DEFMTH4.CVP   920105
 
                  Write protection - hardware
 
Generally, in the microcomputer world, write protection is held
to mean write protection implemented by hardware.  Although it
is a truism that "whatever the hardware people can do, the
software people can emulate, and whatever the software people
can do the hardware people can emulate", it is physically
impossible to overcome a "sufficient" hardware protection with
software.  Note, however, that not all hardware protection
devices are as safe as they may seem at first glance.
 
First, the universal write protect "tab" on floppy disks.  It
*is* possible to write to *some* write protected drives. 
Certain systems (MS-DOS is not one) check for write protection
in software rather than hardware.  Thus, even though the write
protect device is hardware, the software checking can be
circumvented by a virus.  (In systems where the write protection
*is* effective, it is still the case that the notification of an
attempt to write to the drive is done through software, and so
the warning that something may be going on may be trapped by the
virus.
 
However, even on some MS-DOS systems, write protection may not
be reliable.  Some manufacturers use an optical, rather than
mechanical, sensor for the write protect tab or notch.  Using
"translucent" floppy disks, the "silvered" write protect tabs or
even the shiny black ones on 5 1/4" diskettes, may allow
sufficient light to get through to the sensor as to leave the
disk unprotected.  It is interesting to note that, because of
the two different protect tab designs, the hardware write
protection circuits for 5 1/4" diskettes generally "fail safe"
in a write disabled configuration, whereas 3 1/2" diskette
drives "fail" into a writable configuration.
 
(A pity.  I prefer the ability to protect and enable repeatedly
without building up gobs of tape adhesive around the notch.  And
when I did protect 5 1/4s, I used to use "magic" tape as it was
easier to remove.  These days I'm using "Post-it" notes ...)
 
As in the past, so again I will deplore the failure of drive
manufacturers to provide write protect switches on "fixed media"
hard drives.  Tape and cartridge media do have tabs or switches. 
Those knowledgeable about hardware and drive cabling can
"retrofit" switches, but recent tests at various sites with
hardware write protect switches have indicated problems with
certain types of drives.  No one procedure has been proposed
that works for all types of drives.
 
copyright Robert M. Slade, 1992   DEFMTH4.CVP   920105

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