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From: thacher@unx.sas.com (Clarke Thacher)
Newsgroups: comp.lang.c
Subject: Re: What C compilers have non-zero null pointers?
Message-ID: <1990Jul19.231038.19372@unx.sas.com>
Date: 19 Jul 90 23:10:38 GMT
References: <422@minya.UUCP> <1990Jul10.141208.24902@uunet!unhd> <13520@softg.uucp>
Organization: SAS Institute Inc.
Lines: 17

Actually the Prime 50 series has both types of pointers.
The older (PL1) style of pointer uses segment 7777 (octal) in their
NULL pointers (there can never be a segment 7777), dereferencing
this type of pointer would raise a NULL_POINTER condition.  
The newer C compiler uses a pointer with a segment of 0 and an 
offset of 0.  This pointer is still not bit equal to an integer 0
there are two extra bits (for the ring number) that may or may not
be set.  To solve this, Prime added a TCNP (test c null pointer) 
instruction to the instruction set.  They also added a bunch of other
instructions for the C compiler (mostly having to do with character
operations).

Clarke Thacher        PRIMOS Host developer          SAS Institute, Inc.
sasrer@unx.sas.com    (919) 677-8000 x7703          Box 8000, Cary, NC 27512
-- 
Clarke Thacher        PRIMOS Host developer          SAS Institute, Inc.
sasrer@unx.sas.com    (919) 677-8000 x7703          Box 8000, Cary, NC 27512