Diff between 11/20 and 11/45?

[sms@moe.2bsd.com (Steven M. Schultz)]

Hi -

> From: Dave Horsfall <dave at 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 expressly prohibited?

	But I do know what that 'something' was.   In "user" or "supervisor"
	mode MPFI functioned as "MFPD" - a user program could not read its own 
	I(nstruction) space.    Only for "kernel" mode did MFPI access the
	I space.

> 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 "run"
	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.

	Steven Schultz
	sms at moe.2bsd.com

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From: Johnny Billquist <bqt@Update.UU.SE>
To: Warren Toomey <wkt at cs.adfa.edu.au>
cc: Unix Heritage Society <pups at 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

Johnny Billquist                  || "I'm on a bus
                                  ||  on a psychedelic trip
email: bqt at update.uu.se           ||  Reading murder books
pdp is alive!                     ||  tryin' to stay hip" - B. Idol