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From: town@hpspkla.spk.hp.com (Brian R. Town)
Newsgroups: comp.sys.ibm.pc.hardware
Subject: Re: Memory Parity. Is It Really Needed
Message-ID: <6040012@hpspkla.spk.hp.com>
Date: 20 Mar 91 15:23:52 GMT
References: <steveh.669386099@tasman>
Organization: Hewlett Packard Company, Spokane, Wa.
Lines: 43

steveh@tasman.cc.utas.edu.au writes:

>I installed them into my 386, disabled parity checking and it worked fine. 
>The question is, is parity really needed, and if so what am I going to be
>missing out on.

The parity bit is used for error detection.  Here is a brief description of
how it works:

   Whenever a byte (8 bits) is written to memory, special hardware adds up the
   number of bits set to 1.  The 9th bit is then set to 1 or 0 such that
   the total number of 1's is even (if using even parity) or odd (if using odd
   parity).  Example:
         
        DATA      TOTAL 1's        9th BIT FOR     9th BIT FOR
                  BEFORE PARITY    EVEN PARITY     ODD PARITY
        
        1110010       4                 0              1
        1010100       3                 1              0 
        0001000       1                 1              0
        0011000       2                 0              1

Now, whenever data is read back from memory, the hardware adds up the number of
1's (including the 9th bit).  If the system is using even parity, then the
total should be even.  If a read occurs and the total bits set to 1 is odd,
then a memory parity error has occured and the CPU gets an interrupt.  I am
not sure what PC's do to report this, but most systems will simply report
something like 'Parity error at <address>'.

So, now that you know what it is for, you can be the judge as to whether you
need it or not.  What you will be missing is the ability for your hardware to
detect single bit memory errors.  How a memory failure affects you depends
on if it is in code space of data space.  What you really have to worry about
is having an error that doesn't cause a crash, but corrupts some of your data.
Somehow, it helps me sleep at night knowing that I don't have to rely on a
system crash or data corruption to inform me that I have a memory problem. 8-)
I am of course sunk if I have more than a 1 bit error. 8-(

Now, want to hear about error correcting memory?????

Hope this helps,

Brian (I dream about this stuff) Town