mb from floppy disc media while remaining small and light - and it provides a storage device which can take a fair amount of rough treatment. However, these benefits don't come cheaply with prices starting at about $1900 for a bottom of the range system. While the system is certainly durable, it is also slow and prone to wear since, like the floppy disc, the read/write heads actually touch the media surface. In an effort to reduce wear, the systems spin anywhere between 600 RPM and 1800 RPM. Although this reduces media wea:r, disc performance is severely limited. Combining the performance of a hard disc with the convenience of removable cartridges seems to be the best of both worlds. The removable hard cartridge separates the read/write mechanism from the disc platter, which rests in its own removable cartridge. However, these systems generally have no provision for auto-parking and the cartridges are more prone to damage if dropped. Since the cartridges are typically equipped with hinged or sliding doors, it is inevitable that dust and debris will ultimately find its way onto the disc surface. Although the presence of debris may not cause severe data loss, expected media life is undoubtedly shortened. So what is the answer? If you can take it out, eventually it will be dropped. Therefore you should choose a heavy duty product and from this angle, the flexible disc seems to be the best choice, particularly if you regularly travel or send information by courier/post. On the other hand, if you want to store a lot of programs and data and be able to access it very quickly at any time, a coventional 40 megabyte hard disc drive is a very hard proposition to beat. 1§;1 Notice to Subscribers Are you about to change your address. Please advise us by the middle of the month to ensure that your new issue arrives at your new address. It helps us if you can send your old address label too. If you have any queries about your subscription call us or write to us at: SILICON CHIP, PO Box 139, Collaroy Beach, NSW 2097. SEPTEMBER 1989 77