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From: molly@killer.UUCP (Molly Fredericks)
Newsgroups: comp.arch
Subject: Re: *Why* do modern machines mostly hav
Message-ID: <1332@killer.UUCP>
Date: Fri, 14-Aug-87 03:50:21 EDT
Article-I.D.: killer.1332
Posted: Fri Aug 14 03:50:21 1987
Date-Received: Sun, 16-Aug-87 02:35:49 EDT
References: <2807@phri.UUCP> <101200004@datacube>
Organization: Plain'O Girls Home
Lines: 45
Summary: No, DEC-10's and 20's actually let you at 36 bits.

In article <101200004@datacube>, rick@datacube.UUCP writes:
> 
> /* Written  5:18 pm  Aug  5, 1987 by ram@elmgate.UUCP in datacube:comp.arch */
> >They use 18 bit words ( 9 bit bytes).
> >.. used a 10 bit byte.  And a nine bit tape.  This was weird
> 
> 
> 	Not really 9,10 bit words though. You don't get to use these
> 	bits, the board does, (for parity and/or EDC). The price you
> 	pay for correct data always is extra memory. This explains
> 	also 36 bit long words (4 * 8 data bits =32 data bits)+(4*1 parity 
> 	4 parity bits) =36 bits total.
> 
> rick@datacube.UUCP ( please forgive incorrect ettiquete I am new to the net)
> /* End of text from datacube:comp.arch */

DEC-System 10's and 20's actually had 36 bits in the word, all of which
where addressable.  The addresses where 18 bits long, which I think
explains the 18 bit Unibus addresses on early PDP-11's

The 10's had 2 or 3 character modes.  The first has 6 bit.  Six characters
were packed into a 36 bit word.  That gave 6 characters per word, no
parity.

The next mode was 7 bit ascii.  5 characters were packed into 36 bits with
the last bit wasted.  This was used as I understand for storing data in
datafiles.  The six bit stuff tended to shift all of your characters into
upper case, so that wasn't so hot for text files and anything else you
didn't want to become monocase.

There may have been an 8 bit ascii mode, but I don't ever recall doing
anything with it.

The memory checking was done someplace else as I once got loaded into a
bad memory box and crashed the system with a parity error.  The DEC-10
we had had PDP-11/23's working as node concentrators and I had the
privilege of getting the error message first ...  (Before everyone else
got `Path to remote system is lost')

Molly
-- 
       Molly Fredericks       UUCP: { any place real }!ihnp4!killer!molly
    Disclaimer:  Neither me, nor my cat, had anything to do with any of this
  "I love giving my cat a bath, except for all those hairs I get on my tongue"
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