Re: DECSYSTEM 20

Article View: comp.arch

Article #1666

#1666

From: Bill Pechter
Date: Wed, 26 Jul 1989 22:09

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In article <3893@phri.UUCP> roy@phri.UUCP (Roy Smith) writes:
>In <BRUCE.89Jul23155646@heather.pooh.com> bruce@heather.pooh.com
>(Bruce Robertson) writes:
>> I like the way the PDP-11 treats the program counter as a general
>> register.  You can play a lot of tricks with that!
>
>	I always wanted to try (I don't know why I never did) using
>auto-decrement mode on r7 (i.e. the pc).  I wonder what would happen?  

Try this one -- toggle 014747 into memory location 10000 (or any other 
memory location in the lower 28kw).  Set the pc to that location and start
it.

The pc gets copied into pc-2  (autodecremented). Next that 014747
executes (pc-2 --> pc -4 ).  What you have is the old DEC Field Service
single instruction memory exerciser.  I fixed a flat down 11/70 with this
a year ago at W.W. Grainger in Niles, Ill.  

The program works in low memory which must be working to bootstrap the 
XXDP+ diag package.

The program will halt, trap to 4, or trap to 10 if anything goes wrong
(usually).

One thing of interest...  I've seen it wrap through the gpr's on some 
11's but not on others.  Any explanation on this would be appreciated.

Bill
(ex-Field Service)
Now just hacking and administering systems.  
No pager, no standby, no problem.

-- 
Bill Pechter -- Home - 103 Governors Road, Lakewood, NJ 08701 (201)370-0709
Work -- Concurrent Computer Corp., 2 Crescent Pl, MS 172, Oceanport,NJ 07757 
Phone -- (201)870-4780    Usenet  . . .  rutgers!pedsga!tsdiag!scr1!pechter
  **   MS-DOS is CP/M on steroids, bigger bulkier and not much better  **

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<3256@wpi.wpi.edu> <EGDORF.89Jul21195924@zaphod.lanl.gov> <443@anasaz.UUCP> <BRUCE.89Jul23155646@heather.pooh.com> <3893@phri.UUCP>

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Path: arc1-ca.pugleaf.net!utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!bellcore!att!tsdiag!scr1!pechter
From: pechter@scr1.UUCP (Bill Pechter)
Newsgroups: comp.arch
Subject: Re: DECSYSTEM 20
Message-ID: <433@scr1.UUCP>
Date: 26 Jul 89 22:09:57 GMT
References: <3256@wpi.wpi.edu> <EGDORF.89Jul21195924@zaphod.lanl.gov> <443@anasaz.UUCP> <BRUCE.89Jul23155646@heather.pooh.com> <3893@phri.UUCP>
Reply-To: pechter@scr1.UUCP (Bill Pechter)
Organization: CONCURRENT COMPUTER,OCEANPORT NJ
Lines: 38

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