Cheet Sheets 1995 February/CHEET38.ZIP/INSTALL.DAT/1ISKS.TXT

Text File | 1995-02-07 | 14KB | 372 lines

~Disk Drives.

An explanation of what they are and how they work.

Written by Wayne Roberts back in 1989 !!!  So some of the information
is definately out of date, and remember this is way before the CD-ROM
was used on home computers...  Before the 386 was born, just around
the time when VGA was introduced and the time when a 40Mbyte hard
drive was a luxury !!


~What are disks ?

Disks allow the computer to permanently or temporarily
remember, or store, information.  Unlike information in ROM
(Read Only Memory) chips, this information can be readily
changed.  There are several types of disks.  Two examples of
disk storage follow :-


~Floppy Disks (or diskettes).

A floppy disk works in a similar way in which a cassette does
but is structured like a record.  The computer's disk drive
writes information on it just like a tape player records
music on tape.  When the computer puts information on a disk,
it is said to be writing to the disk.  When the computer uses
this information, it is said to be reading from the disk.
The floppy disk itself is portable.  Currently there are two
popular sizes of floppy disk - the 5.25" and the newer
smaller 3.5" size.  In the past 8" and 3" have been used but
these sizes have slowly diminishing.

Floppy disks are used with a floppy disk drive.  This drive
reads and writes to the disk and must be the same size as the
disk being used.  Besides the obvious physical difference, a
3.5" disk is more durable than the older 5.25" disk.  And the
3.5" disk can surprisingly hold often as much as twice as
much information than 5.25" disks.  3.5" disks are rapidly
becoming the standard size disk for computer systems.

The disk's storage capacity, or amount of information that
can be written to a disk, is called disk memory.  Disks have
memory just as computers do.  For instance, a standard 5.25"
disk can hold 360 Kilo bytes, while the new 3.5" can hold
720K.  The newest IBM PC, the PS2, uses 3.5" disks which can
hold 1.44 Mbytes of information instead of the older 720K
standard.

As well as the amount of memory a disk has there are many
other different aspects of a floppy disk.  Below is a brief
explanation of these.

The density of the disk can be either Single, Double, High,
or Quad, the latter of these being the best as more
information can be stored on them.

Disks can also either be single or double sided.  The double
sided often being able to store a higher amount of
information.

The diskette itself is divided up into tracks and sectors.
The more the tracks and sectors the greater the storage size.
Eg. If a 3.5" floppy disk is Double Sided, has 80 tracks, 18
sectors and each sector is 512 bytes of information then the
memory available on the disk would be 1.44Mbytes (see below).

Sides x tracks x sectors x bytes per sector = memory on disk

`  2   x   80   x   18    x      512         = 1,474,560  (1.44M)


~Hard Disk Drives.

Hard disks are often referred to as fixed drives or non
removable disk, the hard disk drive is just like a floppy
disk drive except that it is not portable.  It is also stiff
rather than floppy, hence its name and is always situated in
the computer.  The advantages to a hard disk drive include
speed and the amount of information that can be stored on it.
The smallest hard disks are much faster than floppies and can
hold over 10 to 20 times as much information as a single
floppy.

The following tables shows some typical technical disk
information that can be acquired through disk utility
programs or by simply typing chkdsk (Check disk).  The
information given in the tables is kept as simple as possible
thus making them easier to follow.

+-----------------------------------------------------------------+
|Drive C:(Hard Disk)
|
|
|
|Basic storage capacity:
|
|
|
|  20 million characters (MegaBytes)
|
|  7% of disk space is free
|
|
|
|Logical dimensions:
|
|
|
|  Sectors are made up of 512 bytes
|
|  Tracks are made up of 17 sectors per disk side
|
|  There are 4 disk sides
|
|  The disk space is made up of 610 cylinders
|
|  Space for files is allocated in clusters of 4 sectors
|
|  There are 10,337 clusters
|
|  Each cluster is 2,048 bytes (2 Kbytes)
|
|  The disk's root directory can hold 512 files
|
+-------------------------------------------------------------------+
+-------------------------------------------------------------------+
|Drive A:(Floppy disk)
|
|
|
|Type of disk:
|
|
|
|  Double-sided, Quad-density, 9-sector. (DS-QD)
|
|
|
|Basic storage capacity:
|
|
|
|  720 thousand characters (Kilobytes)
|
|  69% of disk space is free
|
|
|
|Logical dimensions:
|
|
|
|  Sectors are made up of 512 bytes
|
|  Tracks are made up of 9 sectors per disk side
|
|  There are 2 disk sides
|
|  The disk is made up of 80 tracks
|
|  Space for files is allocated in clusters of 2 sectors
|
|  There are 713 clusters
|
|  Each cluster is 1,024 bytes (1Kbytes)
|
+-------------------------------------------------------------------+


~Other storage devices.

As well as floppy and hard drives there are many other types
of storage devices that are often removable from the
hardware.  These are normally tape backup up systems but are
also disk and even CD based.  Names given to these types of
storage are Bernoulli, Tape streamers, removable disk, Fault
Tolerant systems, WORM or CD-ROM (or disks).

CD-ROM is becoming wider used for storage of information as
the research into optical storage has been slowly renewed.
At the moment optical storage of information is done using
two methods WORM and CD-ROM.

WORM (Write Only Read Many) drives can have a capacity of up
to two Gigabytes, whereas compact-disk technology can have
capacities of up to 600 Mbytes, but at the moment it is of a
read-only nature.  Both WORM's and CD's are removable and
retail at a price between £5 and £50 when blank.

Recently the first re-usable optical disk has been pronounced
and are expected on the market by the end of 1989.


~General features of Hard and Floppy Disks.

Things to consider when buying a hard disk include the type
of magnetic medium on it.  Usually this was a ferric oxide
coating similar to that used on cassette tapes.  On some hard
disks a thin coating or pure metal which when plated on to
the disk provide a hard surface.  This makes the disk less
prone to damage and speeds up the Disk Access Time.


Disk Access Time - This is the time interval between the
command is given to receive data from the computer memory or
disk to the moment the transfer is completed.  Consequently
when a read or write command is received, any point to be
accessed within one second, whether it is about to pass by
the head or has just passed (in the latter case, the drive
was for the designated point to come around again in one
revolution).  The access time for a floppy disk is generally
between 90 milliseconds (1 millisecond= 1/1000 of a second)
and 500 milliseconds.  It is also possible to read from a
certain spot on the disk and move directly from there to
another spot.  This can be done very quickly - between 3 and
50 milliseconds.

In most systems the retrieval of data involves three
indentifiable tasks :-

     a) Seek time - This is the time taken to move the
     read/write head to the required address of the data.

     b) Rotational Delay - This is when a read/write
     instruction is issued, the head is not usually
     positioned over the sector where the required data is
     stored, there is some rotational delay while the disk
     rotates into the proper position. On average, the time
     taken will be half a revolution of the disk pack.  This
     average time is known as the "latency" of the disk.
     c) Data Transfer Time - This is the time taken to read a
     block of data into main memory.

Disk Controllers - Each disk drive must have means of
communicating with the computer that it serves, this is done
by the disk controller.  The purpose of the controller is to
take commands from an application (such as "get next record")
and, in conjunction with the operation system, translate them
into commands that will be understood by the disk drive.

When selecting a controller for a disk drive there are two
major considerations you should take into account.

     a) The controller and its disk drive must use the same
     recording convention.

     b) The controller and its disk drive must use the same
     interface protocol.


Sequential and Random Access files - Tape systems can record
or read data only in sequence.  In other words, they can only
read recorded data in order, and we call this a sequential
file.  A structure of a disk does not have this drawback.  It
has a structure which allows data to be quickly recorded onto
or read from any position on the disk.  This is called a
random access or a direct access file.  There are many
occasions when one wants to locate and use information
quickly on a computer or word processor.  This random access
file is very convenient for successively handling data that
was not produced in a particular order.
Advantages of Disk Drives (Floppy and Hard).

The necessity of a Hard disk drive is not so if you are using
a computer with a lot of memory or you are running programs
that do not take up disk space.  A second floppy drive is
often a substitute for a hard drive as the cost difference
can be a lot, but the speed and size of programs that are run
is often a let down.

Large programs such as ORACLE and MS EXCEL need a hard disk
to run as they can not run off floppy, plus they would run
extremely slow.

Probably the most obvious advantage of a hard disk is that it
can store a great deal more information and software
applications.  Eg. A floppy disk can hold probably hold one
software application whereas the hard disk could hold
anything from 8 software applications plus user files.

So, in short, Speed, price and storage capacity decides
whether you buy a hard disk rather than a second floppy
drive.


~What sort of drives are there you can buy ?

When buying a disk system you need to decide whether you want
a 3.5" drive or a 5.25" drive.  Probably your best choice
would be a 3.5" drive as they are quickly becoming the
standard size disk and if you use 5.25" disks elsewhere then
it would be advisable to purchase a 5.25" portable drive for
transferring of data.  5.25" floppy disks are just over half
the price of 3.5" disks and can store about half the amount
of information, thus in the long run 3.5" disks are a greater
value for money.


~Disk Management.

Storage systems are very prone to corrupt if they are not
looked after in the proper ways.  This section of the leaflet
illustrates some points which need to be taken towards
keeping your storage systems error free.

a) Floppy disk need more care taken of them than actual
   drives as they are very delicate.  Care of 5.25" is
   commonly illustrated on disk sleeves, (see overleaf), they
   are normally as follows :-

     i)   Never touch disk surface.
     ii)  Never leave near magnetic materials (eg.
     speakers,monitors).
     iii) Never bend the disk.
     iv)  Never write on disk with hard nibbed instruments
                    (pencils,biros).
     v)   Store in temperatures between 10-50C (50-122F).
     vi)  Store in envelopes and/or storage boxes.

b) The Floppy and hard disks need to compressed using a disk
   utility program.  This tidies up the disk by moving
   programs to the beginning of the disk where free spaces
   have been left by programs deleted, all these free spaces
   are then put to the end of the disk.  A compressor also
   lowers the chances of incurring bad sectors on a disk.
   You can also, by using a compress program, analyse the
   disk surface for bad sectors etc.

c) The floppy disk drive needs to be cleaned regularly using
   a disk head cleaner.  About once a month would be an ideal
   period in which you should clean your drive heads.

d) The hard disk should also have care taken that it is not
   damaged in any way.  Moving the system without parking the
   heads could quickly corrupt the disks operation.  On most
   systems you have a command "PARKHEAD", which parks the
   heads on the hard drive and allows you to move the system
   without damaging the drive in any way.

e) Their are many errors that can occur when using disk
   drives, some of the more common include :-

   1) NOT READY ERROR READING DRIVE A - Which means that
      either disk door is not closed or floppy disk is not
      inserted properly.

   2) GENERAL FAILURE ERROR READING DRIVE A - Which means
      that the disk is of a wrong format or it can not be
      read.

   3) INVALID DRIVE - Which means that drive specified does
      not exist.

   4) BAD COMMAND OR FILENAME - Which means that command
      entered does not exist or can't be found.

   5) NON-SYSTEM DISK OR DISK ERROR - REPLACE DISK AND PRESS
      A KEY - When re-booting computer if a system disk (with
      DOS commands etc.) is not present then this error will
      occur.

Other DOS errors that may occur could be found in a DOS user
manual and other disk maintenance hints can be found, in
detail, in books.  One such book is Hard-Disk Management
Published by TAB Books Ltd.


~There you are, a little look back to 6 years ago, when PC's were
~a little less used in the home.  It's nice to see what we were using
~years ago......