From: Dave Horsfall <dave(a)fgh.geac.com.au>
Do you recall the PC-board hack on the sep-ID machines that changed
No, I do not recall seeing a PCB hack (hacking on an 11/70 was
frowned upon ;))
the MFPI instruction to do something that was
But I do know what that 'something' was. In "user" or
mode MPFI functioned as "MFPD" - a user program could not read its own
I(nstruction) space. Only for "kernel" mode did MFPI access the
Something about allowing a user program to access
something else, for
some obscure hack or other...
It was aimed at providing "execute only" code - a program could
but not "read" its code space.
This caused problems though if trap handlers (floating point exception
handling comes to mind) needed to retrieve the faulted instruction
for inspection/analysis. Thus in 2BSD there is a system call that
programs can issue to request the KERNEL to do the 'mfpi' for them
and return the value.
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From Johnny Billquist <bqt(a)Update.UU.SE> Thu Sep
9 19:58:19 1999
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Date: Thu, 9 Sep 1999 11:58:19 +0200 (MET DST)
From: Johnny Billquist <bqt(a)Update.UU.SE>
To: Warren Toomey <wkt(a)cs.adfa.edu.au>
cc: Unix Heritage Society <pups(a)minnie.cs.adfa.edu.au>
Subject: Re: Diff between 11/20 and 11/45?
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On Tue, 7 Sep 1999, Warren Toomey wrote:
These executables were written for a PDP-11/20. Are
there any significant
USER-MODE differences between the 11/20 and later PDP-11 models? I'm
thinking missing instructions, different addressing mode behaviour etc.
As far as I can remember, there aren't any huge differences. However, some
stuff behave differently in the 11/20. On the other hand, some stuff
behave differently in just about every implementation...
Condition flags on some instructions specifically. And the 11/20 might
have had some limitations on using the PC which differed as well.
I have a processor handbook for the "modern" -11s, which has a chart with
all differences between different models. I started writing it down, to
place in the PDP-11 FAQ, but haven't come that far yet...
Johnny Billquist || "I'm on a bus
|| on a psychedelic trip
email: bqt(a)update.uu.se || Reading murder books
pdp is alive! || tryin' to stay hip" - B. Idol