487 lines
28 KiB
Plaintext
487 lines
28 KiB
Plaintext
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HARD DISKS - THE ESSENTIAL ACCESSORY
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A simple observation: the first accessory any computer user
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should buy is hard drive. On a dollar for dollar basis nothing
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speeds up processing and expands convenience like a hard drive.
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The bad news? The substantial storage capacity of a hard drive
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contains the seeds of data catastrophe if you don't understand
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how to CAREFULLY maintain a hard drive. Some reference
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information pertaining to larger desktop hard drives as well as
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smaller laptop drives has been retained since drives in both
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computers are similar in function although different in form and
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size.
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Many computer operations tend to slow down at the critical
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bottleneck of information transfer from computer memory (RAM) to
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disk. The faster the transfer, the faster the program operates.
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Nine times out of ten it is the bottleneck formed when
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information flows to or from a disk that you and your program
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must wait. This is where a hard drive really shines - speed.
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Given the best possible treatment, a hard drive should last from
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eight to fifteen years. Drive manufacturers typically suggest
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30,000 to 70,000 hours of routine life for a hard drive before
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failure. If you kept your PC on for a 40 hour work week for 50
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weeks - you could expect about 15 years of service for a drive
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rated at 30,000 hours. Some hard drive users even suggest
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leaving the drive on continuously or alternatively turning it on
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in the morning and off at night to minimize motor and bearing
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wear since it is the starting shock which wears most heavily on
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a drive. However, given marginal treatment or abuse, you can
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expect about fifteen minutes of service followed by a $250
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repair bill. Obviously a little information about hard drives
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and their care can't hurt.
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TECHNOLOGY 101 - BOOT CAMP FOR HARD DRIVE USERS
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What is a hard drive? If you have worked with a floppy disk you
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already understand something about hard drives. Basically the
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hard drive unit is a sealed chamber (sealed against dust and
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dirt) which contains rapidly spinning single or multiple stacked
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platters. The platter(s) are similar to a floppy disk in that
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they store information magnetically - data can be erased and
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rewritten as needed. The trick is, however, that the storage
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capability is immense on a hard drive.
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A floppy typically holds about one third of a million computer
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characters (360,000 or 360K bytes). The hard drive can commonly
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hold 20 to 40 million (or more!) bytes or computer words. In
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addition, the hard drive motor spins the magnetic platter
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quickly so that information is transferred rapidly rather than
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the tedious rate of the leisurely spinning floppy. A small
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read/write head hovers and moves above the hard drive magnetic
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platter much like a phonograph needle above a record. The
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difference is that the read/write head of the hard drive rides
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slightly above the platter on a thin cushion of air. In the
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floppy drive mechanism, the read/write head is in direct contact
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with the floppy. All hard drives are installed in two parts: the
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drive (a box containing the disk and read/write head) and the
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controller (a circuit board) which may be integrated into the
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drive or a separate circuit board. The hard drive stores the
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information. The controller assumes the role of a high speed
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"translator/traffic cop" to help the hard drive move its massive
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amount of information smoothly.
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Back to the magnetic platter for a moment. The read write heads
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are mounted on a moveable arm and each position of the head
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above the platter defines a circular TRACK just like the track
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of a phonograph record. As the arm changes positions, different
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circular tracks are traced magnetically upon the surface of the
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platter. Most hard drives have several read/write heads which
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service both the top and bottom of each platter. A set of tracks
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on different platters define a vertical CYLINDER somewhat like
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the surface of a tin can whose top and bottom are missing. Large
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hard drives can have six or more platters and therefore 12 or
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more sides for information storage. The tracks can also be
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defined as divisions of equally divided data called SECTORS
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which are something like portions of the outer edge of a circle.
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Finally, the sum collection of tracks, sectors and cylinders
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define the entire VOLUME of the hard disk.
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Each piece of data has an address which tells the read/write
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heads where to move to locate that specific piece of
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information. If you tell the read/write heads to move to and
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hover over a specific track, sooner or later your data will pass
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beneath it. Since you can move the heads directly to a given
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track quickly, the early nomenclature for a hard drive was the
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DASD or DIRECT ACCESS STORAGE DEVICE.
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Movement of the read/write head arm takes a little time. For
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this reason an ACCESS TIME is associated with hard drives and
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stated in advertising and specification sheets. Generally this
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time is stated as the AVERAGE ACCESS TIME and is frequently in
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the thousandths of seconds or millisecond range which is fast
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indeed. The old IBM XT class machines featured access times
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around 85 milliseconds with the AT class machines featuring
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access times around 40 seconds. Newer hard drives post times in
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the 28 to 15 millisecond access range. Remember, the faster you
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can move the read/write heads, the faster you can get to your
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data.
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The AVERAGE WAIT TIME is a less frequently discussed number but
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equally interesting. Once the read/write head is positioned over
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the track holding your data, the system must wait for the
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correct sector to pass beneath. Obviously, the average wait time
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is one half the time it takes for a full rotation of the
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platter. This figure is rarely given in advertisements and is
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usually comparable for most drives of the same type and is
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generally much shorter than the access time. Speed matters to a
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hard drive! Average wait time is published if you dig it out of
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the specification sheet or write to the manufacturer.
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An extension of this logic brings us to consider the INTERLEAVE
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FACTOR for a disk. Generally a hard drive reads and writes
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information in sectors of the same, repeatable size such as 512
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bytes. However programs and data files are usually much bigger
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than this and obviously must be scattered onto many sectors. The
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problem is that the disk rotation is much too fast for a large
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file to be written in perfectly contiguous sectors on the same
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track. If you tried to write the data onto a track, one byte
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after the next, the central processing unit chip or CPU could
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not absorb the data fast enough.
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The solution is to place sectors to be read in ALTERNATING
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fashion which gives the CPU time to digest the data. Thus if a
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circular track on the platter had 8 sectors you might number and
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read them in this order: 1,5,2,6,3,7,4,8. This way the CPU has a
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"breather" in between each sector read. The number of rotations
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it takes the heads to read ALL tracks in succession is the
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INTERLEAVE FACTOR. Slow CPU chips can force a disk to use an
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interleave factor of 3 or even 4. A faster processor might be
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able to handle a disk interleave of 1:2 (such as 80286 processor
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chips) or even 1:1 (such as 80386 processor chips.) It is
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possible to low level format a disk and change its interleave
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factor; but if the CPU cannot keep up, the adjustment is
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worthless. To the processor operating in millionths of a second,
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the time drain of waiting for a hard drive which operates in
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thousandths of a second or floppy drive which operates in tenths
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and full seconds is wasted time. The obvious point of logic is
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that when using a hard drive you need to organize files for
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minimum time delays for the processor.
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The first outer track on a disk is always the boot record which
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loads the main portions of DOS into the machine. Following this
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is the file allocation table or FAT which we discussed in
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earlier tutorials. The FAT maintains data in CLUSTERS which, for
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an XT class machine are 4096 bytes. On the AT class machine the
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cluster size is 2048 bytes which is much more efficient and less
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wasteful of disk space. Following the FAT are the sectors for
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the root directory of the hard drive. Each directory entry is 32
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bytes in length. Curiously, and to our good advantage, unused
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entries in the directory have a unique first character byte.
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When a file is deleted though DOS, ONLY the first character is
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reset.
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Fortunately this allows various utility programs to attempt to
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recover the deleted file since ONLY the directory data is
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altered but NOT the file itself. However, as time goes on and
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additional files are added to the disk, the original file is
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overwritten by new information. This is why you need to act
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immediately if you discover you have accidentally deleted a
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file. An advantage to the use of the FAT is that files do not
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have to be given a fixed amount of space on a disk - they can
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use as many or few clusters as needed. The downside is that the
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file pieces can be scattered wildly over the surface of the disk
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in a non contiguous fashion which only the FAT can track. This
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means more read/write head motion and more wasted time as far as
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the CPU and the performance of your program is concerned.
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Additionally, if you have many deleted files within the
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directory, DOS must search tediously through each one from top
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to bottom of the directory to find a match for the file you are
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trying to locate. Obviously, then, programs and data of high use
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should have their directory entries located near the top of the
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directory to speed the search. Each time the read/write head
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moves takes time: searching the directory and finding the pieces
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of the scattered file all take movement of the read/write arm.
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There are several ways to unfragment files which boost disk
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performance, and we'll talk about those techniques it a bit.
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HARD DISKS - STRATEGIES FOR TURBOCHARGED RESULTS
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Before we examine methods for improving hard drive performance,
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several simple "care and feeding" precautions should be
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mentioned.
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Hard drives are touchy if mistreated! Once brought up to speed,
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a hard drive should never be bumped or moved. The read/write
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head (similar to the phonograph needle resting on a record) will
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smash or chip into the surface of the spinning hard drive
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platter and take your data with it. Either the head or the
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magnetically coated platter can be permanently damaged. Allow
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the hard drive to some to a complete stop before moving the
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computer.
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In addition always use a "parking" software package to move the
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read/write head to the safety zone before turning off the
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computer. A parking program usually accompanies most computers
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which have hard drives installed or can be obtained from
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commercial or shareware sources. A few drives automatically park
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the heads when turned off but this tends to be a rare feature
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seen mostly on high priced hard drives.
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Always maintain copies of data and programs outside the hard
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drive by "backing up" onto a floppy or tape. How often should
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you back up your files? Daily if you use the computer to produce
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many changes to important documents. Weekly backup is probably a
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bare minimum considered reasonable for occasional computer
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users. Other computer users maintain vital data on floppies or
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other backup systems and use the hard drive to store programs or
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applications only such as a spreadsheet or database. Backups are
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a good idea even for floppy disk systems which have no hard
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drive.
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Make two copies of every file regardless of whether you have a
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hard drive or not. Some shareware and commercial utilities ease
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the backup chore by only copying those files to a floppy which
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have been changed or updated since the last backup has been
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performed. They ignore files which have not changed and thus do
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not require copying again. This can save a lot of time when
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backing up valuable files from your hard drive to a floppy for
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safekeeping.
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Hard drives should periodically be reorganized (files
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unfragmented) to ensure speedy retrieval and access to data.
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Inexpensive or free software programs known as "disk file
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unfragmenters" do this job nicely. As disk files are created and
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deleted, blank spaces and unused sectors begin to build up.
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Gradually files are broken into pieces and scattered over the
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many tracks and sectors of the disk. This happens to both
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floppies and hard drives, but is especially annoying on hard
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drives because of the dramatic increase in time it takes to load
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a program or data file. The File allocation table is the
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culprit, sense all data is packed away in the first and handiest
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sector on the drive which the FAT can find.
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The FAT allows files to be fragmented down to the cluster level.
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One way to unfragment a disk is to copy all of the files off to
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floppies and then recopy them back to the hard drive - a tedious
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nuisance at best. You would do this with the DOS XCOPY or COPY
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commands but not DISKCOPY since this would retain the tracks and
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their fragmentation as you first found them.
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Defragmenting programs perform this task without requiring
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removal of the files from the hard drive. They perform their
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magic by moving around the clusters of a scattered file in such
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a way as to reassemble it into contiguous pieces again. Some
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customization is permitted with the more sophisticated
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"defragmenting" programs. For example, subdirectory files can be
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relocated after the root or below a different subdirectory or,
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in another example, high use files might be placed higher in the
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directory listing for faster disk access.
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The first time a defragmenting program is run may require
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several hours if a hard drive is large and badly fractured with
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scattered files and clusters. It is a good idea to backup all
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essential files prior to "defragging" just in case there is a
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power failure during a long "defrag". Subsequent runs of the
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"defragger" produce runs of only a few minutes or so since the
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heavy work was done earlier. Essentially, "defragging" the hard
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drive should be done regularaly, perhaps weekly. Defragging is
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not a substitute for caching, ramdisks, or buffer - instead it
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is a maintenance function which should be done regularly.
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Yet another possible avenue to improve disk performance is that
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of changing the disk interleave factor which we will discuss a
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bit later in this tutorial. By way of brief introduction: the
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disk interleave indicates how many revolutions of the magnetic
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platter are required to read all the sectors of data from the
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spinning track. A ratio of 1:1 means all data can be read
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sequentially. One sector of data after another.
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There is some overhead time required for the read/write head to
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zip to the FAT area of the disk (if it is not in a cache or
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buffer) to determine location of the next sector along the disk
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track.
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For example, five clusters of data on a track might require four
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trips back to the FAT track to find the cluster addresses even
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on a completely defragmented disk. We will talk more about
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cluster and defragmenting a bit later in this tutorial.
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Nevertheless, depending on the speed of your central processor
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or CPU, using a program which tests and alters the interleave
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factor, IF THIS CAN BE DONE, may yield better performance. Most
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interleave adjustment software first performs a test to
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determine the current interleave, the possible changes and of
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course how much performance time might be gained. A few of these
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packages can alter the interleave with the files in place but
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you should backup truly essential files before starting the
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process. Interleave factor adjustment are mainly derived from
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the CPU speed NOT the disk speed. Thus a fast AT or 80386
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equipped machine will more likely be able to take advantage of
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an interleave adjustment.
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Tinkering with a hard drive for optimum results might best be
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divided into two categories: DISK SUBSTITUTION and DISK
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ALTERATION. DOS allows two clever ways substituting RAM memory
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for disk memory.
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In the first, using BUFFERS, the small CONFIG.SYS file on your
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hard drive is modified to contain a buffers statement. A sample
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might be: BUFFERS=20. A DOS buffer is an area of RAM memory
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capable of holding a 512 byte mirror image of a disk sector.
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This allows DOS to quickly search the buffer area for frequently
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used data instead of the slower disk. In the older XT class
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machine, if you did not specify a buffer size, DOS defaulted to
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2 buffers while later versions of DOS default to about 10
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buffers. Most users settle on about 20 buffers but you can
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specify up to 99 with current releases of DOS. But you don't get
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something for nothing. If you used the full 99 buffers
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available, you would soak up 45K of your main RAM memory! The
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downside of using buffers is that more is not necessarily
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better.
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Unfortunately, DOS searches the buffer area of RAM sequentially
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rather than logically so if DOS requires data which is in the
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buffer area, it will search each 512 byte area in sequence from
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top to bottom even though the data it needs may be at the end of
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the buffer. Logically, then, there is an optimum number of
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buffers - too many used with a small program and you can slow
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things down, not enough and DOS will be forced to go out to the
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disk to retrieve what it needs. If you rarely use the same data
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within a program twice but load lots of different programs and
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data, a large number of buffers won't help. However if you need
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frequent access to a certain data file or portion of that file,
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buffers will help. Portions of the FAT are kept within the
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buffers area, so dropping your buffers to zero has the damaging
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effect that DOS must always go to the disk to read the FAT which
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isn't helpful either.
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Another way of substituting RAM memory for disk memory involves
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using a RAMDISK. The idea is to create in RAM memory an entire
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disk or a small portion of a disk. This works like magic on many
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machines since the reading of tracks and sectors takes place at
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the high speed of RAM memory rather than the mechanically
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limited speed of the read/write heads on a floppy or hard drive.
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But be careful. Three areas of difficulty can arise. First you
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must remember to take the data from a floppy or hard drive and
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move it into the RAMDISK. Many people do this automatically from
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within an AUTOEXEC.BAT file or may have several floppies, each
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with a different RAMDISK configuration depending on the task at
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hand. Copying data to the RAMDISK usually moves along briskly.
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Secondly you must sacrifice a large area of memory for the
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RAMDISK which can no longer be used by your main program. Users
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of computers with extended or expanded memory usually choose to
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put their RAMDISK in the extended or expanded memory area of RAM
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so that precious main memory is not lost. Still, a small RAMDISK
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can soak up 64K of RAM memory and one or two MEG RAMDISKS area
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common for many users. The third and most serious problem when
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using RAMDISKS is that they are volatile - switch off the
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machine or experience a power failure, and your data is lost
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forever! Rather than residing safely on a magnetic disk, the
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data is "floating" in RAM memory and should be - MUST BE! -
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written to a disk before the machine is powered down.
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Many applications fly with a RAMDISK. Users of word processors
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find that moving the spelling checker and thesaurus to the
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RAMDISK speeds up things considerably since these are used
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heavily in a random manner. Spreadsheet users find that reading
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and writing short data files to RAMDISKS is a boon. Programs
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which use overlay files or temporary files as well as
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programming compilers benefit from RAMDISK use. Batch files
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which are disk intensive as well as small utilities really
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sprint when placed on a RAMDISK. Basically, any program file
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which is frequently used and loaded/unloaded repeatedly to a
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disk during normal computer operation is an excellent candidate
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for RAMDISK placement. DOS contains a RAMDISK which is called by
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using the statement DEVICE=VDISK.SYS or DEVICE=RAMDRIVE.SYS (if
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you are using MSDOS) which is placed in your CONFIG.SYS file.
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Your DOS manual details the specifics such as stating the size
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of RAMDISK and giving it a drive letter. You must still copy
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your target files into the RAMDISK and place it in the search
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path (with the PATH= command) as we mentioned in a previous
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tutorial. And the RAMDISK should always be the first drive
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letter mentioned in the path command so that DOS searches it
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first for optimum results.
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Yet another area of investigation is that of CACHE software.
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Essentially a CACHE is an extension of the buffers idea we
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discussed earlier. But the twist is that the CACHE is searched
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intelligently by a searching algorithm within the CACHE software
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rather than from top to bottom as with the more typical DOS
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buffer search system. Disk CACHE software can be obtained as
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either commercial software or shareware. As with a RAMDISK, the
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CACHE requires a chunk of RAM memory to operate. This can be
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extended memory, expanded memory or main RAM memory. Some
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manufacturers include a CACHE program with the software package
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or DOS disk. A CACHE is a sophisticated type of RAMDISK, in a
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rough sense.
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CACHE software allocates a large area of memory for storage of
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frequently used disk data. This data is updated by an
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intelligent CACHE search algorithm in an attempt to "guess"
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which tracks of a disk you might read or need next. The CACHE
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also stores the most frequently used disk data and attempts to
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remove less frequently used data. Whenever DOS requests disk
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data, the CACHE software first tries to fill the order from data
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currently stashed in the CACHE which prevents a slower disk
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search.
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When data is written from the program to the CACHE, first a disk
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write is done to prevent data loss in case of power failure and
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then the data is stashed in the CACHE in case it is needed
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again. Usually the hard drive data is the target of the CACHE
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activity, but a floppy disk could also be cached. All CACHE
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software allows you to allocate the size of the CACHE as well as
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the drive or drives to be cached. And some even allow you to
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specify exact files or data to be cached. The key is that high
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use data lives in RAM memory which keeps tedious disk access
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times low. In general, if your computer has a megabyte or more
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of memory and a speedy processor such as an 80286 or 80386
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either or both a CACHE or RAMDISK option does improve
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performance.
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As we leave hard disk boot camp, let's finally look at hard
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drive formatting processes. Two basic formatting operations are
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of concern: physical formatting or low level formatting and
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logical or high level formatting. When you use the format
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program on a floppy disk both low level and high level
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formatting is accomplished. On a hard disk, formatting performs
|
||
only logical or high level formatting. On a hard disk, low level
|
||
formatting is usually done to a disk before shipment. As an
|
||
aside, the FDISK command of DOS has little to do with either
|
||
type of formatting, but is a method of partitioning or arranging
|
||
the data onto the hard drive tracks. Each disk platter is
|
||
separated into circular concentric tracks where data is stored
|
||
as we saw earlier. During physical formatting the tracks are
|
||
divided into further subdivisions called clusters and further
|
||
yet into sectors. High level formatting involves the specific
|
||
ordering of the space for the exclusive use of DOS and is a bit
|
||
more analogous to the formatting of a floppy disk.
|
||
|
||
Some software programs of use by hard drive owners:
|
||
|
||
The following two programs perform low level formatting and
|
||
simple diagnostic routines on a hard drive:
|
||
|
||
Disk Manager and CheckIt
|
||
|
||
Data recovery and "unerasing" programs also containing
|
||
diagnostic routines are:
|
||
|
||
PC Tools Deluxe, Norton Utilities, Mace Utilities
|
||
|
||
Extensive diagnostic and maintenance/data repair functions as
|
||
well as interleave alteration and head parking are offered by:
|
||
|
||
SpinRite II, Optune, Disk Technician
|
||
|
||
Shareware programs with unerase functions include:
|
||
|
||
Bakers Dozen
|
||
|
||
Shareware programs with defragmentation capabilities include:
|
||
|
||
SST and PACKDISK.
|
||
|
||
Tutorial finished. Be sure to order your FOUR BONUS DISKS which
|
||
expand this software package with vital tools, updates and
|
||
additional tutorial material for laptop users! Send $20.00 to
|
||
Seattle Scientific Photography, Department LAP, PO Box 1506,
|
||
Mercer Island, WA 98040. Bonus disks shipped promptly! Some
|
||
portions of this software package use sections from the larger
|
||
PC-Learn tutorial system which you will also receive with your
|
||
order. Modifications, custom program versions, site and LAN
|
||
licenses of this package for business or corporate use are
|
||
possible, contact the author. This software is shareware - an
|
||
honor system which means TRY BEFORE YOU BUY. Press escape key to
|
||
return to menu.
|
||
|
||
|