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From: jbn@wdl1.UUCP (John B. Nagle)
Newsgroups: net.micro
Subject: Parity checking
Message-ID: <384@wdl1.UUCP>
Date: Tue, 14-Aug-84 22:49:57 EDT
Article-I.D.: wdl1.384
Posted: Tue Aug 14 22:49:57 1984
Date-Received: Thu, 16-Aug-84 03:49:42 EDT
Lines: 22


     Machines without parity checking must be considered only 
slightly above the toy level.  Intermittent errors are a continual
nagging problem in such machines.  The IBM PC has parity checking;
the PC Jr does not.  The TI Professional does not, and suffers from
intermittent problems because of it.  Any machine costing over $1000
should unquestionably have parity checking; below that level, there is
some argument for economy, but personally I would go for parity all the
way down to the appliance control processor level.
     In a small computer general-purpose operating system, parity errors in 
user space should kill the job involved and display a message, not crash
the machine.  Parity errors in system space should crash the machine with
a message.  More elaborate strategies are possible; this is a minimum.
     Power supplies should be designed such that if the output voltage
deviates from the rated value, the machine goes down.  A zener in the
right place will accomplish this.  It is better to crash fully than
have an undetected error.  Again, more elaborate strategies are possible,
such as power fail interrupts, but just plowing on is a bad idea.
     If you build an unreliable machine, it will not sell.  Remember
the Coleco Adam?
     					
				John Nagle