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From: lstowell@pyrnova.pyramid.com (Lon Stowell)
Newsgroups: comp.protocols.ibm
Subject: Re: hard disk write protect
Message-ID: <156131@pyramid.pyramid.com>
Date: 20 May 91 19:51:53 GMT
Sender: daemon@pyramid.pyramid.com
Reply-To: lstowell@pyrnova.pyramid.com (Lon Stowell)
Organization: Pyramid Technology Corp., Mountain View, CA
Lines: 22


 Some older systems had hardware write protect to prevent
 accidental corruption of boot and load image files.  These had
 address switches....the hard drive was write protected from 
 0,0,0 up through the address set in the switches.  A separate
 switch enabled the protection...allowing updates when in the
 defeat position.

 You may need to prevent both write and erase current....the
 implementations I am familiar with actually performed the write
 protect in the disk controller...by inhibiting the Write Enable
 signal...which inhibited ALL write procedures for protected
 addresses.  A failure in the disk write circuitry could destroy
 data, but the purpose of Write Protect was to protect the
 hardware from software...not hardware faults.

 If you were to implement it today, you could provide write
 inhibit for a range of addresses...using software to load
 these.  You could then set an "expert" flag instruction in
 software to prevent ordinary mortals...such as root...from 
 overwriting the data.
 '