Path: utzoo!utgpu!water!watmath!clyde!att!att-ih!pacbell!ames!umd5!purdue!i.cc.purdue.edu!j.cc.purdue.edu!pur-ee!uiucdcs!uxc.cso.uiuc.edu!uxe.cso.uiuc.edu!mcdonald
From: mcdonald@uxe.cso.uiuc.edu
Newsgroups: comp.lang.c
Subject: Re: Referencing through a null pointer
Message-ID: <225800028@uxe.cso.uiuc.edu>
Date: 27 Apr 88 13:50:00 GMT
References: <4729@cup.portal.com>
Lines: 22
Nf-ID: #R:cup.portal.com:4729:uxe.cso.uiuc.edu:225800028:000:1180
Nf-From: uxe.cso.uiuc.edu!mcdonald    Apr 27 08:50:00 1988


/* Written  6:04 pm  Apr 25, 1988 by henry@utzoo.uucp in uxe.cso.uiuc.edu:comp.lang.c */
> I imagine that PDP-11s without split I&D had *(char *)0 == 7 or 8, and
> *(short *)0 == 0407 or 0408 (OMAGIC and NMAGIC respectively).

Actually, no.  The a.out header was not part of the actual core image, so
the first instruction of the program was first; in practice this was the
"setd" that got the floating-point processor into the right mode (or tipped
the software off that the processor lacked hardware floating point), which
gave *(char *)0 == 011 and *(short *)0 == 0170011.  The programs which
made assumptions about *0 were generally the big ones, which ran split-space
of necessity.
-- 
"Noalias must go.  This is           |  Henry Spencer @ U of Toronto Zoology
non-negotiable."  --DMR              | {ihnp4,decvax,uunet!mnetor}!utzoo!henry
/* End of text from uxe.cso.uiuc.edu:comp.lang.c */
Actually, yes. I think Henry is making a unstated assumption: his use
of the phrase "a.out" implies he is thinking of Unix. Most PDP-11's run
RT-11. The memory location 0 in RT-11 is indeed 7. Thus if you're
not using relocation, *(char *)0 is 7. I just tried it. 

Doug McDonald