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From: howard@cpocd2.UUCP
Newsgroups: comp.arch
Subject: Re: catering to bad code
Message-ID: <436@cpocd2.UUCP>
Date: Thu, 19-Feb-87 15:51:44 EST
Article-I.D.: cpocd2.436
Posted: Thu Feb 19 15:51:44 1987
Date-Received: Sat, 21-Feb-87 04:29:24 EST
References: <14833@amdcad.UUCP> <14837@amdcad.UUCP>
Reply-To: howard@cpocd2.UUCP (Howard A. Landman)
Distribution: world
Organization: Intel Corp. ASIC Services Organization, Chandler AZ
Lines: 44

In article <14833@amdcad.UUCP> phil@amdcad.UUCP (Phil Ngai) writes:
> In a Unix system I am designing, I am considering catering to bad
> code. That is, like the VAX I propose to make location 0 contain a
> readable 0.
> What do people think of this? Is it kind of disgusting?

I agree with Rob and others who have said No (don't do it) and Yes
(it's disgusting).

It's not just location 0.  It's all small integers, positive and
negative.  I once made a typo which left legal code that managed to
pass lint but still dereferenced address 1 (or was it 3?), treating
it as a (char *) and printing the string (i.e. the contents of low
memory) out onto the users terminal.  The terminal was emulating a
VT100 and low memory just happened to contain several copies of
the "Control String Initiator" character.  The result was that the
terminal would hang waiting for a "Control String Terminator" that
never came.  Explaining this bug to customers who had just lost
minutes/hours of work was not pleasant.  Explaining to management how
it had occurred and gotten by me unnoticed AND GOTTEN THROUGH SIX
WEEKS OF SOFTWARE QA UNNOTICED *AND* *BEEN* *SHIPPED* *TO* *EVERY*
*CUSTOMER* was even less enjoyable.  On a machine without read access
to page 0 this bug would have caused a simple core dump and been
easy to find and fix.

Approximately,
	INTENDED CODE:	printf("%s is %d","name",value);
	ACTUAL CODE:	printf("%s is %d",value);
(Note: this was before "printfck", which finds this error easily.)

It is safer to make page 0 *and* page -1 both inaccessible to catch all
uses of small integers as pointers, no matter which sign.  And perhaps
even all the pages addressable by shorts.  Actually, you'd like it to be
true that you couldn't accidentally use ANY integer in place of a pointer,
but this is not easy on an untagged machine unless you are willing to make
most or all of your address space inaccessible.  ;-)  Perhaps this is an
argument for tagged architectures.

Use of location 0 is a bug.  It is not portable to many machines and
operating systems (e.g. VMS).  And lots of early UNIX programs do it. :^(
-- 

	Howard A. Landman
	...!intelca!mipos3!cpocd2!howard