Path: utzoo!mnetor!uunet!lll-winken!lll-lcc!ames!nrl-cmf!cmcl2!brl-adm!brl-smoke!gwyn
From: gwyn@brl-smoke.ARPA (Doug Gwyn )
Newsgroups: comp.unix.wizards
Subject: Re: Aliasing text and data segments of a process
Message-ID: <7208@brl-smoke.ARPA>
Date: 29 Jan 88 21:37:01 GMT
References: <11476@brl-adm.ARPA> <459@minya.UUCP>
Reply-To: gwyn@brl.arpa (Doug Gwyn (VLD/VMB) <gwyn>)
Organization: Ballistic Research Lab (BRL), APG, MD.
Lines: 26

In article <459@minya.UUCP> jc@minya.UUCP (John Chambers) writes:
>>  From: Doug Gwyn  <gwyn@brl-smoke.arpa>
>>  > This cannot possibly work on an architecture that enforces the
>>  > distinction between Instruction and Data spaces.
>I tried it out on a PDP-11/75 that I had handy.  The machine definitely had
>separate I and D spaces, and the program quite definitely worked.  I didn't
>tell the compiler anything special, and I doubt the linker recognized that
>_main was special and belonged in I space.  But neither the compiler nor the
>linker was fazed by having main as a data array.

You don't listen very well, do you?  Just because the underlying hardware
CAN enforce the distinction between I&D space doesn't mean that it always
DOES so.  In fact, the usual UNIX C implementation for a PDP-11 defaults
to a single shared address space, and only programs that need more space
(such as the f77 compiler) request split I&D spaces by specifying the
cc -i option when linking.  Try running these example programs with I&D
separation enforced, AS I SPECIFIED, and see what happens.

As someone (rbj?) said, the PDP-11 isn't the best example, due to its being
possible to set it up to blur the I&D distinction by default.  (Some cheaper
models couldn't be set up to enforce the distinction!)  I used the PDP-11 as
an example because it seemed the machine you were most likely to have access
to.  I have seen segment-based architectures (Burroughs B5500 comes to mind)
where the default behavior is to enforce the distinction, and I would be
very surprised if IBM's System/38 or the H-P 3000 didn't also do so.

The way to understand an issue is not to resort to blind experiments.